GEOGRAPHIC VARIABILITY IN COMPONENTS OF THE MALE

ADVERTISEMENT CALL AMONG POPULATIONS OF THE NORTHERN

SPRING PEEPER. PSEUDACRlS CRUCIFER CRUCIFER

Bryan William Rogers

A thesis submitted in confiormity with the requirements for the degree of Master of Science,

Graduate Department of Zoology, in the University of Toronto

O Copyright by Bryan William Rogers 2000

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Abstract

Geographic vwiability in components of the male advertisement cal1 among populations of

the northem spring peeper, Pseudacris crucifer crucger. Master of Science, 2000. Bryan

William Rogers, Graduate Department of Zoology , University of Toronto.

1 studied variation in three panmeters of the male advertisement cail across nine

populations of the spring peeper, Pseudacris crucifr. 1 discovered significant inter-

populational variability in two of those components: the midpoint of the dominant fiequency

and cal1 duration. Frogs fkom the southwestern Unites States, living in open habitats, have a

lower MDF and a longer cal1 than their northeastern brethren living in forested habitats;

possibly due to different transmission parameters in open versus forested habitats. There was

also a significant difference in body size among populations in the northeast, although the

reason for such size variation awaits furtber investigation. Based upon my results, 1 believe

that it is important for researchers investigating the effects of female preferences on the

evolution of male advertisement cal1 to aiso consider the effects of envuonmental selection in

shaping the structure of the c d .

Acknowledgernents

1 would like to convey my sincere thanks to : Dr. Deborah A. McLennan for

providing the opportwiity to pursue this research, and for the invaluable contributions she

made to this project and without whom this project would not have been possible, nor my

aspirations realized; Dr. Daniel Brooks for his assistance in the field, enthusiasm and

discussions which helped this research take fom; Dr. Glen Morris for his helpfuI

discussions relating to acoustic communication and the analysis thereof; Seth Seegobin for

his invaluable assistance in the statistical anaiysis, Company and bravery in the face of

inhospitable weather conditions, blood sucking insects, "local thugs" and Canadian wildlife,

but most of al1 for his motivation and encouragement when it was needed most. 1 would like

to give a special thanks to Michelle Mattern for her help with various computer applications

and her patience with my technological shortcomings; David Zamparo for his moral support

and assistance in maintainhg focus; My parents, Ronald and Helen Rogers for their

financial support, providing an environment that was conducive to the pursuit of my passions

and moa of al1 for believing in me. I would like to acknowledge the assistance of the

Deparmient of Ichthyology and Herpetology at the Roy& Ontario Museum for the use of

their resources; the department of Biology at the University of Arkansas for their hospitality

and assistance in locating ided Pseudach crucifer habitats. Research was funded by a

Naturai Sciences and Engineering Research Council of Canada grant to D. A. McLennan.

Table of Contents

Abstract .............. ....................................................... i i

Acknow ledgements ....................................................... iii

................................................................. Introduction 1 O 25

.................................................................... General Introduction

The animais: Who are the spring peepers

....................................... Taxonomy and Systematics

................................. General Biology ... ..........

Where are s p ~ g peepers found?

....................................... Regions of North America

Distribution of Pseudacris crucifer within regions ............

Why do we expect to find variability in advertisement call pmeters?

................................. Do we. in fact, find such variability? ..... .

................................................. Materials and Methods

Locations .................... .,. ...........................o...........* Recording ................... .... ... ... ................................. Cd1 Andysis .................................................................. Statistics ..................................................................

......................................................... Generai Observations 39 -40

............ Changes in male body size and advertisement cal1 variables 40 - 74

Snout-vent length (SVL) ....................................... 40

Cal1 Repetition Rate (CRR) ............................. .. .... 42 . 51

Mid-point of the dominant fiequency (MDF) ...................... 41 = 58

CailDuration(CD) ....................... .................. 58-63

Cornparisons of adjusted variables across geophysicd areas and

habitat types ................................................ 63 . 74

Discussion ............. ...... ......... ................................. 75 - 97

............................. General Introduction .......................... ... 75

Midpoint of the dominant fiequency ........................................ 75 O 83

Cal1 repetition rate ............................... .. ........................ 83 . 87

Cdlduration ......................... ..... ..................................... 87-93

....................................... Male body size (snout-vent length) 93 . 97

Summary .................................................................. 97 - 99

Literature Cited ..........................................................

.................................................................. Appendices

..................... Appendk 1 . Scanned images of site localities .. ......

Appendix 2 . Raw data for fiog c d . morphological. and ecological

variables .................................................................. 120 - 122

Appendix 3 . Cal1 variables adjusted for air temperature and snout-vent

length ..................................................................... 123 - 125

Appendix 4 . Oscillograms of male advertisement cal1 for each voucher

specimen ................................................................. 126 - 171

Tables

Table 1. Environmental variables fkorn North American regions in which

Pseudacris crucifer lives .............................................. 1 9

Table 2. Studies documenting inter-poprilational variability in parameters

........ ................... of the anuran male advertisement call ,.

Table 3. Means and standard deviations for variables measured at each

population ....................... .. ... .... .........................

Figures

Figure

Figure

Figure

Figure

.................. 1. Distriiution of Pseudacris crucifer subspecies

.............................. 2. Lins drawing of Pseudacris crucijer

3 . Map of North America highlighting the physiographic regions

........... ................... relevant to Pseudacris crucfer .... .................... . 4. Map showing locations of collecting sites ...

Figure 5. Scanned image of a oscillogram highlighting midpoint of the

dominant fkquency, repetition and duration components of the

.,... ......*.*......... male advertisement call ................... .. ,.,

Figure 6. Differences in snout-vent lengths of calling Pseudacris

crucifer males Grom 9 different populations (South Portage sites

arecombined) .......................................................... Figure 7. Differences in unadjusted (raw) values for call repetition rates

among 9 different populations (South Portage sites are combined) 46

Figure 8. Regression of call repetition rate against air temperature ...... 48

Figure 9. Differences in call repetition rate (adjusted for air temperature)

among 9 populations of Pseudacris crucifer (South Portage

............... .................... sites are combined) ..............œ.

Figure 10. Differences in unadjusted (raw) values for midpoint of the

dominant fkquency among 9 populations of Pseudacris crucifer

...... (South Portage sites are combiaed) ...................... ..

Figure 1 1. Regression of midpoint of the dominant fiequency

........... against (a) air temperature and (b) snout-vent length

Figure 12. Differences in midpoint of the dominant fiequency (adjusted

for air temperature and body size) among 9 populations of

Pseudacris crucifer (South Portage sites are combined) .......

Figure 13. Differences in unadjusted (raw) values for cal1 duration

among 9 populations of Pseudacris crucifer (South Portage

................................................... sites are combined)

......... Figure 14. Regression of c a l duration against air temperature

Figure 1 S. Differences in cal1 duration (adjusted for air temperature)

among 9 populations of Pseudacris crucifer (South Portage

................................................... sites are combined)

Figure 16. Differences arnong the three cd1 variables grouped by

geological area. (a) Cal1 +tition rate adjusted for air temperature

and body size; (b) Mid-point of the dominant fiequency adjusted

for air temperature and body size; (c) Cd duration adjusted for air

............................................................. temperature 67

Figure 17: Cali duration (adjusted for air temperature) plotted against

(a) longitude and (b) latitude . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . .......... . . 70

Figure 18: Midpoint of the dominant fkequency (adjusted for air temperature

and body size) plotted aga& (a) Longitude and (b) latitude . . . . . .

Figure 19. Differences arnong cal1 variables grouped by the density of

vegetation. (a) Cd duration adjusted for air temperatme;

(b) Mid-point o f the dominant fiequency adjusted for air

temperature and body size . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .

Researchea have ken fascinated with the evolution of male advertisement cails in frogs

for decades (Blair, 1958 a, b, 1962,1964; Martoc 196 1 ; Loftus-HUS and Littlejohn, 197 1 ;

Gerhardt, 1974,1976,1978a, 19784 1982,1987,1991; Rosen and Lemon, 1974; Oldham

and Gerhardt, 1 975; Brown and Brown, 1 977; Doherty and Gerhardt, 1 984; Forester and

Czarnowsky, 1985; Forester and Harrison, 1987; Schwartz, 1987; Sullivan and Leek, 1987;

Asquith et. al., 1988; Platz, 1988, 1989; Forester et. al., 1989; Ryan and Rand, 1990;

Gerhardt et. al., 1989; Sullivan and Hinshaw, 1990; Sanderson et. al., 1992; Ryan et. al.,

1996; Penna and Solis, 1998; Schwartz and Gerhardt, 1998; Brenowitz and Rose, 1999). One

result of that fascination has been the painstaking documentation of two different functions

for the call. The first bction pertains to species recognition: when given a choice between a

conspecific and a heterospecific call, fernales of many anuran species are able to discriminate

between hem, showing preference for the cails of conspecifics. Those preferences are based

upon inter-çpecific differences in a variety of call parameten, including pulse repetition rate

(two Australian hylids, Hyla verre& and Hyla ewingi: Littlejohn and Loftus-Hills, 1968;

Lo Aus-Hills and Littlejohn, 197 1 ; the neotropical frog, Hyla microcephala: Schwartz, 1987;

the gray tree fkog, Hyla versicolor: Klump and Gerhardt, l987), fkequency (the green tree

fiog, Hyla cineru: Gerharât, 1974,1978b, 1982; Oldham and Gerhardt, 1975), pulse rate and

fiequency (Woodhouse's tosd, Bufo woodirowei: Sullivan and Leek, 1987), and the presence

of a unique c d component (the barking tree fro& Hyla gratiosu: Oldham and Gerhardt,

1975; the Thgara fiog, Physalaemus pt ls l~iow: Ryan 1980,1985; Ryan and Rand, 1990,

1993a; Wiiczyiiski et al., 1995).

The second hction of the male advertisement call involves fernale choice of a mate

within her own species based upon intra-specific variability in a call parameter or parameters.

These responses include preference for (i) higher call rates and lower dominant fkequencies

(the northem cricket fiog. Acris crepitans: P e d l and Lower, 1994), (ii) a longer call

duration, an additional component [the chuck] added to the cd1 and a lower f'undarnental

fiequency for that component (the &gara frog, Physafuemus pustuIomrs: Ryan, 1980; Ryan

and Rand, 1990, 1993a, 1993b), (iii) higher call rates (Woodhouse's toad, Bufo woodhousei:

Sullivan, 1983, l987), (iv) longer call duration (the gray treefkog, Hyla versicolor: Klump

and Gerhardt, 1987; Gerhardt, 1 W ) , (v) a longer, more persistent cal1 with more pulses, a

higher repetition rate, and varying fkequency bands (high and low fiequency bands but not

intermediate levels: the green treefiog, Hyfa cinera: Gerhardt, 1 978b, 198 1, 1 982, 1987). and

(vi) calls containing additional secondary notes (Hyla microcephala: Schwartz, 1987).

Al1 of the preceding studies have contributed to our understanding of forces infiuencing

the evolution of the male advertisement call. Given the charismatic nature of frogs and their

reputation as rnighty singea, however, it is sulprising that information has been collected For

so few species (of approximately 4,360 described species). Even more surprishg is the

relative lack of attention that has been paid one of eastem North America's most ubiquitous

singers, the spring peeper, Pseudacris crucifer* Research on this species has Uidicated that

fexnaies identify conspecifics by call duration (approximately 150 - 300 rnilliseconds;

Doherty and Gerhardt, 1984) and are tuned to a kquency of approximately 2800-3360 Hz

(Schwartz and Gerhardt, 1998). Studies have demonstrated that femaie spring peepers

respond preferentially to diffierences ammg conspecific males in cal1 ~petition rate

(Forester and Czarnowsky, 1985; Sullivan and Hinshaw, 1990), c d duration (Doherty and

Gerhardt, 1984), c d intensity (Doherty and Gerhardt, 1984; Forester and Cuunowsky,

1985; Forester and Harrison, 1987), callhg persisteme (Forester et al., 1989) and dominant

nequency (Doherty and Gerhardt, 1984; Forester and Czarnowsky, 1985; Forester and

Harrison, 1987; Schwartz and Gerhardt, 1 998). Interpretation of those results, however,

rernains controversial because of misunderstandings about the way in which female spring

peepers actually hear male calls and problems with the confouuding effects of temperature on

the production of those calls (see discussion in Schwartz and Gerhardt, 1998). To date,

studies on P. crucifer calling behaviour have been conducted using populations fiom Maine,

Maryland, Missouri, and Quebec. In this thesis, 1 will add to that database by focussing my

attention on the amount of variation in the male advertisement c d within and among

geographically disjunct populations of spring peepers fiom Ontario, Arkansas and Missouri.

The animais: Who are the spring peepers?

Taxonomv and Svstematics

Pseudacris crucifer is a member of the class Amphibia (amphibians), order Anura (fiogs

and toads), family Hylidae (tree fiogs), and the genus Pseudacris (chorus fiogs). Pseudacris

crucifer, descnied by Wied-Neuwied in 1838, was divided into two subspecies by Harper in

1939: P. crucifer cnîcifer (northem spring peeper: Wied-Neuwied 1838) and P. c.

bar~umiana (southem spring peeper) (Wright and Wright, 1995; Frank and Ramus, 1996).

The northem spring peeper (or peeper, Pickering's tree fiog, Pickering's tree toad,

Pickering's Hylodes, Pickering's Hyla, peepiag fiog, or the castanet tree hg) , characterized

by a plain or Wnially plain kiiy, is found throughout most of eastem North America. It

Figure I : Distribution of Pseudacris crucifer subspecies. Pseudacris crucifer crucifer in

the north, and P. c. bur~amiana in the northern half of Flonda and southern Georgia

(adapted fiom Wright and Wnght, 1995)

ranges fiom the Gaspé Peninsula to Manitoba and Minnesota south to northeastern Kansas,

Arkansas, Louisiana, Texas, Mississippi, Alabama, Piedmont, Georgia, South Carolina to

New Bmswick (Figure 1). The southem spring peeper (also the Florida peeper, southem

peeper, Bartramian peeper, Sabalian peeper and the coastal peeper), characterized by a belly

that is strongly marked with dark spots, is found fiom the Coastal Plain of southem Georgia

to the northem end of Florida (Figure 1 : Conant and Collins, 199 1 ; Wright and Wright, 1995;

Harding, 1997). The differences in abdominal colouration are presently the only characters to

have been documented as being consistently geographicdly different between these two

subspecies. Clearly additional work needs to be done before we can fully understand the

nlationship between these two subspecies.

Pseudacris crucifer was first named in "Cantonment Leavenworth" (now Fort

Leavenworth), Leavenworth County, Kansas U.S.A. in 1838 (Wied, 1843). This anuran was

originally placed in the genus Hyla due to some characters that it shares with Hyla, including

the possession of more than ten premaxillary teeth, the sphenethmoid not projecting forward

between the nasals, having the anterior end of the dorsal ridge of the urosty le perpendicular

to the long axis of the urostyle, a large ilial protuberance, a well-developed ilid ridge,

moderately well-developed toe webbing, hylid "like" leg muscle prote&, a well-developed

encounter cal1 and singly deposited eggs (Hardy and Borroughs, 1986). Hedges (1 986)

suggested that Hyla crucifer be transfèrred to the genus Pseudams based on analysis of

electiophoretic data. Using genetic distances, both phenetic and parsimony (ûistance

Wagner) analyses indicated that Hyla cmc@ier mis more closely related to fkogs within the

genus Pseudami than fiogs found in Hyla. Moving H. crucifer into Pseudacris was

supported by other evidence, including albumin immunologicai (Maxon and Wilson, 1975),

karyotypic (Wiley, 1982) and hybridization data (Mecham, 1965; Ralin, 1970). Interestingly,

Blair (1958b) had already proposed that the short cal1 (averaging 0.14 sec.) repeated at

intervals of 0.8 to 1 .O seconds with a dominant fiequency averaging 2467 Hz, of P. crucifer

"show [ed] certain interesthg similarities" with other species of Pseudacris.

The species in the genus Pseudacris typicdly possess relatively mal1 digital pads, have

spherical or ovoid testes surrounded by darkly pigmented peritoneum (Ralin, 1970), exhibit

terrestrial behaviour and breed during the cold weather of winter or spring (Hedges, 1986).

Pseudacris crucifer 'Sts the bill" for the genus Pseudacris with two minor exceptions. The

digital pads of this frog are intermediate in size between hylids and those of Pseudacris

(Hedges, 1986); and the fiogs are not entirely ground-dwellers (Conant and Collins, 199 1 ;

Behler and King, 1997; Harding, 1997), spending a great deal of t h e in s h b s and tall

grasses. 1 feel that these two characters are related, with the intermediate sized toe pads

permitting this fiog to live a semi-arboreal lifesty le. As a result of Hedge's (1 986) study

"cmcfer" was moved into the genus Pseudads.

Addt spring peepers are relatively small fiogs ranging in length fiom 1.8 to 3 .7 cm (males:

1.8 - 3.5 cm; fernales: 2.0 - 3.7 cm; Conant and Collins, 1991; Wright and Wright, 1995;

Harding, 1997). Both sexes have a pointed m d e , projecting considerably beyond the lower

jaw (Wright and Wright, 1995). Unlike other members of the genus Pseudacris, the smooth

or nearly smooth skin of peepers is not distinctiy striped, monled or spotted. They do,

however, have an oblique, dark line on their backs approximating the shape of a cross

(Figure 2). The cross may be incomplete, having spurs and, in some cases, side bars

associated with it (Harding, 1997). It is believed that these variations are p h a i l y genetic and

pdaiiy due to variables such as temperature, health, diet and time of year. Peepers range in

colour fiom brown, tan, or gray to yeilow or olive, with females generally lighter than males

(Wright and Wright, 1995). Although individuals are capable of darkening or lightening the

shade of their skui in response to either interna1 "mood" or extemal surroundings, very littie

research has been done on the exact mechanisms underlying this ability (Conant and Collins,

199 1 ; Harding, 1997; Behler and King, 1997; personal observation). A V- shaped marking

is usually visible between the eyes, and a dark stcipe often runs Erom the nostnl through the

eye to the tympanum, and in some individuals extends down the side. The upper surfaces of

the legs of this species have crossbars, while the under surfaces of the hind legs and groin are

plain yellow or pink. In contrast, the belly can be white, cream coloured and sometimes pink.

The feet have marpinally expanded pads on the toe tips and the feet are not webbed (Wright

and Wright, 1995). As mentioned previously, the sexes of P. crucifer cm be disthguished,

althou& not easily, because males tend to be slightly smailer and slightly darker than

females. During the breeding season sexing individuals becomes much easier because males

develop visibly mottled, dark green loose skin under their chhs and on their throats,

representing the vocal sac (Conant and Collins, 1991 ; Harding, 1997; Behler and King,

1997).

Pseudacris crucifr breeds in temporary and permanent ponds, marshes, ditches and

flooded areas. Males tend to cal1 nom the waters edge while partialiy submerged, fiom

perches in the ta11 grasses, or 60m clumps of s h b s that ate in or near water (Harding,

1997). Males may begh calling fiom breeding ponds in late March or early ApriI in northem

Figure 2. Photographs of Pseudacris crucifer

A Vocalizing Pseudacris crucifer Male

Represen tative of Pseudacris crucifer

10

habitats. In fact these fiogs are quite often the first ones to call in many areas, finishing thek

breeding season by the end of June (Harding, 1997). In contrast, southem areas experience

the bteeding ritual of the spring peeper during fd and winter rains fiom November and

March (Behler and King, 1997). Although the presence of adequate water, often linked to

increased &al1 creating temporary ponds, is an important prerequisite for breeding,

breeding itself appears to be triggered primarily by gradually inc~asing temperature

(Duellman, 1986).

The cal1 of the spring peeper consists of a hi&-pitched ascenâing whistle or "peep"

(dominant fiequency averages 3000 Hz, range of 2670 - 3300 Hz: Doherty and Gerhardt,

1 984; Schwartz and Gerhardt, 1 998) repeated at intervals of about one second by infiahg a

single large vocal sac. These frogs are also capable of producing a trill-like call that

resembles the sound made by dragging your thumb down the teeth of a comb. This signal is

usually associated with male - male aggressive interactions during breeding and is usually

used to establish the cali hierarchy within a chorus and not to attract female conspecifics

(Rosen and Lemon, 1974). Pseudacris crucifer is an antiphonal species, often calling in

duets, trios and well-orchestrated choruses. Once a female has indicated her interest by

approaching a calling conspecific, amplexus is initiated by her physicaily corning into

contact with the male (Harding, 1997). In order for this sexual act to be successful the

female mua ovulate prior to amplemis (Bragg, 194 1 ; Jameson, 1955; Gosner and Rossman,

1959; Oplinger, 1966). The female attaches up to 900 (1.10 mm) eggs in a strand to

submerged vegetation as the male simdtaneously fe r t ihs hem (Gosner and Rossman,

1960). Both the eggs and larvae of peepers are fiilly aquatic? with the embryos developing

into filee-swimming tadpoles in approximately six days. These tadpoles feed on primarily

algae and soft decaying plant material for another 45 days prior to undergohg

metamorphosis. It will be one to two years before they thernselves can breed (Gosner and

Rossman, 1960). Once metamorphosis is complete, adult peepers switch fiom eating algae to

a diet consisting of invertebrates such as spiders, mites, ticks, pi11 bugs, ants, beetles,

springtails and caterpillars as weli as other selected arthropods (Harding, 1997).

Peepers are very rarely seen outside of the breeding season, when they disperse into

woodlands, old fields, and s h b b y areas to beginkontinue the terrestrial portion of their life

which lasts approximately ten months out of the year (Harding, 1997). They can be spotted

occasionally at this the , but only when it is rainlng and they are bmly visible against the

leaf litter. During the colder months they often over-winter beneath logs, bark or fallen

leaves. These small fiogs are actually able to survive subfieezing tempematures because they

have high levels of glycerol in thek body tissues and fluids. Glycerol, which is not found in

their tissues at any other time of the year, causes ice to fom in the intercellular spaces rather

than the cells thernselves. This decreases ce11 damage due to freezing, allowing peepers to

tolerate and function at temperatures as low as -6°C for up to five days, at which time

approxirnately 35% of their body fîuids will freeze (Schmid, 1982; Harcling, 1992; Pinder et

ai., 1992).

Where are spring peepers found?

Renions of North America

Geograptiically, North America may be divided into five physiographic divisions

(Lobeck, 1948). Each of these divisions is subdivided into provùices which are then M e r

subdivided into smaîier units. Each physiographic division and its smailer countefparts has

its own unique combination of ecologicai factors including temperature, precipitation,

elevation and vegetation. The combination of al1 these factors results in profound differences

in the habitats available to arnphibians. Pseudacris crucifer is found in four of the

physiographic divisions, the Interior Plains, the Atlantic Coastal Plain, the Appalachian

Highlands and the Lamntian Upland @uellman and Sweet, 1999).

Interior Plains (Figure 3; Table 1): extends from the Laurentian Uplands and

Appalachian Plateau in the east to the Rocky Mountains in the West, and frorn the Arctic Sea

nearly to the Gulf of Mexico where it meets the Atlantic Coastal Plain (Lobeck, 1948).

The spring peeper is found in ail tbree phy siographic divisions of the Interior Plains

(Stebbins, 1985; Conant and Collins, 199 1): (i) the Great Plains: a long region with a

surface area of approximately 1,489,000 km2, spreading fiom approximately 5S0N.

southward to the Atlantic Coastal Plain. I t reaches a height of about 3 00 m in the east, where

it meshes with the Interior Lowlands, to an average of 1600 m in the West by the Rocky

Mountains. This difference in elevation creates a strong latitudinal gradient in both

temperature and precipitation. Although there are five natural regions recognized within the

Great Plains (Shortgrass Steppe, Tallgrass M e , Prairie Parkland, Aspen Parkiand and

Edwards Plateau), P. crucifer has been reported in only two , the Prairie Parklands, a wide

ecotone separating the Tallgrass Prairie and the eastem deciduous hardwood forest in the

Interior Lowlands, and the Taiigrass Prairie. Despite a great deal of the area king under

cultivation, some stands of old forest coupled with grasses and sedges of the Tallgrass Prairie

still remain; (6) the Interior Lowlandr: has a d a c e area of approximately 2,646,000 krd

and encompasses al1 of the Great Lakes with the exception of Lake Superior. The lowlaeds

Figure 3 : Map of North America highlighting the phy~io~mphic regions relevant to

Pseudacris crucifer. Borders of regions are showri by dotted lines; borders of physiographic

divisions are shown by heavy , solid lines* Physiographic divisions and provinces are

identified as follows: II= Intenor Plallis; V= Laurentian Upland. The Everglades and

Flonda Keys are part of the Peninsdar Florida Region (adapted fiom Duellman and Sweet,

1999).

fonn a broad transition zone between the Appalachian Highlands in the east and the Great

Plains on the west. Its northemmost point consists of the Arctic plains of Alaska and the

Mackenzie Lowlands of northwestem Canada; eastward it borders the Laurentian Shield

down south io the Great Lakes. Physically the region ranges from Bat areas to slightly rolling

hills with elevation generally between 200 and 400 m. The vegetation in ihis region is

variable, ranghg fiom coniferous forest (fi, Abies sp., and spruce, Picea sp., to pine, Pinus

sp., and hemlock, Tsuga sp., in the east) to deciduous hardwood forest (beech , Gmelina sp.,

and maple, Acer sp., in the east with the addition of basswood, Tilia ornericana, in the west).

The southemrnost part of the region is largely oak-hickory forests; and (iii) the Interior

Highlands: an elevated region, of approximately 1 19,000 km' in the south-centrai United

States. The northem Ozark Plateau is divided from the Ouachita Mountains to the south by

the Arkansas River Valley, with an elevation not exceeding 839m. The region has many

spring fed strearns, some of which are thermal, and is comprised of deciduous hardwood

forests (oaks, Querms sp., and hickory, Carys sp., with pine forests existing at higher

elevatioas)@uellman and Sweet, 1999).

Appdachhn Highlands (Figure 3, Table 1): a lengthy northeast - southwest region

approximately 1,355,000 km2 in area which extends fiom the island of Newfoundland to the

Mississippi Embayment and which divides the Atlantic Coastal Plain h m the Interior

Lowlands (Lobeck, 1948).

Pseudacris crucifer thrives in al1 four regions which comprise the Appdachian

Highlands: (i) Tlie Northern AppaIachim: fiom Newfoundland south through the Gaspé

Peninsula and the extreme aortheastem United States (i.e. - New England states and eastern

New York). in the late Triassic a number of basins were created by continental sepmtion,

this barrier in turn separates the Northern Appalachians fiom the Southem Appalachians.

Elevations do not exceed 191 7 m with the habitat consisting of coniferous forests (fi and

spruce) at higher, and deciduous hardwoods (beech and maple) at lower, elevations; (ii) the

Southern Appalachians: extend south-west from the Northern Appalachians, paraileling the

Atlantic coast. The median line of the orogenic belt bends to the West alongside the Gulf

coast and underlies both the Ouachita Region of the Intenor Highlands and the Edwards

Plateau before extending southward into Mexico under the Sierra Madré Oriental. Elevations

reach 2000 m and greater for a number of peaks in the region. Fu trees dorninate the Corests

above 1300 m, with spruce, then deciduous hardwoods becoming dorninant with decreasing

altitude; (iii) Piedmont: gradually descends fiom approximately 300 m at the south-east edge

of the Southem Appalachians. Over 1000 mm of precipitation f d s annuaily providing water

for the deciduous hardwood forests. in the Mer areas the conifer, loblolly pine, Pinus taeda,

is present; and (iv) The Allegheny Plateau: a broad region West of the Appalachian mountains

that descends in elevation from approximately 800 m through a number of escarpments to the

Interior Lowlands. Deciduous trees intermixed with hemlock, spmce and pine cm be found

at elevations in excess of 600 m. Various species of maples are found below this elevation,

but the forests are predorninantly oak (Duellman and Sweet, 1999).

Atiantic Coastal Plain (Figure 3; Table 1): The Atlantic Coastal Plain has a d a c e

area of approximately 9 13,000 km2 and stretches fiom Cape Cod aad Long Island in the

north to Florîda and the Gulf Coast of the United States and Mexico. The region rarely

exceeds 250 m in height and is siightiy lower dong the coast. Withlli the plain there are

many large bays and drowned river mouths (Lobeck, 1948).

Six of the eight distinct coastal regions found within the Atlantic Coastal Plain are

populated by the spring peeper: (i) The Northern Coastal Plain: extends northward to include

Long Island and Cape Cod and is separated fiom the Southern Coastal Plain by the northem

edge of Chesapeake Bay. The vegetation is predominantly deciduous forest with some pine

forests; (ii) The Southern Coastal Plain: tapers as it descends southward to the base of the

Florida Peninsula Deciduous hardwood forest is the most common vegetation in the north

consisting mostly of oaks and hickories. Sandy soils in the south contain mostly pines with

some oak. The remaining areas contain mixed evergreen hardwoods and cypress, Cupresw

sp., swamps; (iii) Peninwlar Florida: the penùisula, which does not exceed 105 m in

elevation, is 600 km long and separates the Atlantic Ocean from the Gulf of Mexico. Frosts

can occur in the north, but mean temperatures are u d l y above 15°C throughout the range in

January, with annual precipitation ofien in excess of 1500 mm. The native vegetation

consists of pine-oak scmb. In southem Florida there are hammocks of oak and cypress

swamps containing a great ded of spanish moss, Tillandsia useneoides; (iv) The GuifCoart

Plain: the area west of Peninsuiar Florida and east of the Mississippi River Valley. The

dominant forest vegetation is pine with some areas containing pure stands of p s t oak,

Queras stelZutu. Hammocks of evergreen hardwoods, cabbage palms, Roystonea oleracea,

and cypress swamps are also common; (v) The Mississippi Emboyntent: a broad plain with an

elevation of 90 m that extends approximately 1000 km iniand fkom the Gulf of Mexico to the

borders of the Interior Highlands and Lowlands. Northem vegetation consists of deciduous

forests while the south parallels flora found in the Gulf Coastal Plain; and (vi) The Pine

Woodm&: a transition zone between the Gulf Coastai Plain and the Mississippi Embayment

to the east, the Rairie Parkland to the northwest and the Coastai Prairie to the southwest. The

Table 1. Envimental variables h m North American regions in which Pseudacris cruf#ier Iives. n= north, s = south, w = west, e = east.

Temp. ( O C ) (Jan.-Jul y)

Recipitation (mm)

Dominant Vegetation

laterior Plrialit Gneat Plains

Interior Lowlands

-25 to 15 (n) 15 to 25 (s)

-25 to 10 (n) 10 to 25 (s)

5 to 25

northern -15 t020 muthem O to 25 - m Piedmont O to 25

& y / 0 - 2 5 Alle en Plateau

750 (e) - 300 (w) 1 ow (el - 750 (w)

750

eastern deciduous hardwd, grasses

up to 1917 up to 2000

3 0 to sea level 800 to 200

tir, spmce, pine, hemlock - beech, maple to oak, hickory O&, hickory

1000 > 1OOO > 1000

750- 1 000

fir, spnice - beech, maple fir, spruce - beech, maple

pine - beech, maple hemlock, spmce, pine - oak, maple

4 5 0 ~1000 oak, maple <250 ~1000 oak, hickory - cypress swamps

100 to sea level pine-oak, saw palmette, wire grass, cypress > M û SWMiD

Gulf coast plain 1 \O to > 25 ( 1 0 to sea level 1 1 500 1 pine forest, cypress swamp Mississippi Embayment ", deciduous hardwood in the north

Pine woodlands 1 10to>25 ~ 2 5 0

Liurnthn U p h d -10 t020

750 (e) -1000 (w)

250 (nw) - 1000 (se)

pine, wire grass

balsam fir, spruce -tamarack - tundra

Pine Woodlands are a Qatland located in western Louisiana and eastem Texas, with

elevations of less than 250111. The vegetation is largely longleaf pine and wiregrass

(Dueliman and Sweet, 1999).

Inhwluction in Cuba: Cuba is the largest island of the West Indies with an area of

105,007 km2. Pseudacris crucifer c m be found in the region referred to as Alturas de la

Habana-Matanzas. The fiog is not native to the island, but was introduced (tirne unknown:

level (Hedges, 1999). This region is located on the no&-west side of the island and is

roughly at sea.

Distribution of Pseudacris crucifer within re~ions

Spring peepea can be found fiom the nearly subtropical conditions of the Gulf Coastal

Plain to north of the Boreal Forest into the Sub-Arctic Parkland. As is apparent fiom Table 1,

the northem extent of the peeper's range (53-5S0N latitude in eastem North America) is

limited by over-wintering temperature. The fiogs are able to survive winten with mean

temperatures as low as -25°C; anything colder for prolonged periods will kill them. Within

its range, the distribution of the peeper is limited by rainfall and the availability standing

water (including pools, fiooded ditches or shallow ponds) during both the breeding season

and the approximately 45 day period between fertilization and metamorphosis. Pseudds

crucifer breeds in open lowland marshes, and wooded, open, sphagnum, or cattail swarnps at

the h e d of streams. Vegetation can be present or absent and the body of water CM be either

permanent or ephemeral (Wright and Wright, 1995). The overall broad distriution of P.

criccifer is one reason why this species is an excellent candidate for an investigation of inter-

population variability.

Why do we expect to find variability in advertisement call parameters?

Variability in the advertisement call can atise from at Ieast three general pathways:

(i) diffenntial adaptation to specific habitats by populations wivithin a species:

Paterson (1 985) argued that signals involved in species recognition should be selected to

enhance mate location. In the case of acoustical cues, females should respond to a call

component that unambiguously aliowed them to orient towards and ultunately corne into

physical contact with a conspecific male. Paterson predicted that the cues (or components of

cues) involved in species recognition - mate location should show relatively little variation

within and among populations, becsuse the emphasis during the evolutionary history of such

cues centered around the effective transmission of unarnbiguous Uiformation. This dynamic

is expected to occur when populations within a species breed in similar habitats. If, however,

one population breeds in a different habitat (for exarnple, an open area with a high density of

predators attnicted to the call) then a change in the mate location/ recognition component of

the call might be favoured. For example, cdls which are conspicuous (due to intensity,

duration, call repetitioo rate etc.) contain more acoustical energy, and thus may be indicative

of good physical condition (Kiump and Gerhardt, 1987). These types of call components are

good candidates for inter-sexual selection (Darwin 1871): that is, femaies can use variability

in such a compownt to nliably assess the vigour of calüng males. Because the degree of

coospicuousness wiU vary with envbnmental parameters (such as density of vegetation),

female choice could drive the evolution of male calling components in different directions,

depending upon the habitat.

(ii) reproductive character displacement: suppose that two closely related species

share a similar, but not identical b's;>ecies recognition" signal. If these species are allopatiic

with respect to one another, they don? have the potential to make any mating rnistakes and,

as described above, we would expect to see Little within species variability in the signal (ail

other thines beine eaual). If, however, the two species overlap in part of theu range, then

mating mistakes, leading to decreased fitness through male-female gamete incompatibility or

decreased survival of the "hybnd" o f f s p ~ g , rnay be possible. Under these circumstances,

any difference in the mate recognition system that allows members of the two populations to

mate essortatively will bc favoured and strengthened by sclection and those components will

diverge. This may lead to a change in one or both of the overlapping populations away from

the "species-specific" cal1 component that would typify allopatric populations (Dobzhansky,

1937; Mayr, 1942; Butlin, 1987).

(iii) phenotypic correlation: particular components of the male advertisement cd1 may

be directiy linked to other phenotypic characters. For example, the cal1 of a larger h g has a

lower dominant fiequency than a smaller fiog because of morphological dBerences in the

larynx (Lykens and Forester, 1987; McClelland et al., 1996). In this case, direct selection on

the morphological trait (for example, male size may play a role in struggles to maintain

amplexus with a f e d e ) will lead to changes in the c d variable. This, in turn, might produce

confusing pattern of inter-populational variation in the c d if we only are considering the

relationship between the cal1 and the suzrounding enviromnent (see e.g., Ryan et al., 1996).

Overall then, the unit that researchers have named the "male advertisement cali" may in

fact represent the complicated endproduct of many selective pressures acting on different call

components simultaneously. Because of this complexity, we would expect to h d some inter-

populational variability in cal1 components, particularly if a species is a widespread one

comprising many separate populations.

Do we, in fact, find such variability?

Given the importance of variability in signals to mate recognition and discrimination, it is

not surprishg that so much attention has been focussed on inter-populational variation in

male songs in birds (Baker and Cunningham, 1985; Young et al., 1994) and insects (Ritchie,

199 1, 1992; Claridge and Morgan, 1993). It is surprising, however, that so little attention has

been paid to documenthg variability in such charismatic singers as fiogs. To date, only five

such studies have been underiaken (Table 2). Al1 of these studies uncovered inter-

populational variation in a variety of male advertisement call components, indicating that this

particular line of research should be a very miitful one to pursue for other anuran species.

Although several researchers have uncovered a potential role for female preference in

the evolution of the male spring peeper's advertisement cail, no one has documented the

amount of variability in that call across populations. Given how geographically widespread

Pseudacris cnicfer is, this is a striking omission! in this thesis, therefore, 1 will begin to

buiid the database for diis ubiquitous fkog by recording cails fiom aine populations of the

northem spring peeper in order to m e r the following three questions:

Table 2: Studies documenting inter-populational vdability in parameters of the anuran male advertisement call.

Pseudacris triseriata (Chorus fiog)

Pseudacris triseriata wmplex (P. t. maculata, hlmi, firiaruni, triseriata

Hylu cinera (Green treefiog)

Acrls crepitans (the cricket h g )

Georgia, New York and Manitoba,

Transect rang ing fiom South Dakota to Oklahoma,

30 populations on a 500 km transect fkom Mexico to Venezuela

6 populations fiom Illinois, Mississippi, Louisiana, Florida.

1 7 populations in Texas

Call parameter

Frequency: higher in Manitoba vs New York/ Georgia. Cal! duratiw: longer in ManitobaMew York than Geotgia.

Call duration: longer in northem regions.

CiII puise rate: lower pulse rate in northern regions.

Whine: number of pulses at the beg i~ ing , difference in amplinide between the first and second harmonic, duration of entire whine, initial, middle and final fiequency Chuck: dominant frequency , difference in amplitude beiween the second harmonic and dominant fiequency To$al caU: dominant fiequency, duration

Dominant frequency: clinal variation on a N-S gradient.

Lower frequency, longer duration, slower call rates in the open versus the forest

Reference

Martof, 1961

Ryan et al., 1996

Asquith et al., 1988

Ryan et al ., 1 990; Ryan and Wilczynski, 1991

1. 1s there any variability arnong geographically distinct populations of Pseudacris crucifer

in male snout-vent length?

2. 1s there any variability among geographicdIy distinct populations of Pseudacris crucifer

in the three most widely measured components of the male advertisement call, midpoint

of the dominant frequency, cal1 repetition rate, or call duration?

3. If so, is there any pattern to the variability. For example, is it organized dong a distinct

north-south or westeast cline? 1s it correlated with habitat variables that might affect

sound transmission such as the density of emergent and surrounding vegetation?

Materials and Methods

Locations

Field work was conducted fiom April6 to June 1, 1998 and April 19 to May 30,1999.

Ten locations were meyed. The site names and descriptions foilow in the order that they

were first sampled (for a map showing locations of sites see Figure 4 at end of this section).

Scanned photographs of some sights are presented in Appendix 1.

(1) Arkansas [AR] (3S0, 45' N, 94' W): approximately 50 kilometers south of

downtown Fayetteville, Arkansas on highway 71 in the O m k National Forest. The body of

water was an unnamed pond, refmed to locaily as "Lightening Pond". Data were collected

fiom April5 to 1 1, 1998 inclusive, with a total of five calls and representative voucher

specimens king taken. The pond was almost uniformiy 75 cm deep and sparsely vegetated,

with a length of 5.5 rn, a width of 2.1 m and a perimeier of 1 1.4 m. Geographically, the

Arkansas area falls on the west side of the Interior Highlands of the Interior Plains. This area

borders the Prairie Parkland.

(2) Missouri w] (Robertsville, Missouri): just east of St. Louis on the Missouri 1

Illinois border. The collection of a frog at this location was purely opportunistic. We heard it

cailing nom the %est Western's pool on the evening of April 15, 1998. The cal1 was

recorded on the evening of April 16, 1998 in a hotel room in Pittsburgh, Pennsylvania.

Needless to say the original "habitat" was a perfectly nctangular body of water lacking

vegetation (a pool). Geographicaily, Robertsviile is centrally located on the Tallgrass Prairie

in the Great Plains of the Interior Plains.

(3) North of Cottage w], Ontario (45', 40' N, 79', 23' W): just nolth of Burkes

Falls, Ontario. The site cm be located by taking highway I I north through Burkes Falls and

then tuming east on Pickerel Lake road. Once on Pickerel Lake road continue for 17.2 km

until a small field on the north side of the road, flooded by a beaver dam, becomes visible.

The fiooded field nuis off through a culvert that has been instailed under the mad and creates

a shallow (2 cm), slow moving creek in a ditch on the south side. Calls and voucher

specimens for this site were collected on May 3 1, 1998. A total of five calls and theu

vouchers were obtained. Geographicaily, North Cottage is located in the southem Boreal

Forest of the Laurentian Upland. It borders right on the Interior Lowlands.

(4) Millord Bay [MB], Ontario (45', 04' N, 79', 28' W): located approximately 18

kilometers north-west of Bracebridge's town center and two kilometers north of highway 1 18

on Milford Bay road, immediately east of Lake Muskoka. The breeding site was a flooded

lowland area on the West side of the road opposite an unnamed river on the east side. Ody

one cal1 and voucher were obtahed from this site (on June 1, 1998) because the population

was small. 1 was also distracted by the ever-present sound of a seemingly very large mammal

jumping into the river across the road, making incressingly louder grunts and splashes as it

approached the sample location. Knowhg that discretion is the better part of valor, 1

hurriedly withdtew fiom the area Milford Bay i s located on the northem most point of the

Interior Lowlands in the Interior Plains.

( 5 ) Waterloo [WA], Ontario (43', 2S7, 30" N, 80' 49' W): located between Waterloo

and Stratford in a srnall hamlet called Ratzburg. The site cm be found by leaving Waterloo

via Erb street and travelling West towards Stratford. Upon reaching Ratzburg, continue d o m

the road 0.1 km past the Faith Mennonite Church sign. The breeding site is an ephemetal

body of water hidden in a small woodland on the south side of the road. This site was

sampled on Apt4 29 and May 5,1999, with two and four voucher specimens and their calls

king collected respectively. The locale is geographicdly located in the northem portion of

the Interior Lowlands in the interior Plains,

(6) Kortright [KO], Ontario (43', 51 ' N, 79', 35' W): located in Kleinburg in the

Kortright Conservation Area. The Kortright Conservation Area is on the West side of Pine

Vaiiey road, approximately four kilometers west of the point where Rutherford road

intersects Highway 400. The body of water is easily located, as it is situated immediately

beside the only two windmills in the area Calls and voucher specimens were collected on

Aprü 27, April29 and May 1,1999 with one, three and two individuils behg sampled

respectively. Kortright is in the northem portion of the Interior Lowlands in the Interior

Plains.

(7) South Portage 1 [SPI], Ontario (45', 12', 20" N, 79', 6' W): 0.4 kilometers

northeast nom the intersection of highways 2 and 9 (Brunel road and South Portage road

respectively) on the north side of the road, in the township of the Lake of Bays. This site was

visited on May 13, May 19, May 29 and iday 30, 1999 with one, six, three and one voucher

(s) and calls being gathered respectively. For an unknown reason, males al1 but ceased calling

from this location on May 30'. They could, however, stiil be heard calling on the south side

of the road, so 1 took samples fiom this second location in order to obtain as large a sample

as possible from the area (south Portage 2).

(8) South Portage 2 [SP2], Ontario: Two additional vouchers and their calls were

obtained from the south side of the road on May 30, 1999. Both SP 1 and SP 2 are located in

the southem Boreal Forest of the Laurentian Upland and border on the Interior Lowlands.

(9) Kaladar 1 [KI], Ontario (44', 39', 30" N, 77', 07' W): Kaladar is located at, or

rather is the intersection of', highway 41 and highway 7. This site is a densely vegetated, old,

stagnant beaver pond. It is situated 0.6 kilometers east of highway 41 on highway 7, just past

the Cornmunity Center on the north side of the road. Two vouchers with their calls were

obtained on May 17 and one male + cal1 on May 18,1999. This location proved to be very

diacult to access and even more difficult to maneuver within due to the large number of

submerged logs, dense network of both living and dead vegetation, makes and the misguided

suspicions of the local authorities. It was decided that a more hospitable location shouid be

found.

(10) Kaladar 2 [K2], Ontario (44'. 39', 50" N, 77', 07', 70" W): The second site in

Kaladar was located 2.1 kilometers north of highway 7 on highway 41 on the east side of the

road. The water here was not stagnant but instead a semi-permanent flooded field that ran

off through a culvert to the West side of the road. The two Kaladar locations were joined by a

comdor of forest that stretched unintempted nom K 1 to K 2. Six vouchers/calls were

collected at this site, al1 on May 18, 1999. Both K I and K 2 are geogmphically situated in

the north-eastern portion of the Interior Lowlands in the Interior Plains.

The Waterloo, Milford Bay, North Cottage and both Kaladar locations were chosen

based on historical population data obtained fiom the Royal Ontario Museum's Herpetology

department. Kortright, Waterloo, Arkansas, Missouri and both South Portage sites were

chosen by ident*ing what I thought would be mitable breeding habitats within the species

range. Potential breeding sites were identified fiom field guides (Conant and Collins, 199 1 ;

Behler and Wayne, 1997; Harding, 1997) as well as nom locations where I had heard

Pseudacris crucifer choruses in the past

Recording

Al1 calls were recorded using a Sony WM-D6C professionai Walkman stereo cassette

recorder and a Sony ECM-MS907 electret condenser microphone at the 90' setting. Cds

fiom Kortright Conservation Center and the Waterloo were recorded on 90 minute Maxell

SL II IEC Type II, hi& (Cr 9) audio cassette. C d s from Lightning Pond in Arkansas were

recorded on a Radio Shack Hi-definition HD 60 audio cassette. AU remaining c d s

(Missouri, North Cottage, Kaladar, Milford Bay and South Portage) were recorded on TDK

SA60 high bias IEC II I Type II audio cassettes. Al1 cails except for the one h m Missouri

and two h m Arkansas were recorded in the field. Male 25 16 fiom Missouri was recorded

Figure 4: Map showing locations of collecting sites. Lettea refer to the following

sites: A = North Cottage, South Portage 1 and 2; B = Milford Bay, Waterloo, Kortnght,

Kaladm 1 and 2; C = Arkansas, D = Missouri.

from within a plastic bag in the hotel room, while males 25 1 1 and 25 12 fiom Arkansas were

recorded fiom within a plastic bag in the hotel parking lot. Because Blair (1958a), a pioneer

in fiog call d y s i s , based bis study of Floridian hylid frogs on one to nine calls per species

(using only one for Pseudacris crucifer) 1 decided to include the sites with only one recorded

call, at least in my initial analysis.

1 established a protocol for gathering data for al1 locations pnor to beginning field work. 1

began by approaching the chorus as slowly and silently as possible, attempting not to look

directly at any individual frogs. During this approach, 1 was eventually able to orient towards

an individual male based on his call, much as an incoming female would act. Once a male

was located, 1 directed the light fiom my heacüamp on him in order to confum that he was

indeed the individual emitting the call. Having confirmed this as quickly as possible, 1

exthguished the light and waited in the dark for the male to begin calling again. This could

take anywhere fiom almost irnmediately to ten minutes, depending upon how strongly the

calling male was disturbed by the light or my approach. I recorded the call by holding the

microphone approximately half a meter away fiom the calling male. The recorder was

adjusted so that the peak level reading was approximately +3 dB, the standard level used for

Crû2 il type tapes, as per the instructions in the recorder manual (Sony, 1983). Each call was

recorded for a minimum of thirty seconds. Once the call had been recorded, the cding male

was caught by hand, assigned a voucher number, measured for snout-vent length (Sm) and

put in a clear plastic bag dong with some water and representative vegetation for transport

back to the laboratory. Individuai males were euthanhd with aa overdose of MS-222 and

retained as voucher specimens. Monnation pertaiaing to location of site, date, the, voucher

identification number, a i . temperature, water temperatute and general observations

(including perch location, weather conditions, moon shape, vegetation present, and whether

or not othet species of fiogs were present d o r cding) was recorded at the tirne of capture.

Both air and water temperatures were taken using a standard mercury thermometer and

measured in degrees Celsius to the nearest half degree. Snout-vent length was measured in

miilirneters using Mitutoyo dial calipers.

When possible. at least six individuals were sampled per location. Exceptions were

Missouri, Milford Bay (one voucher each), North Cottage and Arkansas (five vouchers each)

where numbers were limited by population size, fiog "skittishness" andot time restraints.

Call Analysis

1 downloaded individual calls f?om the tape recorder to the cornputer (Macintosh

Perfoma) when 1 retumed to the laboratory. The software package "Protools for the

Digidesign sound c d converted the tape recorded calls into a digital format. This

conversion eliminated any unnecessary background noise that would have occurred during

attempts to analyze the c d s directly fiom the tape cassettes. Call files were then viewed in

the signalyzeN Sound Analysis cornputer program, which isolates individuai calls and

depicts them as oscillogram-wavefomis. 1 used the oscillograms to mesure the following

three c d parameters (Figure 5):

(i) Mid-point of the dominant frcquency (MDF): the frequency that contains the

gnatest energy in the d l , it also has the lowest hquency in the sound produced. This

parameter, detemiined h m the Fourier trarisformation (a wave that represents the sum of a

series of sine waves: Ludel, 1970) of the entire caii (Cocroft and Ryan, 1995), is measured

by placing the cursor at the midpoint of the widest portion of the oscillogram for a given call

(signalYzem in ttun provides the corresponding frequency reading).

(ii) Cal duration (CD): the t h e fiom the beginning to the end of one call (one pure

tone) expressed in milliseconds (Cocroft and Ryan, 1995). This cal1 parameter was measured

by highlighting the wavefom presented on the ~ i g n a l ~ z e ~ screen. In order to avoid any

ambiguity due to an individual "WamUng-up" or "cooling-down" at the beginning and end of

the bout, 1 rneasured cal1 (pure tone) parameten nom one call in the middle of an

individual's bout. Choice of "the middle" was tempered by the need to analyze the cal1 with

the least amount of background noise, so the middle cal1 generally represented "the call

closest to the middle of the bout with the lowest acceptable level of background noise".

(iii) Cal1 repetition rate (CRR): the total number of calls in one bout, where "a bouty'

= the longest continuous repetition of the call. The t h e for the bout is measured fiom the

beginning of the first call to the end of the last call. This measurement is generally converted

to "number of calls I minute" (Forester and Cmowsky , 1 985). When the cal1 being

analyzed was less than one minute in length, the following calculation was performed: no.

calls 1 milliseconds x 60 S.

Mid-point of the dominant frequency and call repetition rate were chosen for analysis

because past studies have shown that female Pseudacris crucifer may respond to intermale

variability in these call panuneters (Fonster and Czamowsky, 1985; Forester and Harrison,

1987; Sullivan and Hinshaw, 1990; Doherty and Gerhardt, 1994; Schwartz and Gerhardt,

1998). Although females do not appear to respond preferentally to call duration (Forester

and Cmowslry, 1985), it has also been niggested that if a male increases his cal1 duration

upon the approach of a gravid female, he transmits information about his ability to make the

Figure 5. Scanned image of a oscillognun highîighting midpoint of the dominant frequency,

repetition and duration components of the male advertisement d l . (A) Call repetition rate I

min; (B) Call duration: the X axis is t h e in ms, the Y axis is fiequency in Hz; (C) Mid-point

of the dominant kquency: the X axis is frequency in Hz, the Y axis illustrates the fiequency

spike.

A Cal1 repetition rate / minute (CRR) Cd Gmup

I I

B Ca11 duntion (CD)

C Mid-point of the dominant frequency (MDF)

increased energy investment; somediing that may be important to a femde at close range

(Rosen and Lemon, 1 974; Fellers, 1 979; Forester and Czarnowsky, 1 985; Forester and

Daniel, 1986; Forester et al., 1989). It is also possible that the evolution of cal1 duration may

ais0 be influenced by the presence of acoustically-orienting predators. As discussed

previously, the evolution of the "male call" represents the outcome of numerous selection

vecton (e.g.. female choice. male-male cornpetition for access to a breeding site. predation,

transmission panuneters of a pdcular habitat), and possibly also chance operating on the

components of the call simultaneousIy. Given the complexity of that interaction, I decided to

rneasure call duration in addition to dominant frequency and call repetition rate in order to

document the extent of inter-populationai variability in the male advertisement cal1 of the

s p ~ g peeper.

S tatistics

Previous research has indicated that many anuran species demonstrate a significant

correlation between (i) dominant frequency and both temperature and snout-vent length

(Pseudacris crucifer: Doherty and Gerhardt, 1 984; Forester and Czamowsky, 1 985; S divan

and Hinshaw, 1990; other anurans: Ryan, 1984 1983; Sullivan, 1984, 1989); (ii) cal1

repetition rate and both temperature and snout-vent length (Brown and Brown, 1977;

Forester and Czarnowsky, 1985; Sullivan and Hinshaw, 1990); and (iii) cal1 duration and

temperature (Brown and Brown, 1977; Sullivan and Hinshaw, 1990). There does not appear

to be a significant correlation between c d duration and snout-vent length (Forester and

Cuunowsky, 1985; Suiiivan and Forester, 1990). In order to compare c d variables among

populations, therefore, ali variables were standardized for air and water temperature (to a

temperature of 14 degrees Celsius [CDi4/Fis = CDmb./'F. - (Tmb, - 14)(m)] ) and snout-vent

length (30 mm: with the exception of cal1 duration as discussed above) using a least squares

regression analysis following the methodology outlined by Platz (1989). In addition to single

standardizations, dependant variables were simultaneously standardized for multiple

independent variables using serial least square regressions.

A mode1 II single factor analysis of variance (ANOVA) with a significance level oCO.05

was used to search for significant dzerences among sites. In cases where the nul1 hypothesis

was rejected, Tukey's analysis was used to detemiine which of the populations were actually

different fiom one another. Al1 Tukeys tests are reported at a significance level of p = 0.05.

Correlations between data were examined using a Pearson moment correlation test

(conelation values are reported as "r"). Al1 of the preceding tests were chosen because they

give relatively conservative interpretations of relatiowhips (Sokal and RoW, 1969). In cases

where mal1 sarnple sizes were a problem, nonparametric tests were used (Kruskal Wallis and

S pearman rank correlation tests).

General observations (Appendix 2)

1 recorded calls fiom forty-five individual Pseudacris crucifer males. Of those males, (i)

eleven were recorded calhg while partiaily submerged in the water, aithough of ody seven

of those had more chan 10% of theu body in contact with the water , (ii) ten were recorded

calling while floating on a mat of vegetation in the water (although the vegetation was damp,

these individuals did not have any part of their body in the water), (iii) fourteen were

recorded calling fiom emergent vegetation (cattails, gnisses, smail shnabs) within the flooded

area above the water and (iv) ten were recorded calling fiom emergent grasses and shrubs

dong the shore or idand a few feet fiom the pond. Overall then, only 1 1 1 45 (24.4%) of the

males actually had any part of their body in water when they were calling. Of the remaining

males, twelve were actually removed fiom the water by anywhere fkom 1.5 - 7 feet. indeed,

three males were calling fiom nearby trees! Given the biology of the species and the

difficulty in determinhg what actually constituted "contact with the water", 1 decided to

investigate the relationship between cal1 variables and water temperature for the most

conservative subset of the data possible: the seven males cailing with at least some part of

their body in contact with the water. Reducing the sample size to only seven males h m aine

populations effectively reduced the probability that 1 would find any significant interactions

between cal1 variables and water temperature. In the hture, many more males will have to be

collected in order to study this interaction.

Changes in male body size and advertisement call variables

The raw data for the call (cal1 repetition rate; call duration; mid-point of the dominant

fiequency of call), rnorphologicai (snout-vent length = SVL), and ecological variables (air

and water temperatws) are presented in Appendix 2. Values for call variables corrected for

air temperature and snout-vent length, and a combination of temperature and morphology are

in Appendk 3. Oscillogntms of individual calls are presented in Appendk 4.

As mentioned in the Methods section, South Portage 1 and 2 were essentiaiiy the same

site; the males were just calling from the flooded area on either side of the road. Mann-

Whitney testing did not uncover any siWcant differences between males collected from

SPI (N = I l ) or SP2 (N = 2), so the data were combined to give one South Portage site (N =

13). Kaladar I and 2, on the other hand, wen separated by at least 2 km. Although they were

connected by a forested corridor, 1 analyzed the two sites separately because we know so

little about s p ~ g peeper dispersa1 patterns.

Snout-vent leneth (SVL)

The average snout-vent length for d l of the males measured in this study was 27.8 mm,

with a range from a mean of 30.1 mm at SP to 24.5 1 mm at NC (Table 3). ANOVA testing

uncovered a significant difference among populations (df = 8, F = 5.09, p < 0.0003: Figure

6). Further statistical analysis using a Tuky test (at p < 0.05) indicated that there was a

significant clifference in SVL between: North Cottage and KO / SP and between Kaladar 2

and KO / SP. There thus appears to be a continuum in male snout-vent length across

populations, with South Portage f&g at the hi& end for male body size, and Kaladar 2 and

North Cottage falling at the low end.

* e c - w p i * I 3 0 ar

E cn

Cal1 Rewtition Rate (CRRI

The mean call repetition rate for al1 sites was 71 -96 calldmin. The population with the

greatest average CRR was Missouri (107.41 calldmin), however, that site was represented by

only one male. Of the populations with at least two specimens, the highest average CRR was

found at K1 (95.32 calldmin), and the lowest average CRR occurred at NC (45.16 calls/min:

Table 3). ANOVA testing uncovered a significant difference among populations (df = 8. F =

3.14, p c 0.009: Figure 7). Further statistical d y s i s by Tukey's testing indicated that there

was a significant difference in CRR between NC and both K2 and K1.

As expected raw values of cal1 repetition rate were positively correlated with raw values of

air temperature (0.615, p < 0.0001). Regression analysis indicated that call repetition rate

changes by a factor of 2.87 with increasing air temperature (Figure 8). There was no

correlation with water temperature for the small subset investigated (0.59, p < O.OS), althougb

there is a positive correlation if that subset is expanded to include any male who was calling

fiom the water or fiom vegetation floating on the water (0.63, p <0.04). The dzerences

noted among populations based upon the raw CRR scores disappeared when corrections for

were made for air temperature (df = 8, F = 2.12, p < 0.06: Figure 9).

There was no correlation between snout-vent length and cd repetition rate ( raw values:

z = 0.012, p < 0.99; corrected for air temperature: z = 1.3 1, p < 0.19). Raw measwes of call

repetition rate were negatively correlated with raw measures of call duration (-0.57, p <

0.000 1) and positively correlated with raw meames of dominant frequency (0.3 1, p < 0.04).

Once again, these correlations disappeared when dl variables were adjusted for temperature

and body size, with the exception of a weak, but aot significant negative relationship between

Figure 6. Diffetences in snout-vent lengths of calling Pseudacris crucifer males from 9

different populations (South Portage sites are combined). Largest and smallest refer to

populations at either end of the continuum in male snout-vent length.

Figure 7. Differences in unadjusted (raw) values for cal1 repetition nites among 9 different

populations (South Portage sites are cornbined). Lowest and highest refer to populations at

either end of the continuum in cal1 repetition rate.

Figure 8. Regression of cal1 repetition rate against air temperature*

Air temperature (OC)

Figure 9. Differences in cal1 repetition rate (adjusted for air temperature) among 9

populations of Pseudacris crucifer (South Portage sites are combined).

CRR and cal1 duration (air temperature doue: -0.28, p < 0.058; air temperature I snout-vent

length: - 0.29, p<0.057).

Mid-point of the dominant fieauency (MDF)

The mean mid-point of the dominant frequency for ail sites was 2919.57 Hz. The

population with the greatest average MDF was K2 (3323.30 Hz), while the population with

the lowest average was K0 (2750.33 Hz: Table 3). ANOVA testing uocovered a significant

difference arnong populations (df = 8, F = 8.49, p < 0.0001), while Tukey's testing

highlighted significant differences between Kî and KO, Arkansas, NC, SP and Waterloo

(Figure 10).

There was no correlation between mid-point of the dominant frequency and water

temperature (r = -0.29, p < 0.43). Mid point of the dominant fiequency, however, was

positively correlated with air temperature (0.448, p < 0.00 1 8). MDF changes by a factor of

20.43 with increasing air temperature (Figure 1 1 a). As expected, there was also a strong

negative correiation between MDF and male body size (-0.508; p < 0.003); MDF changes by

a factor of 40.5 8 with changes in male body size (Figure 1 1 b). Significant differences in

MDF arnong populations persisted following corrections for air temperature (df = 9, F =

6.02, p < 0.000 1) , body size (df = 9, F = 4.86, p<0.0003), and the combination of air

temperature / body size (df= 9, F = 3.49, p < 0.004). Tukey's analysis retained the

ciifferences between population K2 and KO, AR, NC, SPI, and WA as noted in the preceding

paragraph foilowing air temperature and body size adjutments individually. Combining the

two adjustments (Figure 12) highlighted the differences between Kdadar 2 and either

Figure 10. Differences in unadjusted (raw) values for midpoint of the dominant fiequency

among 9 populations of Pseudacris crucifer (South Portage sites are combined). Highest

refers to population at the high end of the continuum in MDF.

Figure 1 1. Regression of midpoint of the dominant frequency against (a) air temperature and

(b) snout-vent length.

2400 1 1 1 1 1 1 1 1 1 1 1 1

2 4 6 8 10 12 14 16 18 20 22 24

3800 7 ( a ) Air temp (O C)

20 22 24 26 28 30 32 34 ( b ) Snout-vent length (mm)

Figure 12. Differences in midpoint of the dominant frequency (adjusted for air temperature

and body size) among 9 populations of Pseudacris crucifer (South Portage sites are

combined). Highest and lowest refer tu populations at both ends of the continuum in MDF

(there is a significant difference between the two ends).

highest

L

lowest

Ki K2 W KO A MI NC Mi3 SP

Population

Arkansas or Waterloo, and also indicated that the MDF in Arkansas was significantly lower

than SP.

Raw measures of midpoint of the dominant fiequency were positively comlated with

raw measures of cal1 cepetition rate (0.3 1, p < 0.02) and negatively correlated with raw

measures of call duration (-0.69, p < 0.0001). The relationship between the midpoint of the

dominant fkquency and call repetition rate disappeared when both variables were corrected

for air temperature and air temperature I body size. The aegative relotionship between mici-

point of the dominant fiequency and call duration, however, was maintained following al1

corrections (air temperature alone: - 0.586, p < 0.0001; air temperature / body size: - 0.57, p

< 0.001).

Cal1 Duration -Dl

The average call duration between al1 sites was 142.1 5 ms. The site with the longest cal1

duration average was KO (1 77.93 ms) while the site with the shortest average cal1 dwation

was K2 (8 1.88rns: Table 1). There was a significant Werence among sites with respect to

call duration (df= 8, F = 4.96, p < 0.0003). Tukey testing reveaied that differences between

populations once again centered around K2, which dEered fiom KO , SP, Arkansas and NC

(Figure 13).

There was no correlation between c d duration and male body size (0.26, p c 0.09). As

expected, cd1 duration was negatively correlated with air temperature ( - 0.64, p < 0.0001).

Regression anaiysis indicated that CD changes by a factor -5.2 with air temperature (Figure

14). There was no correlation between caii duration and water temperature ( -0.59, p 4).08),

although, once again, that reiationship was signifîcant (and negative) if males calhg fiom

Figure 13. Differences in unadjusted (raw) values for cal1 dmtion among 9 populations of

Pseudacris crucifer (South Portage sites are combined). Shortest refen to the population at

the low end of the continuum in cal1 duration.

40 ' K1 K2 W KO A MI NC Ml3 SP

Population

Figure 14. Regression of cal1 duration a g a . air temperature.

Air temperature (OC)

floating vegetation were added (-0.6 1, p<0.02). Significant inter-populational differences still

existed after CD was adjusted for a standard au temperature (df = 8, F = 5.27, df = 9, p <

0.0002). Prior to standardization, signifîcant differences centered around K2. Following

adjustment to a 14OC standard air temperature, the focus shifted to Arkansas, which differed

significantiy fiom K1, K.2, KO, SP, NC and Waterloo (Fig. 15).

Corn~arisons of adiusted variables across eeo~hvsical areas and habitat types

Four geophysical areas were represented in this study: (i) tall grass Prairie (Missouri),

(ii) interior Highlands (Arkansas), (iii) Intenor lowlands (Milford Bay, Waterloo, Kortright,

Kaladar 1 and 2) and (iv) Boreal Forest (North of Cottage, South Portage). Because Ta11

grass Rairie was only represented by one individual, and that individual was collected from a

swimming pool, the Missouri "population" was eliminated fiom the analysis. Anova testing

indicated that there was no difference in body size ammg males from the three different

areas (df = 2, F = 0.88, p = 0.42). There was, however, a significant difference amoag

regions based upon (i) call repetition rate comcted for air temperature and body size (df = 2,

F = 3.63, p c 0.04), (ii) midpoint of the dominant fiequency corrected for air temperature and

body size (df= 2, F = 5.40, p < 0.009) and (iii) call duration corrected for ait temperature (df

= 2, F = 13 38, p < 0.0001 : Figure 16).

1 next asked whether the differences among these regions was related to either

differences in latitude 1 longitude and 1 or clifferences in the amount of vegetation (habitat

type) munding the calling male. Habitat type was assigned a score from 1 - 3 depending

upon the density of vegetation around and in the pond [ 1 = no to very littie vegetation; 2 = a

moderate amount of vegetation, sparse emergent grasses, canails, mounding h b s ; 3 =

Figure 1 5. Differences in cd1 duration (adjusted for air temperature) among 9 popdations of

Pseudacris crucifer. (South Portage sites are cornbined). Arkansas has the longest cail.

Figure 16. Differences among the three c d variables grouped by geological area. (a) C d

repetition rate adjusted for air temperature and body size; (b) Mid-point of the dominant

fiequency adjusted for air temperature and body size; (c) Cal1 duration adjusted for air

temperature. The results of Tukey 's tests (at p < 0.05) are included on each graph.

cal1 duration (msec) n n U mid-point dominant frequency (Hz) = u

cal1 repetition rate

ni-

water area densely vegetated both in and around the pond]. This was a very subjective

measure of plant cover, so the results are prelimuiary , at bat, pending a more objective way

to measure cover.

There were no significant correlation between snout-vent length and latitude, longitude,

or density of vegetation at the calling site. The= was, however, a strong correlation between:

(il call duration adjusted for air temperature and latitude (Rho corrected for ties = 0.47. p <

0.002: Figure 17), and (ii) mid-point of the dominant frequency adjusted for body size and

air temperature and both longitude (Rho corrected for ties = 0.37, p < 0.02: Figure Ma) and

latitude (Rho conected for ties = - 0.52, p < 0.0005: Figure 18b).

Because vegetation density assessments were so subjective, a KniskaCWallace test was

used to hvestigate differences in call parameters among the three density types (high,

medium, and low). That test uncovered a significant difference between the density of

vegetation around the cailing site and both call duration (adjusted for au temperature: df= 2,

H corrected for ties = 1 1.40, p < 0.003) and MDF (adjusted for air temperature and body

size: df = 2, H corrected for ties = 9.02, p < 0.01). Frogs calling h m open habitats having

longer (Figure 19a) and lower fiequency (Figure 19b) caiis than those singing in areas with

medium - high density. The variables longitude, latitude and vegetation type were

confounded because the most open habitats were also the most south westerly ones. In the

discussion 1 will concentrate the amount of vegetation in the calhg area, since that cm

dramatically atfect cd1 transmission.

Figure 17: Cd1 duration (adjusted for air temperature) plotted against (a) longitude [not

sigd7cant] and (b) latitude. Site names are as follows: A = Arkansas; K1 = Kaladar 1; K2 =

Kaladar 2; KO = Kortright; MI = Missouri; MB = Milford Bay; SP = South Portage; W =

Waterloo. The outlier, K2, is highlighted with a box.

Cal1 duration (msec) adjusted for air temperature

Figure 18: Midpoint of the dominant frequency (adjusted for air temperature and body size)

plotted against (a) longitude and (b) latitude. Site names are as follows: A = Arkansas; K1 =

Kaladar 1; K2 = Kaladar 2; KO = Kortright; MI = Missouri; MB = Milford Bay; SP = South

Portage; W = Waterloo.

Longitude

84 87 90

Latitude

Figure 19. DBerences among cal1 variables grouped by the density of vegetation. (a) C d

duration adjusted for air temperature; (b) Mid-point of the dominant frequency adjusted for

air temperature and body size.

MDF adjusted for air temperature/body size Call duration adjusted for air temperature

Discussion

Females fiogs fiom a variety of species are able to discriminate between heterospecifc and

conspecific males and among conspecific males based upon variation in the male

advertisement c d , leading researchers to hypothesize that the male call is ûansmitting

information to the female (Blair, 1958 a, b; Loftus-Hills and Littiejohn, 1971 ; Gerhardt,

1974,1978 a, b, 1982,1987,1994; Narins and Capranica, 1978; Gatz, 198 1; Forester and

Czamowsky, 1985; Lykens and Forester, 1987; Schwartz, 1987; Sullivan and Leek, 1987;

Rand, 1988; Ryan and Rand, lWO,l993 a, b; Sullivan and Hinshaw, 1990; Brenowitz and

Rose, 1999). What message could be encoded in the male's call? The most obvious answer to

this question is that the message is carrying reliable information about the health, status, and

vigour of the individual male because of the ciramatic energetic costs associated with calling

(Ryan, 1985; Taigen and Wells, 1985; Taigen et al., 1985). To date, most researchen have

looked for correlations between such markers of male fitness and two components of the

advertisement call - dominant fiequency and call repetitioa rate.

Midpoint of the dominant kauency

Al1 of the studies conducted to date have uncovered a significant inverse conelation

between mid-point of the dominant hquency and snout vent length in Pmdacris mucifer

(ppuiations nom Maryland [r = - 0.521 Forester and Czamowsky, 1985; Maine [r = - 0.581

Sullivan and Hinshaw, 1990; Missouri [r = - 0.421 Doherty and Gerhardt, 1984). This snidy

was no different, noting a general correlation coefficient of - 0.51 Ui the combined data set of

populations h m Ontario to Arkansas. This effect is the resdt of larger h g s having greater

vocal cord mass and in tum lower dominant fkquencies (Sullivan and Hinshaw, 1990; Ryan,

1988). The effects of temperature oa MDF have been more controversial. Doherty and

Gerhardt (1 984) found no correlation between the two variables (measuthg only air

temperature "at the calling site"), while Sullivan and Hinshaw (1990) reported a significant

positive correlation between MDF and "site temperature". My results concur with the latter

study. 1 discovered a significant positive correlation (r = 0.45: Figure 1 la) between MDF and

air temperatwe around the calling male, but no correlation with water temperature, regardless

of whether "calling fiom the water" was defined as "males who had at least some part of

their body in contact with the water" (N = IO), or included "males calling fiom floating

vegetation mats" (N = 16). The positive relationship between air temperature and MDF is in

line with results reported for bufonids (Zweifel, 1968; Sullivan, 1984, 1985, 1989). Given the

contradictory nature of the various studies, 1 believe that is important for researchers

collecting fiog calls in the field to always take meamrements of surrounding air temperature.

It may be necessary to correct for the effects of temperature on MDF in order to compare

those values within and across populations.

There were significant differences in the mean MDF among the 9 populations recorded in

this study, even after MDF had been adjusted for temperature and body size. Cd1 fiequencies

fell dong a continuum with Kaladar 2 at the high end (30 15-40 Hz) and Arkansas at the low

end (2603.22 Hz) of the scale (Figure 12). This pattern of overlap across populations with

only significant differences between the two end points of the continuum has also been

reported for anuran communities in the southwestern Unites States (Ryan and Wilczynski,

199 1 ; Owens and Dixon, 1989). On a larger scale, MDF (adjusted) was negatively correlated

with latitude and positively correlated with longitude (south-central fiogs c d at a lower

fiequency than their north-eastem brethren: Figure 18). The fact that dEerences among

populations remain even &et adjusting for body size, and that body size does wt Vary

predictably with either latitude or longitude, indicates that variation in the dominant

fiequency from east to west and north to south is not due to changes in body size (Ryan and

Wiczynski, 1991).

It is likely that the Merences among populations are influenced by habitat type. The

northern populations live and cail within the moderately to densely forested areas of the

Interior lowlands (Milford Bay, Waterloo, Kortright, Kaiadar 1 and 2) and Boreal Forest

(North of Cottage, South Portage) regions (Figure 16). The southem populations, on the other

hand, were calling from open habitats of the Interior Highlands (Arkansas) and ta11 grass

Prairie (Missouri). Because the sound waves are propagated in air, the density, height,

distribution and type of vegetation will interfere with the transmission of those waves. This

type of interaction can be very cornplex, involving the distortion of sound waves as they

bounce off of objects, the masking of those waves by other sounds, including other species'

vocalizations, the rush of wind through trees or water over rocks, and changes in the wave

transmission medium (in this case, air) caused by changes in humidity, baromeüic pressure,

and temperature. In general ternis, higher fiequemies are absorbed more quickly by the

atmosphere, more easily scattered by objects in their path, and afTected more strongly by air

turbulence than lower fkquencies. Low fiequency sound waves, on the other hanci, are more

prow to disruption by reflections off of the substrate when the singer is close to the gmund.

At its worst, this reflection cm bounce up and cancel out the wave travellllig horizontally

through the air, with a net effect of no sound reaching the receiver. At its best, however, the

reflection can match the horizontal wave perfectiy, thus acting to amplify the cdl, but this

apparently does not happen very ofien (Hunter and Krebs, 1979; Gish and Maston, 198 1 ;

Bowman, 1983; for an excellent review of this topic see Catchpole and Slater, 1995).

Problems with ground effect are not as extreme in marshy environments because water tends

to absorb, rather than reflect, sound waves. Singers cding close to water, therefore, are fieed

somewhat frorn the problems associated with ground reflection, and are therefore expected to

use lower frequency calls whenever possible (Cosens and Falls, 1984). These complicated

interactions lead to the production of "sound windows", fkequencies at which sound waves

are optimally transmitted, that Vary according to habitat type. For example, Morton (1975)

found that fiequencies between 1600-2500 Hz were transrnitted optimally for ground singen

in wooded areas, while Penna and Solis (1998) reported that frequencies above 1000 Hz

attenuated very rapidly in boggy [wet earth] environments.

Birds ofien solve some of the problems with sound ûansmission by singing fiom high

perches, high up in the forest canopy, or during advertisement flights. Most munuis, limited

in their ability to climb and incapable of flight, are constrained to calling near the ground,

with al1 of its associated transmission problems. Given this constraint, is there any evidence

that frog calls are subject to environmental selection to enhance transmission under these

difficuit conditions ? Ryan et al. (1990) and Ryan and Wilczynski (1991) reported that

cricket fiogs [Acris crepitans] calling fiom open habitats had lower fkequency calls then

those calhg in the phe forests of the southwestern Unites States. Based upon the

observation that both calls performed equally well in the open habitat (measured by amount

of cal1 degradation as a fhction of distance), they argued that enWonmental selection on

dominant ikquency was telaxed in the open habitat and stronger in the fores The results of

this study are congruent with the cricket fkog research: maies caIling from the open ponds

had lower fiequency caiis than males in forested north (26 19.60 Hz versus 2868.27 Hz

respectively: Figure 18). The frequency range for the forest species is somewhat outside the

sound window reported for birds by Morton. There are two possible explanations for this

discrepancy. First, c d fkequency is strongly constrained by body size through the effects of

size on the larynx. Penna and Solis (1998) detected no relationship between properties of the

habitat that affect souad transmission and the spectral structure of anuran cdls in those

habitats for five species of temperate forest Chilean leptodactylids (Euisophus emiliopugini,

Batrachylu antartandica, B. leptopus, Hylorina sylvatica, and Pleurodema thmi). The two

Bafruchyia species showed the most obvious mismatch between cal1 fnquency and

environmental trammission parameters: although kquencies above about 1 O00 Hz were

rapidly attenuated in the bggy habitats where these fiogs lived, they consistently produced

calls substantially higher than 1000 Hz because they are srna11 fiogs (2092 Hz to 2445: mean

SV1 3 1.2mm). The lack of precise coupling between dominant fkequency and environmental

transmission parameters in this and other studies is thus, in part, a side effect of

morphologicai constraint.

Pseudacris crucifer males may have found a way to partially compensate for the effects

of small size. Many of the calling males recorded in this study were calling Eom hi& up in a

reed, or even the shmbs and saplings in or around the water (see Appendix 1). Moving up,

away fiom the scattering effects of emergent vegetation, logs, and rocks would allow the

higher frequencies to be transmitted more effectively, thus making the "best of a bad

situation". This still does not explain, however, why the marsh forest species do not sing at

the lower fkequency used by their southern, open dweiiing relatives. There an a number of

factors in addition to the "density of vegetation" that might be infïuencing the evolution of

MDF. For example, Martens and Geduldig (1990) suggested that species living in noisy

habitats tend to sing at higher fiequencies than might be considered optimal because many

natumi sounds, like Nnning water or the wind in trees, are concenûated at fiequencies

around and below 1 O00 Hz. Heuwinkel(1990) argued that birds cailing fiom among reeds in

a marsh had a higher than expected fkquency because it rnatched the resonant fiequency of

the reeds. Finally. the temporal heterogeneity of the habitat chanpd very rapidly in the

northern sites. The depth and temperature of standing and floowinng water, the density and

type of emergent vegetation, and the humidity and barometric pressure dl changed within the

life span of the chorus; in some cases on a daily basis (humidity, pressure), and in others,

over a period of a few days (water depth, vegetation). It would be interesthg to investigate

whether the MDF represents a compmmise frequency - one that transrnits most effectively

when averaged across the breeding season in these spatially and temporally complex habitats.

Ovemll, then there are two levels to the interaction between habitat and cal1 frequency:

The fvst is more coarse-grained, the frequency must meet some minimal threshold in order to

be ûansmitted in a particular environment. This first level involves large scale differences in

the properties of calls ttansmitted in forested versus open, bog versus running water

environments. The second level involves more finely-tuned environmental selection shaping

the cail to some theoretical "optimal transmission" level. This is a much more difficult

question to study because it is likely that fiequency is iduenced by a complex assortment of

direct (e.g., air temperature, habitat structure) and indirect (e.g., body size) selection vectors.

The fact that MDF is so strongly correIated with male body size in a number of a n m

species (Ryan, 1980,1983,1988; Forester and Czamowsky, 1985) has led to the hypothesis

that females should discriminate among males based upon cal1 Frequency; preferring the

lower fiequencies within the species range because this should reliably identiQ a larger male.

Why should females prefer larger males? Research has suggested that larger males are older

males, with individuals growing up to their second or third year (Lykens and Fonster, 1987).

Based upoa this, the assumption is then made that the ability to nwive to old age (which is

probably about four years in spring peepers) is an indication of "genetic superiority" on the

part of the large male; Le., he has been better able to evade predators, find food, survive

fluctuating environmental conditions than his smaller compatriots (Eden, 1976; Wilbur et

al., 1978), although this assumption has yet to be studied/proven in an experimental

h e w o r k . Larger males also produce larger daughtea, who, in tum, are capable of laying

more eggs (8-2 1 % more eggs per millimeter increase in snout vent length: Oplinger, 1966).

Overall, then, it is possible that a female who mates with a larger male will (i) produce larger

sons, who are themselves able to hold better calling sites, and attract more mates, (ii) produce

larger daughters, who will lay more eggs and (iii) produce "genetically superiof' ofTspnng

(better competitoa for food, better at avoiding predation, parasitism, disease etc.).

Clearly MDF is not a perfect market of maie size in spring peepers because, as discussed

previously, the evolution of MDF is potentially influenced by so many other environmental

parameters. Interestingly, female spring peepers show a wide range of responses to MDF,

dependhg upon population and conditions. Midpoint of the dominant frequency ranges fiom

approximately 2600 - 3300 for male s p ~ g peepers. When given a choice between males

singing at 3500 Hz vernis 2750 Hz, female peepers h m a population in Maryland

responded preferentially, but not absolutely, to the lower frequency (Forester and

Czarnowslry, 1985). When given a choice between calls at 2600 Hz, 2875 Hz and 4000 Hz,

females fkom Missouri preferred the central, not the lower, fkequency. These same femaies

showed no discrimination among calls at 287SHz versus 3300,3500, or 3700 Hz (Doherty

and Gerhardt, 1984). When the sound of a spring peeper chorus was broadcast

simultaneously with the choice experiments (2600 vernis 3500Hz), however, the females

displayed a preference for the higher fkquency cail, although that preference was not

consistent within an individual femaie across t h e (Schwartz and Gerhardt, 1998). It thus

appears that the female's response to the ûequency cornponent of the male advertisement cal1

depends upon a variety of factors, including the female's own physiological state and the

structure of background "noise" (e.g., the intensity of the conspecific chorus, the presence of

heterospecifics cailing in close proximity, sounds from wind and water). Overall, then, both

the fkquency component of the male advertisement cal1 and the female response to that

component appear to vary both within and across populations. Schwartz and Gerhardt (1998)

suggested that female preference based upon frequency would operate only when the female

was confionted with extreme differences between males. Such extreme differences were

likely to be important within a spring peeper chorus only in a general way (larger maies

versus smaller males), but not in any Eine-tuned sense that would allow females to distinguish

among large verms medium, or among medium-sized males. Indeed if a s m d male tends to

be relegated to the role of satellite (non-caiiing), then kquency ciifferences are even less

likely to play a role in mate choice within a population (Forester and Lykens, 1986; Lykens

and Forester, 1987; Gerhardt, 1988). It is possible, however, that such choice might be

criticai if females need to distinguish conspecific fiom closely-related heterospecific males in

areas of sympatry or parapatry.

If MDF is not king used as a marker of male quaiity by females, what other parameters

of the male calî could be transmitting this type of Uiformation? Male fiogs tend to c d fiom

protected areas at night when visual signals are of limited value. Vocalizations are a more

efficient way to attract mates at night, so it is possible that individual variability in such

vocalizations may increase a male's conspicuousness to femaies, and thus increase his

chances of king noticed and Iocated. Two variables, cd1 repetition rate (Forester and

Czamowsky, 1985; Gerhardt, 1987; Schwartz, 1987; Sullivan, 1987) and c d duration

(Klurnp and Gerhardt, 1987) have been suggested to important components of call

conspicuousness, and thus possibly of mate choice, in anurans.

Cal1 re~etition rate

increasïng cal1 repetition rate may be beneficial to the male for two reasons. First, a male

who calls more frequently than his neighbon increases the number of times that his "resume"

of availability is broadcast. As with advertising any product, the more times the message is

sent, the greater the likelihood that the receiver will detect it (Shannon and Weaver, 1949;

Forester and Czamowsky, 1985). Second, the probability that one male's cd1 will overlap

another's call increases with the size of the chorus and this, in tum, decreases the fernale's

ability to localize a particular male (littlejohn, 1977). An increase in call repetition rate may

offset this problem somewhat, dthough the degree to which a male will be k e fiom overlap

will depend upon the size of the chorus and the clifference in repetition rates among

individuals (Forester and Czamowsky, 1985). Males who increase theu repetition rate are not

getting "something for nothing". There is a cost, in terms of energy expended (Klump and

Gerhardt, 1987) and aiso, possiily, of increased predation risk. For example, amplexhg and

calhg Pseudum's crucifer hindivuais are preyed upon by Divhg beetles (Dytiscidae), giant

and water bugs (Belostomatidae: Lethoccerus americanus: Hinshaw and Sullivan, 1990);

predators on other breeding anunuis include natricine snakes, provided temperatures are

above hibernation levels (Whitaker, 197 1 ; Wassersug and Sperry, 1977; Wilbur, 1980;

Wiibur et al., 1983) and possibly ambystomid salamander larvae (Schaaf and Garton, 1970;

Jaeger, 1976; Groves, 1980; Kluge, 198 1 ; Ryan, 1985). There are cunently no published

data on the level of predation on breeding P. crucifer* although Hinshaw and Sullivan (1990)

observed an 8% predation rate on Hyla versicolor by invertebrates alone.

Calls which are conspicuous (due to intensity, duration, call repetition rate etc.) contain

more acousticai energy, and it has been suggested that they are likely indicative of good

physical condition. Females who respond to such calls should benefit more than those who

mate randomly (Klump and Gerhardt, 1987). Given these assumptions, an increase in call

repetition rate may be selectively advantageous because (i) it increases the likelihood that a

female will detect the message (benefit to the male [sender]) and (ii) it conveys tmthful

information about male condition to the female (benefit to the femaie [receiver]).

Having established the theoreticai h e w o r k for exarnining CRR, what did I discover

about this variable in this study? The raw values of cd1 repetition rate were positively

correlated with air temperature (Figure 8), but were not correlated with water temperature for

the small number of males used fiom the combined data set of populations fkom Ontario to

Arkansas. These results support the hdings of Sullivan and Hinshaw (1990) who stated that

site temperature was more highiy correlated to this variable [r = 0.5q thaa throat temperature

[r = 0.421 in a Maine population (aithough they did not specifying what "site" temperature

actuaiiy meant). In con- Brown and Brown (1977) found that water temperature was the

best predictor of c d repetition rate [r = 0.831 over air temperature [r = 0.681 and body

temperature [r = 0.581 in an Illinois population. This is odd because only six of theu 25

males were a c d y calling fiom water and these six individuals had only mal1 portions of

theù bodies in contact with the water. The authors suggested that male body temperatures

may have been compromised by holding the frogs in a warm human hand during stomach

temperature readings, a plausible outcome due to the high surface area to volume ration of

these small hylids. The authors also conceded that the perch sites of Pseuducris mcifer can

sometimes be s h b s and in turn water temperature may not dways be the best predictive

temperature. In rny study the majority of males were not cailing fiom the water with a large

nurnber calling fiom shrubs, ta11 grasses or even trees accounting for the high correlation

between air temperature and CRR.

There was no correlation between c d repetition rate and male body size (SVL) once the

call variable was corrected for temperature. These findings are similar to those of Sullivan

and Hinshaw (1990) for theù Maine population [r, = 0.3 11; and Forester et al. (1989) who

found that calling persistence and male body size were not signifcantiy correlated; but difTer

from those of Forester aiid Czamowsky (1985) who found a weak positive correlation

between these two variables in theù Maryland population. Finally, once cal1 repetition rate

was adjusted for air temperature, it did not differ arnong populations (Figure 9), nor did it

Vary with latitude, longitude, or vegetation cover. Overall, then, despite theoretical

predictions that cal1 repetition rate could be traasmitting idonnation about male size, and

hence male vigour and health to fernales, this variable does not appear to be reliably

üansmitting this type of infotmation in Pseudacris cmcifer.

Caii repetition rate is a very flexible component of the d e ' s adveaisement call. For

example, a male may increase his repetition rate if another male moves close to him and

begins to c d (Forester and Harrison. 1987) or if a female begllis to approach (Rosen and

Lemon 1974; Fellers, 1979; Forester and Czamowsky, 1985; Forester and Daniel, 1986;

Robertson, 1986). Variability could also reflect differences in the motivatiooal state of

sexually active individuals within and between nights (fluctuations in the endocrine cycle:

Forester et al., 1989). In other words, it is likely that an individual d l adjust his calling rate

through time, both within and among nights depending upon moment to moment changes in

the social context. For example, if oniy a few, distantly situated males are cailing, each one

may conserve energy by using a lower cal1 repetition rate, then suddenly increasing that rate

as females begin to move into the chorus. Aithough femaie spring peepea do prefer calls

with higher repetition rates (69 cpm versus 53 cpm: Forester and Czaniowsky, 1985), there

have been few investigations of how long an individual male (as opposed to a tape recording

of a male) can maintain a high rate of calltig in the field. Sullivan and Hinshaw ( 1 990)

reported that males with a high call rate on one night were more likely to exhibit a high call

nite on subsequent nights, although they did aot report how many nights that included. The

flexibility of cal1 repetition rate is m e r indicated by the negative relationship between

CRR and call duration. This negative relationship, albeit weak is not unexpected since CD

and CRR are opposing call panuneters. As the duration of an individual's cal1 increases in

length it is only reasonable to assume that the number of calls broadcast per minute must

decrease to some degree.

Overall, then, call repetition rate is a flexible parameter, responding to air temperature

(aad thus possibly numerous other enwonmental factors iike barometric pressure, wind

intensity, and available iight: Forester et al., 1989) and social conditions. My data hint that

there may acnially be Merences among popdations once at least air temperature is taken

into account (see Figure 9); however, those differences are not significant. In the fiitwe, we

need to measure variables like the presence of other calling males, the structure of the

competing male's call, and the presence of females in order to eliminate any sources of

variability due to social responses in the calling rate. Once these problems can be conected

for, we will be able to more rigorously investigate geographic differences in cal1 repetition

rate among populations of spring peepea.

Cal 1 duration

Al1 of the s u e s conducted to date have discovered a significant inverse correlation

betvveen cal1 duration and temperature (Maine: throat temperature [ r = -0.741 and site

temperature [r = -0.811, Sullivan and Hinshaw, 1990; Missouri: site temperature [r = -0.473,

Doherty and Gerhardt, 1984; Illinois: water temperature [r = -0.73T], air temperature [r = - 0.6751, and body temperature [r = -0.4361, Brown and Brown, 1 977). Similar fmdings have

been reported for other anurans, including Bornbina variegata for water temperature [r = - 0.7441 (Zweifel, t 959); Bufi ornerieanus and Bt&o woodhousei (Zweifel, 1 968); Bufo W.

fiwleri (Fairchild, 1981); and Pseudacris niseriata (Platz, 1989). This study was in strong

agreement with previous work on the spring peeper; there was a strong negative correlation

between call duration and air temperature in the combined data set of populations fkom

Ontario to Arkansas indicating that c d duration decreases as temperature increases (Figure

14). Just îike previous authors, I also found no relationship between c d duration and snout-

vent length ([r = 0.261 Doherty and Gerhardt, 1984; [r, = 0.291 Sullivan and Hinshaw, 1990;

Forester and Czarnowsky [1985] neglected to include a correlation coefficient; results this

study [r = 0.261 not sign5cant).

There were significant differences in the mean CD among the nine populations surveyed

in this study, even after CD had ken standardiwd Tir temperature. Cali duration feii dong a

continuum with Arkansas at the high end ( 194.37ms) and Kaladar 2 at the low end (106.64

ms) of the scale (Figure 15). This pattern of overlap across populations with significant

differences existing oniy between the two end points of the continuum may be due to the

high level of intra-populationai variabiiity (Ryan et al., 1990; Ryan and Wilczynski, 199 1;

Owens and Dixon, 1989). In such cases, neighborhg populations are expected to overlap

with respect to a given parameter, particularly if there is any movement of individuals

between populations (Sanderson et al., 1992). At the moment, we know very little about the

emigrationf immigration pattern of anurans, especialiy of small anuram iike the spnng

peeper, which are difficuit to locate outside of the breeding season. It would be interesting to

tmck toe clipped individuals h m year to year to determine whether they retum to their natal

ponds to breed. it would also be interesting to use the more sophisticated technologies

offered by molecular biology to determine how much introgression is occumhg among

particular populations. Until those data begin to accumulate, however, al1 I can say is that

there is substantial overlap in c d duration among my populations.

When CD (adjusted) was anaiyzed on an east-west and north-south gradient it was

positively comlated with latitude and negatively correlated with longitude (south-central

fiogs have longer calls than their relatives in the no&-east: Figure 17). It is possible that

these differences that refîect habitat type. As mentioned in the section on mid-point of the

dominant kquency, the northem populations live and c d within the moderately to densely

forested areas of the Interior lowlands (Milford Bay, Waterloo, Kortright, Kdadar 1 and 2)

and Boreal Forest (North of Cottage, South Portage) regions (Figure 16). In contrast, the

southern populations were broadcasting fiom open habitats in the Interior Highlands

(Arkansas) and ta11 gras Prairie (Missouri). At this time it is worth noting that the CD of the

single frog recorded at the Missouri location (Tallgrass Prairie) was 175ms, weil within the

range of those Uidividds recorded in Arkansas (Interior Highland). This value is not

necessarily Uiformative because only one call was recorded at this site and the male was

signaling h m a swimming pool. With this said, the Missouri site is geographically closest in

proximity to the Arkansas locale and the area surrounding the pool was completely void of

trees. While no other spring peepers were recorded in this area, Pseudacris triseriata were

obsewed calling in large numben fiom bodies of water that were "open". If the Pseudacris

crucifer in this area breed in the same or similar bodies of water to that of P. triseriata then

including the value for the Missouri male in the analysis would be justified. In fact, Doherty

and Gerhardt (1984) reported a mean call duration of 165 ms fiom their study population in

Boone County, Missouri, so my one singing male h m the Bea Western does not appear to

be too far off for the area.

My data show a marked and significant difference between the Intenor highlands (open

habitat) and the interior lowlands / Boreal forest (forested habitat: Figure 19). The interior

highlands geophysical ana has a mean CD of 194.37 ms while the Interior lowlands and

Boreai forest have CD's of 129.88 and 137.64 rns respectively. My results mirror that

reported by Ryan and Suilivan for toads (Bufo woodhousei and B. vaIIiceps; 1989) and Ryan

et al. (1990) for cricket f iogs (Acris crepituns). In ali of these species, males sing shorter

songs in the forest than in the open. Why should signals of shorter duration be ttansmitted

better in the forest? Unfominately most research on signai transmission has k e n focused on

frequency, so there is littie known about the effects of environmental parameters on call

duration. Forests are problematical for the sound transmission because of reverberation; the

bouncing and subsequent interference of sound waves off of trees, rocks, etc. (Wiley and

Richards, 1978, 1982). There is some tantalizing evidence fiom great tits indicating that

shorter, simpler songs are transmitted better in the forest because the period of silence

between songs decreases the reverberation problem buts space between successive, reflected

waves: Hunter and Krebs, 1979). This may explain, in part, the differences between males

calling in the open versus in the forest.

Within the forested habitats, the extremely short call duration of males fiom the Kaladar

2 site does not appear to even remotely resemble the values found for other sites in the same

geographical area (KI, MB, NC, SP, W and KO: Figure 17). in particular, the marked

difference between the CD values for Kaladar 2 (1 06.64 ms) and Kaladar 1 (1 3 1.35 ms) is

most peculiar considering that these two populations are less than 2 km away fiom each

other. This phenomena may be a bction of differences in endocrine state among calling

males, as well as environmental influences on the call in addition to vegetation density (e.g.,

changes in temperature, barometric pressure, humidity, wind and light intensity: Forester et

al., 1989). It is unlikely that ciifferences in endocrine state between males in the two localities

are responsible for the observed dserences since the calls for the individuals of Kaladar 2

were recorded only one night after those of Kaladar 1. In the course of just over an hour

during one recording session, however, conditions changed h m being p d y cloudy with no

wind to completely overcast with strong winds and heavy min. The cloud cover obscured

alrnost al1 mooniîght and the bmmeûic pressure dong with the relative humidity were

rapidly altered by the storm. The Kaladar 2 location was uniq~e in that it was ever changing.

The depth of the water, density and type of emergent vegetation, huxnidity and the barometric

pressure were al1 observed changing over the life of the chorus, sometimes differences were

visible from day to day. It is possible that the extremely low call duration for this population

is in part a response to an inconsistent environment. As previously discussed, social factors

such as chorus density and nearest neighbor distance can also influence some call parameters

such as call duration. Uofomuuitely at this tirne an effective way to account for these factors

has not been determined (Wagner, 1989).

Given differences in call duration, is there any evidence that females are responding

selectively to calls of varying lengths? Forester and Czamowsky (1985) found no femde

preference for call duration in their study of individuals fiom a Maryland population. The

calls were experimentally manipulated so that a single peep was chosen and stretched out

over time dlowing the cesearchers to use two cdls that were seemingly identical except for

CD. The authors suggested that their results may have been caused by how female fiogs

actually perceive male advertisement cails (as opposed to how those cails sound to us). In the

experiment, short duration calls had an abrupt rise and fa11 tirne, whereas calls of longer

duration had an abrupt rise thne but a gradua1 and prolonged fa11 time with respect to call

intensity. This resulted in the long call king at its peak intensity for a shorter penod of t h e

than the short c d . If femaies have a threshold intensity for response that falls within that

peak intensity range (Gerhardt, 198 1 [Hyla cinera]), then they may have in fact perceived

the long cal1 as king shorter in duration than the short c d ! This experiment highlighted the

dangers inherent in assuming, even unconsciously, that animals perceive the world in the

same way we do. Unlike the preceding study, Doherty and Gerhardt (1984) found that

Missouri females did exhibit cd1 duration preferences, although that "preference" was a

complicated one. They discovered that females didn't discrimiDate between calls of 1 Sûms

and 300ms, but did prefer calls of 150ms over very short (75 ms and 40111s) or very long

(400ms) alternatives. They hypothesized that cal1 duration may influence perceived call

intensity, at least on a very coasse level of analysis. For example, if the longer call is

perceived as king more "intense" then you should be able to eüminate the preference for

that call over a shorter one by decreasing the actual intensity (meamred in decibels) of the

longer call. This is exactly what the investigators found for the 150 ms versus 75 ms

cornparison. This hypothesis does not explain everytbing, however, because (i) decreasing

the decibel level of the 150 ms call never led females to lose their preference for that duration

over a very short 40 rns call and (ii) fernales acnially preferred the shorter call of 150ms to

the very long cd1 of 400 ms. The authors then suggested that females might be non-

respondent to calls less than and above a specifc length while being particularly sensitive to

the duration I intensity of calls that fd within a duration "window", ratber than being

sensitive to the entire d l . It is dso possible, given the research on the tuning of the spring

peeper's inner ear to particular fiequencies, that peepea are aiso acoustically m e d to

specific c d lengths. Certainly the details of that tuning, response to durations averaging 150

- 300 ms, have profound ramifications in areas where spring peepea are sympatric with

close relatives. For example, spring peepers overlap with one other chorus fiogs, Pseudacris

ornata in the midwestem-southeastem United States. When given a choice between a

conspecific call and the c d h m P. ornata, femaie spring peepers chose their own males

(Blair and Littlejohn, 1960). Blair and Littiejohn could not explain this choice. The study by

Doherty and Gerhardt (1984: see aiso Gerhardt, 1973), however, indicates that fernale s p ~ g

peepea were reacting to differences in call duration; P. ornata males sing on average for

only 30ms.

Although females may use cal1 duration to discriminate between conspecific and

hetemspecifk cails, it is dinicult to determine what the benefit could be accmed to females

using call duration to discriminate among conspecifc males. Cal1 duration is not correlated

with male body size and so is not transmitting information about the most obvious marker of

male health and vigour (Oplinger, 1966; Eden, 1976; Wilbur et. al., 1978; Lykens and

Forester, 1987). In fact, with the exception of males at Kaladar 2, most calls fell within the

100-250 ms range across al1 populations, so females could not use cd1 length to differentiate

among males in the populations 1 surveyed. Overall, then, it seems likely that the evolution of

call duration is directly affecteci by transmission properties of the environment in which

particular populations of spring peepers are Living (e.g., open versus forestrd), and may be

indirectly af5ected by interactions in areas where close relatives interact.

Male bodv size (snout-vent leneffù

This study revealed no relationship between male body size and the three different

geophysical areas, longitude, or latitude. These results contrasts with the findings of Nevo

(1973), who postulated that clifferences in male Acris blanchardi SVL dong an southeastern-

western cline (larger in the west) occurred because the decreased d a c e area to volume ratio

of large males made them more resistant to desiccation in the more arid western sites. Such

dramatic differences in aridity were not obvious in any of my sites. 1 did h d significant

differences in male SVL among populations of P. crucfler in the northeast, wi;h males living

at the North of Cottage (mean: 24.5 1 mm) and Kaladar 2 (mean: 24.80mm) sites being

signincantly smaüer than males at both Kortright and South Portage (means: 29.1 5mm and

30.09mm respectively). Once again, the Kaladar 2 site stands out (Figure 6). 1 wondered

whether the differences in body size might have something to do with collection time;

perhaps smder fiogs were coliected earlier than larger fkogs, and thus had less time to grow.

There is, however, no correlation between male size and date of collection (sites coliected in

1999: r = 0.03, p = 0.89), indeed the slope of the regression line is practically zero (slope =

0.007).

Al1 of spring peeper populations were syrnpatnc with at least one other anuran species

whose breeding time overlapped with P. crucifer at different times (Hyla versicolor, Bufo

amerkanus, Rana sylvatica, Rana clamitans, Rana pipiens, Rana sphenocephala). These

heterospecifics, however, are al1 large frogs (ranging in size fiom 3.2 cm to 12.8 cm: Behler

and King, 1997), so it is unlikely that female spring peepers would ever mistake any of them

for a conspecific male, and thus establish the prerequisite for a character displacement

dynamic to begin. Given this, it is ualikeiy that differences in male body size are the result of

character displacement at Kaladar or North of Cottage (selection for mal1 body size).

Finally, West Eberhard (1983) suggested that females in diffant populations may exert

direct selection upon the signal (call) itself, independent of what individual c d parameters

represent in the context of fitness, resulting in local cal1 prcferences for a given population.

If the selected c d parameters are conelated with male body size, then the mean body siae of

a population would diverge in respunse to those local preferences. In other murans it might

be possible that femde response to call fiequency wodd indirectly select for ciifferences in

male body size (Doherty and Gerhardt, 1984; Forester and Czamowsky, 1985; Lykens and

Forester, 1987; Sullivan and Hinshaw, 1990), but, as was discussed previously,

discrimination based upon MDF is not well developed in spring peeper females. Recall that

female peepers from Missouri did aot differentiate between fiequencies of 287SHz versus

3300,3500, or 3700 Hz (Doherty and Gerhardt, 1984). The average MDF in the Kaladar site

was 3226.24 Hz., well within this envelope of nondiscrimination.

What other factors, besides female mate choice and a response to clinal variability in

factors such as aridity and temperature could explain the small size of males at K2 (and

North of Cottage)? The two most obvious candidates are predation and food availability.

Broadcasting male Pseudacris crucifer are Milnerable to elevated predation levels by diving

beetles (Dytiscidae) and giant water bugs (Lethoccerus americanus: Hinshaw and Sullivan,

1990), and possibly also arnbystornid salamander larvae (Wilbur, 1980) and natricine snakes

(Whitaker, 1971 ; Wassersug and Sperry, 1977; Wilbur et al., 1983). Studies conducted on

satellite males have suggested that small males appear more alert and perhaps more agile

than larger males. Although it has been suggested that Pseuducris cnrci/r Iacks

antipredatory behaviour (Kats et al., 1988) it could be possible that smaller males are more

adept at predator avoidance as a result of their size. In order to examine this proposal further,

we would have to (i) demonstrate that the Kaladar 2 and North of Cottage sites are actually

subjected to elevated levels of predation compared with other sites (focussing in particular on

the sites with the larger males, Kortright and South Portage) and, if this is so, (ii) demonstrate

that smailer males have an advantage over larger males in eluding those predators, at Ieast

under the controlled conditions of a laboratory experiment. The second proposai, that smd

sïze is related to the qwntity and I or qyaiity of the food available at the two sites could be

tested by ( i ) detemiining the invertebrate f a d composition of the northeastern sites and (ii)

raising tadpoles h m the different sites under coatroiied conditions in order to determine

whether the differences in body sue are due to some environmental variable Iike food

availability or temperatme of deveiopment or whether these differences have a genetic

component.

Although body size is an intuitively obvious measure of male health and vigour

(Oplinger, 1966; Ernlen, 1976; Wilbur et al., 1978; Ryan, 1980), there is no consistent pattern

in spring peepers of female choice based upon size. Some authors have reported that a

significant size ciifference between amplexed vernis non-amplexed males (Gatz, 1980), while

others have found no difference (Forester and Czamowsky , 1 985; Sullivan and Hinshaw,

1990). Veneil(1983) suggested that such results could have been confounded by the

presence of smail, sexually parasitic conspecifics referred to as satellite males. Satellite males

do not c d , but sit silently around the periphery of the pond near calling, territorial males.

When a female approaches a calling male, the satellite male will ofien intercept her and

attempt amplexus (Rosen and Lemon, 1974; Pemll et al., 1982; Perrill, 1984; Miyamoto

and Cane, 1980). My own observations of spring peepers in the field indicates satellite mdes

are ofien present and that these males increase in numbers as the chorus density rises. There

is no way to differentiate a female in amplexus with a small male because she chose that

male, fiom a female in amplexus with a small male (satellite) who essentiaily ambushed her

(or one that she mistook for the calling male towards whom she was moving). Because of

this problem, the presence of satellite males will artificidy hilate the ''mal1 males in

amplexus" count, and weaken the positive comlation between female preference and male

body size.

Cd1 variables that are not correlated to SVL (CRR, CD, inteIlSi@ and call persistence)

may be selected by fexnaies based on conspicuousness (Forester and Czamowsky, 1985;

Forester et al., 1989). The tendency for Pseudacris crucifer to cd in large dense choruses

and for males to call in close proximity to one another illustrates the importance of

conspicuous calhg as a strategy that increases a male's chance of king selected by a gravid

female (Forester et al., 1989). Because it is more energetically costly to produce a

conspicuous call (Ryan, 1983; Taigen and Wells, 1985), males are constmtly subjected to

two confiicting pressures, bbconserve energy" versus "be conspicuous and attract a mate"

(Forester et al., 1989). Male Pseudacris crucifer seem to have overcome these confiicting

pressures, at least in part, changing their calls upon the approach of a gravid female (Rosen

and Lemon, 1974; Forester and Czarnowsky, 1985). Such a change can involve either

increasing the call repetition rate but using shorter duration calls, or increasing the call

duration and decreasing the number of calls per bout. Both of these strategies are

hypothesized to advertise the maximum amount of energy in the period following female

movement into the chorus. Since the call repetition rate I call duration correlation is a weak

one, some overlap in this relationship of reciprocals, possibly due to other call variables (e.g.,

call persistence), is expected.

Overall, then, ecological variables that affect signal transmission (e.g., density and kind

of vegetation, temperature, barometric pressure, humidity), female physiology and responses

and morphological constraints (e.g. the relationship between male body size and larynx

structure) have al1 influenced the evolution of the male advertisement call in the spring

peeper, Pseudacris mciJr. Those interactions have prduced complicated pattern of

geographical variability in the males' cal1 among populations of spring peepers ranging h m

Arkansas to centrai Ontario.

Summary

In this study of the geographic variability in the male advertisement cal1 of the spring peeper,

Pseudacris cruci@, I demonsîrated that:

there is signif cant inter- populational variability in the midpoint of the dominant

frequency component of the cail: fiogs fiom the southwestem Unites States, living in

open habitats, have a lower MDF than their northeastern brethren living in forested

habitats. This difference is hypothesized to be the result of different transmission

parameters in open versus forested habitats.

There is significant inter-populational variability in cal1 duration: fiogs fiom the

southwestem Unites States, living in open habitats, have longer calls than theù

northeastern brethren living in forested habitats. Although this may also reflect different

transmission parameters in open vernis forested habitats, this does not explain why one

of the populations fiom the forested area, Kaladar 2, bas such a short cal1 compared to

with other populations in similar areas.

There is signifïcant inter-populational variability in male body size: fiogs h m Kaiadar 2

and North of Cottage are smaller than frogs fiom South Portage. I propose that these

ciifferences might be due to inter-site variability in a variety of factors, including

differential predation, food availability and developmental temperature. This proposal

awaits M e r testhg.

O There was no difFereace in caii repetition rate among the 9 populations surveyed in this

study .

r Midpoint of the dominant fiequency was negatively correlated with male body size

(snout-vent length) and positively correlated with air temperature, call duration was

negatively correlated with air temperature, and cal1 repetition rate was positively

correlated with temperature. So, within any chorus, as it gets warmer with the passuig

days, a given male's call will have a higher dominant fiequency and be shorter in

duration, but there will be more call repetitions per bout. These results corroborated

studies using other spring peeper populations.

There is still a substantial amount of work to be done with these fkogs in order build a

robust enough database for studies of the evolution of male advertisement calling. For

example, are the call variables h m the southeastern subspecies, Pseudacris criccifer

bmtramiana , substantially different nom the variables for the northern subspecies? If so, is

there any evidence for character displacement in the area of overlap berneen these two

subspecies (e.g., Georgia)? What are the exact effects of environmental transmission

parameters (density of vegetation, humidity, barometric pressure) on the shape of male

advertisement call components? At the moment, we oniy have generai correlations between

such environmentai parameters and Merences in cidi structure across populations. Do these

parameters play an important role in female mate choice? For example. is it there an

interaction between the conspicuousness of an individuai male's cal1 and some component of

his fitness (other than a simple advertisement of male body size)?

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Appendix 1:

Scanned photographs from representative sites

Appendix 1 e: Kaladar 2 Site

Appendix 2: Raw data for fhg call, morphological, and ecological variables

Arkansas

2510

Site/ Individual male

2920 Ka*dar * 1 Mav 18.99

Collection Date

SV length (mm)

29.50 28.98 26.85 25.2 1 27.95

Air Temp.

( O C )

29.50 25-45 29.35

27.25 30.60

vegetation in water 29.77 7.0 12.5 143.67 2726.60 62.63 ..

19.0 19.0 1 9.0 19.0 19.0

23.50 24.68 27.60 25.35

Water Temp. (OC)

15.0 17.5 .,

18.0

1 7.99 18.0 18,O - -

19.0 19.0 18.0 18.5

Calling location

- - -

partially submerged CC

on floating algal mat Parking lot Parking lot

- 22.5

- -

Cal1 duration

(ms)

1 S' out on cattail 1.5-2' "

3' '"

109.84 193.70 153.8 1 203.95 180.27

Frequency (Hz)

94.38 136.39 1 18-96

.b

reed a b v e water shrub

2" above water

Cal1 repetition mte/min

2776.60 2776.60 2987.40

3580.20 327 1,90 3248.20 3224.50

60.07 7 1.22 91.61 94.26

101 .70 69.73 7 1 ,35

3129.70 3106,OO 3034.80

57.53 1 13.02 140.48 94.02

99.59 1 04.69 8 1.69

2560.60 2892.60

63.83 54.39

Appendix 2 continued

May 3999 May 13,99

Site/ Individual male

Collection Date

SV length (mm)

32.73 29.36

29. 14 17.0 17.8 barely in water 75.10 2892.60 25.55 1 7.0 ( 13.5 1 on floating dead ( 187.14 1 263 1 3 0

Air Temp. (OC)

3 1 ,O5

16.5 14.8

I I I vegetation I I

Water Temp. (OC)

15.0

30.60

3 1 -97 26.23 20.95

Call repetition ratelmin

- -

Calling location

17.5 L

18.0

16.0 16.0 18.0

6" above water made seedling

Cal1 duration (ms)

only hindlegs in water

15.0

22.0 -

22.0

Frequency (Hz)

136.55 166.83

282 1.50 291 6.30

1 03.63

vegetat ion on damp, floating

vegetation partially submerged

maple sapl ing Dead emergent

2750.30

129.59

1 2032 126.30 102.90

2655.50

2726.60 2940.00 3011.10

Appendix 3: Call variables adjusted for air temperature (A); snout-vent length (SVWO)

Site/ Individual mate

Call Dom. fieq. (A 14)

Dom. freq. (SVL30)

Dom. freq. (A 14- SVL30)

Call rep. rate

(A 14)

Call rep. rate

(SVL30)

Call tep. rate

(A 14 - SVL30)

Arkansas 2508

Kaladar 1 291 7 291 8 2919

Kaladar 2 2920 292 1 2922

Appendix 3 continued

Call rep. rate

(SVL30)

Call rep. rate

(A 14 - SVL30)

Site/ Individual male

Cal 1 duration (A 14)

Dom. freq. (SVL30)

Dom. freq. (A 14 - SVL30)

Call rep. rate

(A 14)

MiHord Bay 2522

Missouri 2516

North Coîtage 2518 2519 2520 252 1 2521x

South Portage 1 2926 2927 2928 2929

Appendix 3 continued

Si te/ lndividual male

Dom. freq. (A 14- SVL30)

Cal1 duration (A 14)

Call rep. rate

(A 14)

Dom.freq. (A 14)

Waterloo 2902 2909 2910 291 1 2912

Cd1 rep. rate

(SVUO)

Dom.freq. (SVL30)

Call rep. rate

(A 14 - SVL30)

137.25 1 50.45 131.25 136.73 123.76

2859.85 2573,76 2685.73 2899.1 3 2929-36

2637.24 267935 2806.54 2787.02 2643.87

284 1.59 2598.1 1 2765.67 2746.1 5 2562.13

Appendix 4

Scanned image of a oscillograrn highlighting midpoint of the dominant frequency, repetition

and duration components of the male advertisement call. Top image (one call): X a i s is time

in ms, the Y axis is frequency in Hz. For the bottom image (MDF of that call) the X axis is

ûequency in Hz, the Y avis illustrates the fieqwncy spike.

- HI*

Appendix 4 - 29: Voueher 2927 - South Portage 1

Appendix 4 - 39: Voucher 2936 - South Portage 2