enamel microwear in caviomorph rodents
TRANSCRIPT
ENAMEL MICROWEAR IN CAVIOMORPH RODENTS
K. E. BETH TOWNSEND* AND DARIN A. CROFT
Department of Anatomy, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA
We developed a new data set of enamel microwear for extant caviomorph rodents (i.e., South American
hystricognaths) and inferred the diet of an extinct taxon, Neoreomys australis, from data on microwear. To
evaluate frequencies of wear features (pits and scratches) in caviomorphs, we employed low-magnification
microwear, which has been used successfully by others to distinguish among the diets of ungulates, primates, and
sciurid rodents. We developed 3 broad dietary categories for caviomorphs based on behavioral observations
reported in the literature: fruit–leaf, fruit–seed, and grass–leaf. Caviomorphs in general all exhibited wear features
indicative of processing hard objects (e.g., seed predation, eating hard fruits, and consuming exogenous grit).
Among our grass–leaf group, we identified an exogenous-grit subgroup that included fossorial and dust-bathing
taxa. We used a discriminant function analysis of wear features to examine post hoc classification of the
caviomorph taxa into the 3 dietary categories. Ours is the 1st study to quantify the distribution of microwear
features among modern caviomorph rodents; it has the potential to clarify the diets of modern forms that have
little behavioral data as well as to infer the diets of extinct species.
Key words: caviomorph rodents, diet groups, dietary inference, hypsodonty, microwear, paleodiets
During its long period of isolation, the mammalian fauna of
South America included numerous endemic lineages and
radiations, most of which are now extinct. The caviomorph
rodents of South America were arguably the most successful
group to survive the stresses of both environmental changes
and competition from North American mammals after the
Plio–Pleistocene faunal interchange (Flynn and Wyss 1998;
Simpson 1980; Webb 1978, 1991). The caviomorphs are
a diverse radiation of rodents that fill niches typically occupied
by nonrodents (e.g., small forest deer, hyraxes, pygmy hippo-
potamus, and rabbits) on other continents (Bourliere 1973;
Dubost 1988; Mares and Ojeda 1982). Caviomorpha, as a group,
mostly includes large neotropical forms; no species weigh ,90 g
and the largest living rodent, the capybara (Hydrochoeris) is
a caviomorph (Dubost 1988; Mora et al. 2003).
Dietary information for living caviomorph rodents is
generally limited to isolated reports of feeding behavior for 1
or a few individuals and is not based upon long-term studies
that can indicate dietary variation due to seasonality,
geography, or other limiting factors. Most caviomorph rodents
are generally cryptic, do not live in gregarious social groups,
and are difficult to observe in the wild (Dubost 1988).
Consequently, unlike most primates and ungulates, which have
well-known dietary ecologies, caviomorph rodents have not
been categorized into broad dietary groups. Placing these
rodents into broad dietary categories allows researchers to
assess trophic diversity patterns among caviomorphs and to
evaluate trophic structures within their corresponding mammal
communities. Establishing a series of ecologically meaningful
dietary categories for modern caviomorph rodents also
facilitates dietary inference for extinct caviomorphs.
Rodents are an important element of late Paleogene and
Neogene South American fossil mammal faunas and they can
provide important paleoecological and biostratigraphic in-
formation (Flynn et al. 2003; Kramarz and Bellosi 2005;
Pascual and Ortiz Jaureguizar 1990; Tauber 1997; Vucetich
1986; Vucetich et al. 1999). Extinct caviomorphs have been the
subject of numerous taxonomic studies, but there have been
few paleobiological studies (Flynn et al. 2003; Walton 1997).
These latter studies are hampered by the paucity of natural
history and functional data for living taxa (Emmons and Feer
1997; Mares and Ojeda 1982; Rensberger 1978).
Crown height (hypsodonty) has been used as a proxy for diet
in fossil rodents, suggesting that rodents with tall tooth crowns
ate more-abrasive foods than those with shorter crowns (Kay
and Madden 1997; Vianney-Liaud 1991; Williams and Kay
2001). Both abrasive diets (grazing) and grit have been shown
to play an important role in the evolution of tall crowns for
rodents and ungulates; hypsodonty in rodents also has been
used to infer paleoenvironments (Croft et al. 2004; Flynn et al.
2003; Janis 1988; Kramarz and Bellosi 2005; Vianney-Liaud
1991; Williams and Kay 2001). Nevertheless, a hypsodont
* Correspondent: [email protected]
� 2008 American Society of Mammalogistswww.mammalogy.org
Journal of Mammalogy, 89(3):729–742, 2008
729
dentition does not necessarily restrict an animal to eating
abrasive foods. In North American Neogene llamas and South
American Miocene notoungulates, the evolution of hypsodonty
apparently widened the dietary niches of these animals,
allowing for a more generalized diet that could accommodate
seasonal changes in food availability (Feranec 2003; Townsend
and Croft 2005, 2008).
The aim of this study was to use dental evidence from
microwear to categorize caviomorph rodents into broad dietary
categories. Then, using microwear data collected from the teeth
of extinct rodents, we can infer their diets by placing them in 1
of the dietary categories developed here. Because the relation-
ship between hypsodonty and diet in rodents is unclear, we
chose to use analysis of enamel microwear. Microwear studies
evaluate the pits and scratches left on the enamel surface of
teeth by the food an animal eats. Numerous methods have been
developed to evaluate microwear, primarily with the objective
of assessing the diets of extinct mammals (e.g., Solounias and
Semprebon 2002; Teaford et al. 1996; Ungar 1998; Walker
et al. 1978). Because long-term socioecological studies on
caviomorphs are rare, the evaluation of enamel microwear also
has the potential to confirm isolated reports of dietary behavior.
In this study, we strive to characterize the diets of a set of
extant caviomorph rodents by assessing enamel microwear
features of the molar teeth. We 1st evaluated ecological and
behavioral data for a group of extant caviomorphs and then we
generated broad dietary categories that would be useful for
classifying fossil rodent diets. We then used a low-magnifica-
tion microwear method to obtain data on microwear, and we
evaluated whether microwear features covary with the dietary
categories (using univariate and multivariate statistical tech-
niques). Finally, we used this data set to infer the diet of a fossil
rodent and we evaluated its use for further paleodietary
inference.
MATERIALS AND METHODS
Diets of rodents.—Rodents exhibit a range of dietary habits;
some are specialized herbivores, whereas others eat insects,
fish, and small land vertebrates, and many are considered
omnivorous (Eisenberg and Redford 1999; Emmons and Feer
1997; Nowak 1999). Among caviomorph rodents, herbivory
appears to be the dominant feeding strategy, although some
forms will eat small amounts of animal matter, mainly insects
(Emmons 1982; Henry 1999; Herskovitz 1972; Mares and
Ojeda 1982; Woods 1982).
Some caviomorph genera are particularly speciose (e.g.,
spiny rats [Proechimys]) and lack precise dietary data for all
constituent species. In Table 1, we have compiled dietary data
for the rodent taxa evaluated in this study. Most of these data
are for genera because data for species were not available.
These data were used to construct broad dietary categories that
were applicable to modern and fossil rodents. Some taxa are
fossorial (tuco-tucos [Ctenomys]) and others are known to bury
their food in what is known as ‘‘scatter-hoarding’’ behavior
(agoutis [Dasyprocta] and pacas [Cuniculus]); these species
therefore may consume exogenous grit. Because microwear
from grit could skew our dietary interpretations, we took
substrate use into account when evaluating the data on
microwear. When data were available, we considered seasonal
changes in diet as significant criteria for assigning taxa to
dietary categories for analysis.
We developed 3 broad dietary categories based primarily on
reports of observed dietary behavior in wild caviomorphs:
fruit–leaf, fruit–seed, and grass–leaf consumers. Of these
categories, the fruit–leaf and fruit–seed categories represent
a continuum of seed consumption or predation. Leaves, soft
fruit pulp, seeds, bark, twigs, and animal material comprise the
diets of taxa in the fruit–leaf group (Emmons and Feer 1997;
Woods et al. 1992; see Table 1). Taxa in the fruit–seed group
feed mainly on seeds and rely on seeds as a major food source,
although they are known to eat other fruits and plant parts as
well (Emmons 1982; Henry 1997; Streilein 1982). Taxa in the
grass–leaf group ate mainly grasses and leafy plants.
Samples and variables.—Fourteen species of caviomorph
rodents were evaluated in this study, 13 extant and 1 extinct
(Table 1). The extant sample includes 97 individuals from 9
families, all from the collections of the Carnegie Museum of
Natural History, Pittsburgh, Pennsylvania. Our sample of the
extinct dasyproctid Neoreomys comes from the late early
Miocene (16.3–17.5 million years ago [mya]—Flynn and
Swisher 1995) Santa Cruz Formation of Patagonian Argentina;
specimens were examined at The Field Museum, Chicago,
Illinois. We limited the sample of extant rodents to taxa with
relatively well-known diets; some of the ecological data are
from individuals in captivity, but most of the dietary data are
from field observations of feeding behaviors, stomach contents
of wild-caught individuals, or fecal samples. We limited the
sample to individuals in which M2 has erupted.
Occlusal patterns.—A wide variety of occlusal patterns are
represented in this sample. Hydrochoeris and Cavia have
laminar plates that are joined by cement (Fig. 1A; Hillson
1986). Chinchilla and Lagidium exhibit tall plates of enamel
that are not joined by cement (Hillson 1986). Ctenomys has
a crown with a single enamel ring outlining the occlusal surface
(Fig. 1B). Some caviomorphs, including Proechimys, Thrich-omys, and Myocastor, have occlusal patterns formed by
infoldings of enamel from the sides of the crown, similar to
what is seen in porcupines, Coendou (Fig. 1C; Hillson 1986).
Because of the difference in occlusal patterns, the region of
interest for microwear varied somewhat with taxon (see below).
Previous studies have successfully compared microwear on
teeth with different occlusal morphologies (Green et al. 2005;
Solounias and Semprebon 2002) and we therefore do not think
this poses a problem for analysis of microwear in rodents.
Data on microwear.—Data were collected from upper 2nd
molars, which were primarily used to compile the data set on
extant taxa. In the case of Ctenomys, the lower 2nd molar was
used when upper molars were not available; the lingual portion
of the single enamel band was read.
Studies of microwear in primates and ungulates have shown
that the dietary signal is consistent across both maxillary and
mandibular 2nd molars (Green et al. 2005; Semprebon et al.
2004). Although these studies did not include rodents, the cheek
730 JOURNAL OF MAMMALOGY Vol. 89, No. 3
toothrow of extant rodents is effectively homodont, especially in
caviomorphs, with minor differences in size among the molars
and single premolar (if present). This suggests that the functional
differences among these elements would not obscure or skew any
dietary signal on any particular tooth. Both mandibular and
maxillary specimens were used for the extinct Neoreomys and
data were collected on the anteroloph–posteroloph region of the
upper molars and the anterolophid–posterolophid region of the
lower molars. Clear epoxy casts were made of the occlusal
surfaces for both the extant and extinct caviomorphs and
microwear features were read from these casts.
All microwear features were read in a 0.3-mm2 standard area
at 70� magnification using a Leica MZ 12.5 light microscope
(Microscopy & Scientific Instruments, Bannockburn, Illinois).
The standard area or region of interest was delimited digitally
on a computer screen based on measurements made using
a stage micrometer. The imaging software Q-Capture version
2.7.3 (QImaging, Surrey, British Columbia, Canada) was used
to digitize the tooth images using a 5-megapixel camera
(QImaging Micropublisher 5.0 RTV; QImaging) and to transfer
the 70�-magnified image to the computer monitor for
assessment. Microwear features were counted on the computer
screen in the region of interest. Because of the small size of
rodent molars, the region of interest in this study was smaller
and the magnification was greater than that used in previous
low-magnification studies of microwear on samples of modern
TABLE 1.—Classification, diet, and substrate use of caviomorph rodents evaluated in this study. Taxonomy follows Huchon and Douzery (2001)
and McKenna and Bell (1997). The fossil taxon is indicated by an asterisk (*).
Taxon Diet
Substrate and food
storage and acquisition Dietary class Dietary referencea
Order Rodentia
Suborder Hystricognathi
Infraorder Caviomorpha
Erethizontidae
Coendou Seeds, herbivore,
frugivore
Arboreal Fruit�leaf 3, 8, 10, 16, 18, 21
Cavioidea
Agoutidae
Cuniculus paca Fruit, seeds, browse,
leaves
Terrestrial;
scatter-horde
Fruit�leaf 3, 6, 7, 8, 10, 14, 17, 18, 19, 22, 24
Dasyproctidae
Dasyprocta Fruits, seeds, nuts,
leaves
Terrestrial;
scatter-horde
Fruit�leaf 3, 4, 7, 8, 10, 18
Neoreomys* Unknown Unknown
Caviidae
Cavia Grasses, grass
inflorescences
Terrestrial; construct
surface runways in soil
Grass�leaf 7, 8, 17, 22
Hydrochoeridae
Hydrochoeris Grasses, aquatic
plants
Semiaquatic Grass�leaf 3, 7, 8, 10, 15, 16, 17
Octodontoidea
Ctenomyidae
Ctenomys Grasses, stems, roots,
bulbs, bark, leaves
Fossorial Grass�leaf 2, 3, 13, 20, 22, 23, 26
Echimyidae
Proechimys Fruits, nuts Terrestrial Fruit�seed 1, 3, 7, 9, 10, 12, 26
Thrichomys Seeds, fruits, nuts Fruit�seed 3, 17, 22, 26
Myocastoridae
Myocastor Grass, stems, leaves,
roots, bark, aquatic
vegetation
Semiaquatic Fruit�leaf 3, 8, 15, 27
Chinchilloidea
Chinchillidae
Chinchilla Grass, any vegetation Terrestrial; burrows Grass�leaf 3, 4, 25
Lagidium Grasses, herbivore Terrestrial Grass�leaf 5, 8, 11, 15, 17, 21, 22
a References: 1) Adler 2000; 2) Altuna et al. 1998; 3) Arends and McNab 2001; 4) Cortes et al. 2002a; 5) Cortes et al. 2002b; 6) Dubost et al. 2005; 7) Eisenberg 1989; 8) Eisenberg and
Redford 1999; 9) Emmons 1982; 10) Emmons and Feer 1997; 11) Galende et al. 1998; 12) Henry 1997; 13) Justo et al. 2003; 14) Laska et al. 2003; 15) Mares et al. 1989; 16) Mones and
Ojasti 1986; 17) Nowak 1999; 18) Peres 1999; 19) Perez 1992; 20) Puig et al. 1999; 21) Puig et al. 1998; 22) Redford and Eisenberg 1992; 23) Rosi et al. 2003; 24) Smythe 1986; 25)
Sportno et al. 2004; 26) Streilien 1982; 27) Woods et al. 1992.
June 2008 731TOWNSEND AND CROFT—ENAMEL MICROWEAR IN CAVIOMORPHS
and fossil ungulates and primates (Godfrey et al. 2004, 2005;
Semprebon et al. 2004; Solounias and Semprebon 2002;
Townsend and Croft 2005). We also wanted to maintain
a measurement standard that was comparable to similar studies
of microwear on squirrels (Nelson et al. 2005).
Variables.—Microwear features were counted twice, once
on the anteroloph or anterolophid and again on the posteroloph
or posterolophid (Fig. 1). Seven microwear features were
evaluated for each specimen: number of small pits; number of
scratches; number of large pits; number of cross scratches;
scratch texture (fine, coarse, mixed, or hypercoarse); number
of puncture pits; and number of gouges (Fig. 2). The size of
puncture pits was also recorded (small, medium, or large).
When an enamel plate or ring was read instead, 2 areas along
the same enamel plate were read. Scratch textures were coded
as fine, coarse, a mixture of fine and coarse, or hypercoarse. In
previous studies evaluating ungulates and primates, small pits
were combined with large and puncture pits for a ‘‘total pit’’count that was averaged for comparison across taxa (Godfrey
et al. 2004; Solounias and Semprebon 2002). We chose to
evaluate the small pits separately from other types of pits in
order to assess their utility as a dietary microwear feature.
Previous authors have developed 2 summary terms to
describe microwear signatures, coarse and fine wear, that have
been used to characterize the distribution of wear features for
a taxon (Godfrey et al. 2004; Green et al. 2005; Semprebon
et al. 2004; Solounias and Semprebon 2002). Features that
leave rough impressions on the enamel, such as large pits,
puncture pits, coarse scratches, and gouges, generally define
coarse wear. In contrast, fine scratches, few to no large pits,
gouges, and puncture pits define a fine wear signature. Hard-
object feeding or processing is another term that has been used
to describe both ungulate and primate seed predators; this term
implies coarse wear, and is characterized by a predominance of
puncture pits (Godfrey et al. 2004). These terms are used
throughout the remainder of this report to describe general
trends in microwear signatures.
Analyses.—Graphical evaluation and univariate and multi-
variate statistical analyses were used to assess the correlation
between microwear features and diet. The 1st series of analyses
were used to evaluate consistency in microwear variables
among taxa within dietary groups. Further testing explored
differences in microwear features among the 3 dietary groups.
FIG. 1.—Variation in occlusal morphology of 2nd upper molars in
3 representative caviomorphs: A) Cavia, B) Ctenomys, and C)
generalized erethizontid molar, adapted from Candela (2004). Dashed
boxes indicate areas where microwear was read on each type of
occlusal morphology. For all taxa with lophate teeth (Cuniculus,
Dasyprocta, Coendou, Proechimys, Thrichomys, and Myocastor) the
protocone region of the anteroloph and the hypocone region of the
posteroloph were read. In Hydrochoeris, Cavia, Chinchilla, and
Lagidium data were collected along the buccolingual surface of the
enamel plate. In Ctenomys, data were collected on the lingual edge of
the tooth. In all illustrations, the thick line represents the enamel band.
FIG. 2.—Digital photographs (taken at 70� magnification) of caviomorph microwear. A) Proechimys guyannensis, B) Hydrochoeris,
C) Dasyprocta, D) Cavia, E) Ctenomys, and F) Neoreomys (extinct). Microwear features illustrated on this figure are as follows: G ¼ gouge,
PP ¼ puncture pit, SPP ¼ small puncture pit, FS ¼ fine scratch, CS ¼ coarse scratch, SP ¼ small pit, HC ¼ hypercoarse scratch.
732 JOURNAL OF MAMMALOGY Vol. 89, No. 3
Graphs were generated and statistical tests were run on SPSS
11 for Mac OS X (SPSS, Chicago, Illinois).
Two types of variables were evaluated: averaged individual
data (e.g., average number of small pits for each specimen of
Cavia) and averaged individual per taxon data (e.g., average
number of small pits for all individuals of the taxon Cavia).
Averaged individual data and individual coded scratch textures
were used in the 1-way analysis of variance (ANOVA), Mann–
Whitney U-test, and principal component analysis (PCA). The
averaged individual per taxon data were used for all other
analyses. The individual coded scratch textures were evaluated
as percentages per taxon for subsequent evaluation in univar-
iate and discriminant analyses.
We performed a 1-way ANOVA to evaluate whether vari-
able means were significantly different among the 13 extant
caviomorph taxa. We used the Mann–Whitney U-statistic to
assess if the scratch texture data differed significantly among
taxa within the 3 dietary groups. In order to control the prob-
ability of making a type I error with the Mann–Whitney Upairwise comparisons, we performed a Dunn–Sidak correction
to reduce the experimentwise error rate.
We performed a PCA to explore variation in microwear
features among extant caviomorphs. An unrotated PCA of the
trait correlation matrix was run for microwear variables of all
individuals in the data set on extant caviomorphs. This allowed
us to identify the dominant microwear features in the data and
assess the dietary spectrum occupied by the extant rodents.
A discriminant function analysis of the newly defined dietary
groups was run to classify individuals into broad dietary groups
(fruit–leaf, fruit–seed, and grass–leaf) and to generate a dis-
criminant model that could be used to reconstruct the diets of
extinct caviomorphs. The goal of this analysis was to determine
those microwear features that characterize broad dietary cate-
gories and to evaluate the placement of some taxa into those
categories based upon their scores from the model. Using
a jackknifed classification method to evaluate the discriminant
model, we tested whether the modern caviomorph taxa were
correctly classified into their dietary groups. We ran 13 separate
discriminant analyses where 1 taxon was treated as an un-
known and was then subsequently classified into a dietary
group by the model. The diet of the extinct rodent Neoreomyswas reconstructed by treating the extinct taxon as an unknown
in the analysis. The microwear profile of Neoreomys was gener-
ated in the same way as for extant caviomorphs and was based
on both the mean and percentage scores of the individuals.
RESULTS
Overview of data on microwear.—Microwear features
visible in the region of interest included fine, coarse, and
hypercoarse scratches, small pits, gouges, large pits, and
puncture pits of large and small size (Fig. 2). The caviomorphs
overlapped each other in the average number of small pits;
Cuniculus, Chinchilla, Ctenomys, and Lagidium exhibited the
most small pits and Coendou exhibited the fewest (Table 2;
Fig. 3). There was also some degree of overlap in the average
number of scratches among the caviomorphs, although Hydro-
TA
BL
E2
.—M
icro
wea
rdat
afo
rex
tant
and
exti
nct
cavio
morp
hro
den
ts.
Aver
ages
are
giv
enas
mea
ns
(SD
).
Tax
on
nD
iet
gro
up
Aver
age
no.
smal
lp
its
Av
erag
e
no
.
scra
tch
es
Av
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oss
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es
Aver
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Av
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en
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ctu
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pit
s
Av
erag
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.
go
uges
%fi
ne
scra
tches
%co
arse
scra
tch
es
%h
yp
erco
arse
scra
tches
%m
ixed
scra
tches
Coe
ndou
9F
ruit�
leaf
16
.38
(7.7
9)
16
.38
(8.1
3)
1.0
0(1
.58
)5
.39
(3.7
4)
2.2
7(1
.77
)0
.06
(0.1
7)
2.7
7(1
.80
)4
4.4
11
.10
0.0
04
4.4
0
Cun
icul
us9
Fru
it�
leaf
44
.55
(28
.9)
13
.94
(4.3
4)
1.3
3(1
.22
)3
.83
(2.1
1)
0.8
9(1
.14
)0
.00
3.8
8(2
.20
)1
1.1
44
.40
.00
44
.40
Das
ypro
cta
10
Fru
it�
leaf
30
.40
(12
.17
)1
2.1
5(5
.71
)0
.30
(.6
32
)2
.40
(3.1
3)
0.9
0(1
.42
)0
.00
3.1
5(2
.94
)2
0.0
04
0.0
00
.00
30
.00
Myo
cast
or4
Fru
it�
leaf
27
.25
(16
.02
)1
5.2
5(5
.27
)0
.13
(0.2
5)
4.0
0(3
.40
)1
.00
(2.0
0)
0.0
03
.88
(2.0
2)
25
.00
0.0
00
.00
75
.00
Thr
icho
mys
9F
ruit�
seed
27
.05
(16
.47
)8
.77
(4.9
1)
0.4
4(0
.73
)5
.33
(3.6
5)
2.6
1(3
.99
)0
.00
3.2
2(1
.48
)4
4.4
11
.10
11
.10
33
.30
Pro
echi
mys
guya
nnen
sis
11
Fru
it�
seed
24
.36
(14
.70
)8
.70
(4.9
6)
0.2
7(0
.52
)2
.40
(2.5
1)
0.0
5(0
.15
)2
.50
(3.9
8)
3.3
1(2
.20
)2
5.0
01
2.5
00
.00
62
.50
Pro
echi
mys
brev
icau
da4
Fru
it�
seed
24
.50
(13
.69
)9
.50
(1.4
7)
0.7
5(0
.86
)4
.13
(3.1
2)
0.0
00
.00
2.7
5(0
.96
)2
5.0
00
.00
25
.00
50
.00
Pro
echi
mys
cuvi
eri
9F
ruit�
seed
20
.16
(9.1
4)
13
.44
(7.6
1)
0.5
6(0
.77
)2
.22
(2.5
0)
0.8
3(1
.70
)0
.00
2.0
0(1
.48
)5
5.6
44
.40
0.0
04
4.4
0
Hyd
roch
oeri
s9
Gra
ss�
leaf
27
.27
(12
.67
)2
6.6
1(7
.11
)0
.56
(1.6
7)
1.6
1(3
.15
)0
.17
(0.3
5)
0.0
01
.33
(1.0
0)
77
.81
1.1
00
.00
11
.10
Cav
ia7
Gra
ss�
leaf
29
.07
(14
.7)
29
.14
(4.9
6)
1.0
0(0
.52
)0
.86
(2.5
1)
0.0
00
.36
(0.9
4)
1.1
4(1
.35
)8
5.7
0.0
00
.00
14
.30
Cte
nom
ys1
0G
rass�
leaf
46
.40
(35
.14
)7
.45
(5.5
4)
0.0
5(0
.16
)5
.25
(2.9
8)
0.6
0(1
.91
)0
.00
3.6
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June 2008 733TOWNSEND AND CROFT—ENAMEL MICROWEAR IN CAVIOMORPHS
choeris, Cavia, and Lagidium exhibited very high average
numbers of scratches. Microwear features typical of coarse
wear such as gouges, large pits, and puncture pits were present
in high numbers among most caviomorphs, but Cavia,
Lagidium, and Proechimys brevicauda lacked large puncture
pits (Fig. 3). All taxa exhibited both fine and coarse scratch
textures except Lagidium, which displayed only coarse
scratches. Three taxa, Thrichomys, P. brevicauda, and
Ctenomys, had individuals that displayed hypercoarse scratch
textures. A bivariate plot of scratches versus pits (Fig. 4) shows
that scratches (x-axis) are effective in separating the frugivores
from 3 of the grass–leaf consumers (Hydrochoeris, Cavia, and
Lagidium). Average number of small pits (y axis) did not
discriminate along a dietary spectrum; Ctenomys, Chinchilla,
Cuniculus, and Lagidium had the highest frequencies of small
pits.
Analysis of variance and Mann–Whitney U-test.—The
1-way ANOVAs showed significant differences among all taxa
for average number of small pits, scratches, cross scratches,
large pits, large puncture pits, small puncture pits, and gouges
(Table 2). The average number of scratches per individual per
taxon showed the greatest differences among taxa (P ,
0.0001). Because these variables were significantly different
among taxa of different diets, we evaluated their diagnostic
utility further via multivariate analyses below.
Scratch textures were evaluated using a nonparametric test
for categorical data. The Mann–Whitney U-test for differences
in scratch textures between pairs of taxa in the 3 broad dietary
groups showed no significant differences among taxa in the
fruit–seed and fruit–leaf groups (Table 3). There was
a significant difference in scratch textures between 2 pairs of
taxa in the grass–leaf group: Hydrochoeris–Lagidium and
Cavia–Lagidium. Using the Dunn–Sidak Bonferroni method
(Sokal and Rohlf 1995) for correcting experimentwise type 1
error, these comparisons were shown to be not significant.
Scratch texture was used as a variable for subsequent analyses
because it proved homogenous within the other dietary groups.
Principal component analysis.—Using PCA, we evaluated
the dominant microwear signal in the data. Four components
explained 67.2% of the original variation in the data. The 1st
component (PC1) explained 25.3% of the variation, almost
one-third of the variance (Fig. 5). The 2nd through 4th
components each explained between 16% and 12% of the
variation. We used the 1st component axis to evaluate the
diagnostic utility of the microwear features. Taxa with high
positive scores on this axis have a coarse-wear signature
including (in decreasing order of importance) many large pits,
many large puncture pits, and many gouges (Table 4; Fig. 5).
Species with high negative scores have fewer coarse-wear
features but have more scratches overall, more cross scratches,
and more scratches of coarse texture (Table 4; Fig. 5). Of the 8
variables used in this analysis, 6 had relatively high loadings
on PC1, and 2—average number of small pits per taxon and
average number of small puncture pits per taxon—had
extremely low loadings. This axis describes a dietary spectrum
from hard-object feeders (at the high positive end) to grass
consumers and folivores (near the negative end; Fig. 5). On the
FIG. 3.—Box plots illustrating the distribution of the 6 quantitative
microwear variables among the 3 dietary groups. The plots show the
median, interquartile range, and outliers for each taxon. Dark gray
bars represent the fruit–seed group, white bars represent the fruit–
leaf group, and light gray bars represent the grass–leaf group. Outliers
are denoted by asterisks and open circles. The x-axis (taxon) for
each graph is coded: T ¼ Thrichomys, Pg ¼ Proechimys guyannensis,
Pc¼ P. cuvieri, Pb¼ P. brevicauda, M¼Myocastor, Ag¼Cuniculus,
D ¼ Dasyprocta, Co ¼ Coendou, L ¼ Lagidium, H ¼ Hydrochoeris,
Ca ¼ Cavia, Ct ¼ Ctenomys, Ch ¼ Chinchilla. The y axis for all
variables represents the average number of that variable per taxon.
FIG. 4.—Bivariate plot of the average number of scratches versus
pits. Note that the grazing subgroup (Hydrochoeris, Cavia, and
Lagidium) separates from the frugivores and exogenous-grit subgroup
(Ctenomys and Chinchilla) along the scratch axis. Filled circles
represent the fruit–seed group, open circles represent the fruit–leaf
group, and diamonds represent the grass–leaf group.
734 JOURNAL OF MAMMALOGY Vol. 89, No. 3
positive end of the spectrum the differences among scores is
not as great. The 2nd component (PC2) explained 15.58% of
the variation in the data; all variables loaded positively and
contrasted little in their loadings (Fig. 5). The remaining
components, PC3 and PC4, did not show strong contrasts
among microwear variables.
Discriminant function analysis.—We applied discriminant
function analysis to the data set to evaluate whether the
TA
BL
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.—M
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P.
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FIG. 5.—Principal component analysis of extant caviomorph
rodents based on the correlation matrix of 8 variables and 97 cases.
A) Plot of principal component 2 (PC2) versus PC1; most individuals
toward group interiors were removed to provide a clearer view of the
dietary groups. Note the separation of the fruit–leaf (open circle) and
fruit–seed (filled circles) groups from the grazing subgroup (open
diamonds, dotted line). Note also the position of the exogenous-grit
subgroup (filled diamonds, dotted line). The grazers have more
scratches and cross scratches, whereas the other taxa are characterized
by more coarse-wear features. B) Loadings on PC1; note the contrast
between scratch variables and pit variables. C) Loadings on PC2; note
the clustering of most of variables toward the positive end.
June 2008 735TOWNSEND AND CROFT—ENAMEL MICROWEAR IN CAVIOMORPHS
variables could be used to classify caviomorphs by diet. We
removed the variable ‘‘average number of small puncture pits’’because only 7 individuals in the data set exhibited small
puncture pits and this variable did not differ among the dietary
groups in previous analyses. Two canonical discriminant
functions captured 100% of the variation in the data; chi-
square tests of the significance of variance explained by the 1st
function (DF1; using Wilks’ lambda) yielded P-values , 0.05.
DF1 explained 99.4% of the variance in the data and its axis
separated the 2 groups of frugivores from the grass–leaf group
(Fig. 6). The 2nd function (DF2) separated the fruit–leaf group
from both the grass–leaf and fruit–seed groups (Fig. 6). There
was no overlap in the group centroids or in the taxa among the
3 groups, as illustrated by a 100% post hoc classification of the
original cases (Fig. 6). The jackknife classification procedure
resulted in no misclassifications of individual taxa into their
dietary groups (Table 5).
The variable ‘‘average number of scratches’’ had the largest
absolute correlation with DF1 (loading negatively), as did
average number of cross scratches, percentage of fine scratches,
percentage of coarse scratches, and average number of small
pits. Taxa with high positive scores on DF1 had more puncture
pits, gouges, large pits, hypercoarse scratches, and mixed
scratches (Fig. 5; Table 5). Taxa that loaded highest on this
function were the frugivores, with the fruit–seed consumers
(Proechimys and Thrichomys) having the highest scores. The
fruit–leaf consumers had lower positive scores because of
fewer ‘‘coarse-wear’’ features and more scratches of different
textures. DF1 separated grass–leaf consumers from the
frugivores by higher frequencies of small pits, scratches of
all kinds, and scratches with coarser texture. Puncture pits and
gouges were more frequent among frugivores than the grass–
leaf consumers. DF2 separated the fruit–leaf group from the
grass–leaf and fruit–seed groups. In contrast to the fruit–leaf
group, the taxa in the grass–leaf and fruit–seed groups had
more scratch features. The grass–leaf group has the highest
frequency of scratch features of all of the caviomorph taxa. The
conditional probabilities for each taxon being placed in the
correct group were all quite high and the posterior probability
for each taxon was 1.00 (Appendix I); this is also indicated by
the 100% post hoc classification success of the model.
The model classified the fossil taxon, Neoreomys, into the
fruit–seed group, with a 0.00 conditional probability of that
TABLE 4.—Dominant microwear signal, principal component
analysis (PCA), with taxa arranged by increasing score on PC1.
PCA scores are mean (SE).
Taxon PCA axis 1 scores Diet group
Cavia �1.25 (0.29) Grass�leaf
Lagidium �1.19 (0.14) Grass�leaf
Hydrochoeris �0.96 (0.13) Grass�leaf
Proechimys cuvieri �0.23 (0.19) Fruit�seed
P. guyannensis 0.01 (0.15) Fruit�seed
Chinchilla 0.02 (0.23) Grass�leaf
P. brevicauda 0.03 (0.24) Fruit�seed
Dasyprocta 0.05 (0.38) Fruit�leaf
Cuniculus 0.14 (0.23) Fruit�leaf
Myocastor 0.25 (0.53) Fruit�leaf
Coendou 0.59 (0.25) Fruit�leaf
Ctenomys 0.67 (0.26) Grass�leaf
Thrichomys 0.85 (0.48) Fruit�seed
FIG. 6.—Plot of the first 2 discriminant functions from an analysis
of diets of caviomorphs using microwear features. The model classified
all extant taxa by dietary group correctly. The fossil rodent Neoreomysaustralis plots between the fruit–seed and fruit–leaf groups.
TABLE 5.—Jackknifed classification matrix and discriminant scores
of extant caviomorph taxa on the 1st discriminant function. F-L ¼fruit–leaf, F-S ¼ fruit–seed, G-L ¼ grass–leaf. The score for the Santa
Cruz fossil rodent Neoreomys is included with the classification
provided by the model.
F-L F-S G-L % correct
F-L 4 0 0 100
F-S 0 4 0 100
G-L 0 0 5 100
Total 4 4 5 13
Taxon Discriminant score Dietary category
Hydrochoeris �17.99571 Grass�leaf
Ctenomys �17.73593 Grass�leaf
Lagidium �17.36286 Grass�leaf
Chinchilla �16.83331 Grass�leaf
Cavia �15.88239 Grass�leaf
Cuniculus 5.15392 Fruit�leaf
Myocastor 5.34501 Fruit�leaf
Dasyprocta 5.40925 Fruit�leaf
Coendou 5.54652 Fruit�leaf
Proechimys cuvieri 13.96034 Fruit�leaf
P. brevicauda 16.03833 Fruit�seed
Thrichomys 16.86027 Fruit�seed
P. guyannensis 17.49657 Fruit�seed
Neoreomys 27.52555 Fruit�seeda
a The model classified Neoreomys into the fruit–seed group with a 0.00 probability.
736 JOURNAL OF MAMMALOGY Vol. 89, No. 3
score being placed in the fruit–seed group. Neoreomys fell out
beyond the extant seed consumers on DF1 because of a very
high positive score on that function (Table 5).
DISCUSSION
The classic microwear variables ‘‘average pits’’ and
‘‘average number of scratches’’ did not provide the stark
dietary signal apparent in studies of microwear in ungulates and
primates (e.g., the traditional browser and grazer split among
ungulates or the ‘‘trophic triangle’’ seen in primates [Fig. 4;
Godfrey et al. 2004; Solounias and Semprebon 2002]). The
caviomorph rodents are unusual in that the average small pit
count was high (.40 small pits) for some taxa in the grass–leaf
category; in other groups, grass-consuming herbivores tend to
have lower pit counts (Godfrey et al. 2004; Semprebon et al.
2004; Solounias and Semprebon 2002). The discriminant
function analysis showed that the 3 groups are internally
consistent, although some taxa stood out within each group.
Detailed descriptions of the microwear profiles associated with
each dietary group are given in Appendix II.
The differences in low-magnification microwear patterns are
subtler among caviomorph rodents than those reported for
ungulates and primates. In these other groups, the dominant
presence of either pits or scratches allows for easy distinction
among frugivores, folivores, and hard-object specialists or
between browsers and grazers (Godfrey et al. 2004; Green et al.
2005; Semprebon et al. 2004; Solounias and Semprebon 2002).
Small pit and total scratch frequencies did separate frugivorous
caviomorphs from grazing ones in a bivariate plot, but detailed
dietary behaviors could be discerned only by evaluating
frequencies of other coarse-wear features (e.g., puncture pits
and large pits).
Presumed diets of extant caviomorphs were well supported
by the microwear data and the discriminant model. Lagidium,
Cavia, and Hydrochoeris are all grazers; these 3 taxa were
characterized by microwear profiles with the highest scratch
frequencies. Viscachas (Lagidium) have a diet dominated by
grasses (89%—Puig et al. 1998). Capybaras (Hydrochoeris)
are committed grazers that requires open grassy fields for
feeding (Mones and Ojasti 1986). Cavies (Cavia) are reported
to be grazers as well, but this taxon will eat grass flowers; these
have small seeds, likely accounting for the small puncture pits
(Table 2; Eisenberg 1989; Eisenberg and Redford 1999;
Nowak 1999; Redford and Eisenberg 1992). Cavia spends
time close to the ground and is known to tunnel through the
uppermost layers of soft soil (Eisenberg 1989; Eisenberg and
Redford 1999; Nowak 1999; Redford and Eisenberg 1992); it
would therefore be expected to exhibit some kind of coarse-
wear signal caused by ingested exogenous grit. The grazing
features apparently masked any signal from exogenous grit
because Cavia exhibited few coarse-wear features such as large
pits, gouges, and small puncture pits. Tuco-tucos (Ctenomys)
are reported to consume as much as 80% grasses (Justo et al.
2003; Rosi et al. 2003). Tuco-tucos had a microwear profile
with the highest small pit counts of all caviomorphs sampled,
very few scratches, very few cross scratches, high numbers of
large pits, and many gouges; all of these features suggest hard-
object processing. Unlike primates or duikers where ‘‘hard-
object processing’’ indicates seed predation or eating fruits with
tough pericarps, it is likely that the tuco-tuco is consuming
exogenous grit during burrowing because it is known to
‘‘chisel-tooth dig’’ (i.e., excavate with the incisors) in hard soils
(Mora et al. 2003; Spotorno et al. 2004; Vassallo 1998). Food
cleaning has been reported for tuco-tucos, where little soil has
been found in stomach contents (Altuna et al. 1998), but
examination of our data suggests that much exogenous grit is
consumed.
The microwear of the 4 individuals of Chinchilla lanigeraevaluated in this study did not suggest a grass-dominated diet,
but pointed toward hard-object processing. It has been reported
that chinchillas show no interest in eating seeds or pods and
few puncture pits were observed in this taxon, so it is unlikely
that we are getting a signal indicating seed predation (Cortes
et al. 2002a). We interpret this microwear as an indicator of
exogenous grit because chinchillas are known to burrow and
dust bathe (Spotorno et al. 2004). C. lanigera is reported to
have a seasonally varied diet (Cortes et al. 2002a); it is
therefore possible that the specimens examined were collected
during a period when fewer grasses and more soft plants and
fruits were being consumed.
Of the fruit–leaf consumers, Coendou has the highest degree
of hard-object processing; it is reported to eat seeds, twigs, and
bark, along with fruits and leaves (Emmons and Feer 1997).
Cuniculus exhibited a microwear profile consistent with the
frugivore pattern in primates (Godfrey et al. 2004; Semprebon
et al. 2004). Because pacas are forest-floor foragers and are
known to consume tubers, the gouging seen in the enamel may
be an exogenous-grit signal. Pacas do eat seeds, but cannot
open them with their incisors and will only eat the softer inner
parts of seeds that have been pried open by other animals
(Perez 1992). Primate frugivores that are seed predators tend to
have more coarse-wear features such as hypercoarse scratches
and, of particular importance, puncture pits (Semprebon et al.
2004). Cuniculus does have a large number of coarse scratches,
but it has no hypercoarse scratches and very few puncture pits
(Table 2). Coypus (Myocastor) spend a great deal of time in the
water and burrow into the banks of rivers and other waterways
(Woods et al. 1992). The coarse-wear features seen on the teeth
of the coypus are not likely indicative of an exogenous-grit
signal and probably indicate a slight degree of hard-object
processing (Woods et al. 1992). Coypus have been reported to
eat bark along with leaves, roots, and stems and are considered
generalists with tendencies towards folivory (Woods et al.
1992). The folivore signal was not as strong in the coypus as
what was seen in the grass–leaf consumers and was likely
masked by the coarse wear found on the occlusal surface.
Cuniculuss (Dasyprocta) are known hard-object specialists that
use their strong anterior dentition to open seeds and tough fruits
(Henry 1999). The microwear profile for agoutis had very few
puncture pits (Table 2) but it did exhibit high frequencies of
coarse and mixed scratches, as would be expected for a primate
or ungulate seed predator. These differences in the seed-
predation signal were likely due to differences in seed
June 2008 737TOWNSEND AND CROFT—ENAMEL MICROWEAR IN CAVIOMORPHS
processing: primates and ungulates open tough fruits, nuts, and
seeds with their molar teeth, whereas Dasyprocta uses the
incisors (Henry 1999). In the fruit–seed group, Proechimys(spiny rats) and Thrichomys (punares) all are known seed
predators and we expected to see a strong hard-object pro-
cessing signal (Streilein 1982). Fecal samples of P. brevicaudaindicate that it eats seeds, stems, fruit parts, and fungus
(Emmons 1982; Emmons and Feer 1997). P. brevicauda is
known to eat palm nuts and wind-dispersed seeds that are only
found on the ground (Emmons 1982); therefore, the coarse-
wear features in this species (and, by analogy, the other spiny
rats in our sample) could be due to both seed consumption and
exogenous grit. Emmons (1982) noted that a high proportion of
the stomach contents of P. brevicauda consisted of fungi, but it
is not possible at this time to determine the wear signal left
from fungivory. Henry (1997, 1999) noted that P. cuvieripreferred the soft pulp of fruits. The microwear profile of these
spiny rats certainly supports their known dietary habits of fruit
consumption as do their scores on PC1. Punares (Thrichomys)
are known to eat seeds preferentially and they live in the arid
habitats of the Caatinga in Brazil, which could also contribute
to their microwear signal (Streilein 1982). It is likely that
punares feed on seeds to a greater degree than do the spiny rats
because they had the profile with the highest frequencies of
coarse-wear features. Proechimys, like Dasyprocta, uses its
anterior dentition to open seeds, but Emmons (1982:287)
commented that Proechimys may not be an efficient seed
predator (in terms of eating the entire seed) noting that ‘‘the
morphology of the echimyid jaw is poorly adapted for scooping
hard materials.’’ Henry (1997, 1999) echoed this statement in
regards to P. cuvieri. If this is the case, and spiny rats cannot
access tough seeds, it may have had some effect on the
microwear seen in Proechimys. In contrast, the strong hard-
object processing signal seen in Thrichomys would suggest that
punares masticate seeds using primarily the molar teeth.
The low-magnification microwear method has been shown
to pick up dietary subtleties among species within the same
genus (e.g., Alouatta—Semprebon et al. 2004). Spiny rats are
a highly speciose group, and multiple species can be found in
very close proximity because their food resources are de-
pendent on highly productive microhabitats (Emmons 1982;
Emmons and Feer 1997; Patton and Gardner 1972). This is
reflected in very small home ranges that are characterized by
high densities of food. Within these microhabitats, however, it
appears that these animals are eating similar foods because
species of Proechimys are known to have very similar diets
from 1 locality to another (Adler 2000; Adler et al. 1998;
Emmons 1982; Henry 1997, 1999). Examination of our data on
microwear supports the hypothesis that species of spiny rats
have similar food preferences.
Substrate signal or seed predation?—All of the frugivorous
rodents in our study are reported to eat seeds to some degree.
Many of these taxa are terrestrial as well and it has been
established that coarse-wear features are present in all taxa. It is
essential to determine whether microwear profiles dominated
by coarse-wear features represent seed predation or hard-object
feeding, or both, or the effects of substrate (i.e., exogenous
grit); this is especially important when making inferences about
paleoenvironments using dietary reconstructions based on
microwear.
Recent low-magnification microwear work on squirrels
showed that the average number of pits and coarse features is
useful in distinguishing among frugivorous tree squirrels and
ground squirrels (Nelson et al. 2005). The study of microwear
in squirrels suggested that ground squirrels exhibited more
coarse-wear features due to more exogenous grit in their diets,
along with seeds and insects with tough exoskeletons (Nelson
et al. 2005). In primates, the intensity of seed predation is
reflected by the frequency of puncture pits and a spectrum
ranging from no seed consumption to heavy or ‘‘exceptional’’seed predation is apparent (Semprebon et al. 2004). The
recognition of such a spectrum has to do with the extraordinary
behavioral literature on primates combined with large dental
samples available for numerous species. The literature on diets
of caviomorphs is not as extensive as that of primates. Using
the primate model, where the frequency of puncture pits is
correlated with the degree of seed predation, we can make
similar statements about the degree of seed predation in
caviomorph rodents.
Most of the caviomorphs in our study are terrestrial and
a coarse-wear signal was represented in all taxa, ranging from
slight to dominant (Tables 1 and 2). The strongest coarse-wear
signals were found in Thrichomys, Ctenomys, Coendou, and
Myocastor. Of these, Ctenomys is the only form that lives most
of its life underground and is known to feed predominantly on
grasses (Altuna et al. 1998; Puig et al. 1999; Rosi et al. 2003).
Because the microwear profile of Ctenomys showed few
puncture pits (0.60 average pits per individual per taxon), we
are confident that the coarse-wear signal is due to substrate use.
Thrichomys is restricted to the semiarid Brazilian Caatinga
(Redford and Eisenberg 1992; Streilein 1982), and it could be
argued that an exogenous-grit signal is present due to living in
a dry habitat with more dust on food items. Given the strong
behavioral evidence for seed consumption by Thrichomys and
the very high number of puncture pits, we are confident that its
coarse-wear signal is due to seeds (Table 2). Coendou,
a porcupine, is the only arboreal form evaluated in our study.
Coendou is known to eat bark and twigs and is restricted to
humid forest (Eisenberg and Redford 1999; Emmons and Feer
1997); its microwear is clearly due to feeding on hard objects,
probably including seeds, since it had the 2nd highest
frequency of large puncture pits. Although dietary data for
the semiaquatic Myocastor are sparse, bark consumption has
been reported for this genus (Woods et al. 1992), and we
attribute the coarse wear to eating tough plant parts. The
puncture pits present in the profile of Myocastor might suggest
it eats seeds as well, but with such a small sample (n ¼ 4) it
would be tenuous to attribute these pits to seed predation.
The remaining terrestrial frugivores did not have microwear
dominated by coarse wear. The frugivorous spiny rats,
Proechimys, are terrestrial seed predators, but their microwear
profile was not coarse enough to be attributed to either grit or
seeds (Table 2). Dasyprocta, the agouti, is known to bury its
food in a scatter-hoarding behavior and then retrieve it during
738 JOURNAL OF MAMMALOGY Vol. 89, No. 3
times of food scarcity (Henry 1999). Therefore, we expected to
see a grit signal in Dasyprocta but did not. Cuniculus, the paca,
forages on the ground, but will eat only the soft inner portions
of tough fruits (Dubost et al. 2005). It is unclear whether the
slight coarse wear (especially few puncture pits) seen in
Cuniculus could be attributed to eating food from the ground or
to minimal hard-object processing. Because caviomorphs do
much seed processing with their incisors (Emmons 1982;
Henry 1997, 1999; Perez 1992), it is challenging to accurately
assess this behavior based on microwear of the cheek teeth.
Taxa in the grass–leaf group show evidence of coarse wear.
Hydrochoeris, Lagidium, Cavia, and Chinchilla exhibited
gouging, large pits, and a few large puncture pits. Considering
that Hydrochoeris grazes on both dry and wet grass, it is likely
that this animal may be picking up grit on grass blades. Caviatunnels in surface soil and eats grass flowers (Redford and
Eisenberg 1992), both of which could contribute to the coarse-
wear signal seen in this animal. Chinchilla is a known dust
bather and burrows (Spotorno et al. 2004), so it is likely that
exogenous grit affects its microwear. Lagidium is another
committed grazer living in arid habitats and could be
consuming grit on grass as well (Puig et al. 1998).
Paleodietary inference.—Quantitative paleobiological stud-
ies of neotropical rodents are few and have primarily been based
on inference with extant forms (e.g., Kay and Madden 1997;
Scott 1905; Walton 1997). Scott (1905) noted resemblances
among the teeth of Neoreomys, Capromys, Myocastor, and
Cuniculus and suggested that Neoreomys might be a remnant
form of the ancestral stock from which all of these lineages
descended. Fields (1957) stated that Neoreomys huilensis from
the La Venta Fauna (11.8–13.8 mya—Flynn and Swisher 1995)
could be related to Myocastor, based upon similarities in the
lower dentition. Based on analogy with living rodents, Kay and
Madden (1997) listed N. huilensis as a small-seed consumer.
Other authors have noted the similarities in molar occlusal
morphology between Neoreomys and living dasyproctids,
implying that it may have been a forest-floor dweller exhibiting
similar dietary behaviors (Fig. 2; Kramarz and Bellosi 2005;
Walton 1997). Dietary inferences for fossil caviomorphs based
upon analogies with modern forms and crown height (hyp-
sodonty) have been used to inform paleoenvironmental
reconstructions (Flynn et al. 2003; Kramarz and Bellosi 2005).
Although our discriminant model assigned a 0.00 probability
to the placement of Neoreomys in the fruit–seed category,
microwear indicative of frugivory was apparent on its occlusal
surface. As with other studies of microwear, scratches proved
to be the most diagnostic variable discriminating between
folivores and frugivores, and browsers and grazers (Godfrey
et al. 2004; Solounias and Hayek 1993; Solounias and
Semprebon 2002). Based on scratch frequency alone, Neo-reomys certainly was not a grazer; the remaining microwear
features resemble those of Dasyprocta. Expanded studies of
extant and extinct taxa may permit more detailed dietary
inferences for Neoreomys.
Studies of microwear using low-magnification microscopy
hold great promise for correlating dietary behavior of rodents
with wear signatures. Examination of our data shows that
recognizable dietary categories exist among caviomorph
rodents and that microwear can be used to classify taxa into
these categories. In evaluating habits such as seed predation,
greater sample sizes are needed to more accurately distinguish
the exogenous-grit signal from the hard-object processing
signal in frugivorous caviomorphs. For taxa with few dietary
data such as Thrichomys and Coendou, our studies of
microwear have confirmed reported dietary behavior. Unlike
features of microwear in primates and ungulates, individual
features other than scratches are apparently not very in-
formative for inferring diet in caviomorphs but yield insights
when considered together (although this may change with
greater sample sizes and additional taxa, particularly for
arboreal rodents).
The utility of our data set for paleodietary and paleoenvi-
ronmental inference is clear. The microwear profile of
Neoreomys is similar to that of Dasyprocta, suggesting that it
was some kind of frugivore; assessment of more specimens
could verify that it was a seed predator as suggested for the
species N. huilensis from La Venta (Kay and Madden 1997).
When diet is used to infer paleohabitats, studies of microwear
such as ours would be a useful approach to ensure accurate
dietary reconstructions. Furthermore, studies of microwear
underscore the importance of museum collections as sources of
ecological data, particularly because mammalian osteological
collections are generally used for taxonomic or functional
studies.
The data set that we have developed will be expanded in
future studies by adding more taxa and more specimens per
taxon. Additional variables such as incisor microwear would
further clarify food processing and consumption patterns in
caviomorphs. Nevertheless, this data set is valuable for under-
standing the diversity of modern caviomorph diets and shows
potential for interpreting the diets of fossil rodents.
ACKNOWLEDGMENTS
Funding for this study was provided by Case Western Reserve
University School of Medicine. We are grateful to both the Carnegie
Museum of Natural History and The Field Museum for access to
modern and fossil specimens. We thank J. Wible and S. McLaren
for access to specimens at the Carnegie Museum of Natural History;
S. McLaren was extremely helpful in setting up specimens for
molding. W. Simpson of The Field Museum was especially helpful in
suggesting ideas for preparing fossil specimens for molding. We also
thank the Department of Vertebrate Paleontology at the Cleveland
Museum of Natural History for allowing us to use space in the fossil
preparation laboratory for molding and casting; D. Chapman provided
valuable insights in this regard. Discussions with G. Semprebon about
microwear methods were very insightful.
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June 2008 741TOWNSEND AND CROFT—ENAMEL MICROWEAR IN CAVIOMORPHS
APPENDIX IIMicrowear Profiles for Dietary Groups
Grass–leaf group.—Within the grass–leaf consumers 2 subgroups
were apparent: grazers and exogenous-grit consumers. High scratch
frequencies and few to no puncture pits defined the grazer subgroup
(Hydrochoeris, Lagidium, and Cavia), although this subgroup did
have other coarse-wear features (Table 2). These taxa lie at the
‘‘scratch’’ end of the 1st principal component axis, suggesting a diet
composed of more abrasives than that of the other caviomorph taxa
(Table 4). Hydrochoeris exhibited a microwear profile indicative of
a diet dominated by grasses: high frequencies of scratches that are
mostly fine in texture and few coarse-wear features (Table 2).
Lagidium had a high number of small pits, the highest frequency of
scratches of all caviomorph taxa sampled, and all scratches were
coarse in texture; other than scratch texture, this taxon exhibited few
coarse-wear features. Cavia exhibited almost equal frequencies of pits
and scratches (Table 2); most of the scratches were fine in texture and
this taxon exhibited few coarse-wear features.
Ctenomys and Chinchilla comprised the exogenous-grit subgroup
and exhibited microwear profiles defined by coarse-wear features and
fewer scratches. A bivariate plot of average scratch frequency and
average small pit frequency shows that these 2 taxa fall well within the
range of the fruit–seed and fruit–leaf consumers (Fig. 4). The tuco-
tuco (Ctenomys) had the 2nd highest positive score on the 1st principal
component axis, suggesting hard-object processing. Chinchilla also
exhibited both low scratch and cross-scratch frequencies and high
small and large pit counts; it scored positively on principal component
1 (PC1), indicating fewer scratch features (Fig. 5).
Fruit–seed group.—The fruit–seed consumer group is composed of
3 species of Proechimys (which exhibit a consistent microwear profile)
and the punare (Thrichomys). As a group, they had relatively high
small pit counts (more than 20) and rather low scratch counts
(frequencies of both scratches and cross scratches; Table 2). The
presence and high numbers of large pits and gouges suggest seeds in
the diet, particularly in Thrichomys, which exhibited high puncture-pit
and large-pit frequencies (Table 2). Proechimys guyannensis also
exhibited small puncture pits and displayed the greatest number of
these pits among the 3 caviomorph taxa that had this feature (the
others being Coendou and Cavia). The presence of small pits in this
species suggests seed pits, which have been noted in both ungulate and
primate taxa; Proechimys is known to horde seeds for later
consumption (Forget 1991; Godfrey et al. 2004; Semprebon et al.
2004; Solounias and Semprebon 2002). The spiny rats exhibited
a range of scratch textures; Proechimys did not exhibit hypercoarse
scratches, but these were seen in Thrichomys. Coarse scratch textures
were a dominant feature in Proechimys, as evidenced by the
percentages of both coarse and mixed (both fine and coarse scratches)
textures apparent in their microwear profiles. The 3 species of
Proechimys did not fall near the hard-object processing end of PC1,
indicating that they had lower large-pit, gouge, and puncture-pit scores
than other hard-object feeders (Table 4; Fig. 5). The punare
(Thrichomys) had the highest PC1 score, indicating that it was
a definitive hard-object feeder (Table 4; Fig. 5). The most outstanding
feature of the microwear profile of the punare is that it had the highest
frequency of large puncture pits of all the caviomorph taxa in this
sample and it is the only taxon to exhibit hypercoarse scratches.
Fruit–leaf group.—All of the members of the fruit–leaf group
except Coendou exhibited a microwear profile consistent with
frugivory, as noted in both primate and ungulate microwear studies:
high frequencies of small pits and low frequencies of scratches. This
group had high frequencies of large pits, large puncture pits, and
gouges, with low frequencies of scratches and cross scratches. The
fruit–leaf consumers scored positively on PC1, indicating more coarse-
wear features than grass–leaf and fruit–seed consumers (Table 4; Fig.
5). The paca (Cuniculus) scored near the middle of PC1 between the
coarse-wear and grazing ends of the microwear spectrum, indicating
that it had fewer large pits and puncture pits than other more hard-
object feeders and fewer scratches than seen in the grass consumers.
The paca did exhibit a large number of gouges and large pits, many of
them along the edges of the enamel bands (Fig. 2). The agouti
(Dasyprocta) had low scratch values, low pit values, and high numbers
of large pits and gouges present, all features seen in frugivores (Table
2; Godfrey et al. 2004; Solounias and Semprebon 2002). The PC1 score
for the agouti was in the middle range, indicating that it was not a hard-
object specialist. The coypu (Myocastor) exhibited a microwear profile
similar to that of Cuniculus or Dasyprocta. In addition to exhibiting
high pit values and low scratch values, the coypu also exhibited a high
number of large pits, large puncture pits, and gouges. Most individuals
(75%) exhibited a mixed scratch texture signature (both fine and coarse
scratches). Myocastor scored on the positive end of PC1, reflecting the
higher frequency of large pits, gouges, and puncture pits on its enamel.
The arboreal porcupine (Coendou) exhibited the fewest pits and
scratches of the fruit–seed consumer group and these features were
represented with equal frequencies. The arboreal porcupine had the
highest number of large pits among all taxa represented in the study and
had the highest number of puncture pits relative to other members in its
dietary group (Table 2). It also exhibited an equal number of fine and
mixed scratch textures, and a few individuals exhibited only coarse
scratches. On PC1, Coendou scored high on the positive end of the
spectrum, with high counts of coarse-wear features, suggesting hard-
object processing (Table 4; Fig. 5).
Fossil taxon: Neoreomys.—The microwear of Neoreomys was most
similar to that of the agouti (Dasyprocta) in terms of small-pit and
scratch frequencies; it was classified by the discriminant function as
a member of the fruit–seed group. However, the conditional probability
of this classification using the extant caviomorph model is 0.00. This
result is apparent from the bivariate plot of the 2 discriminant functions
(Fig. 6) that Neoreomys falls beyond the fruit–leaf and fruit–seed group
on both the 1st and 2nd axes. The frequencies of coarse-wear features
are lower than those exhibited by members of the fruit–seed group.
These results should be considered preliminary because our sample of
Neoreomys is relatively small.
APPENDIX IProbabilities of group membership from discriminant function
analysis of microwear variables.
Taxon Group
Conditional
probability
Posterior
probability
Cuniculus Fruit�leaf 0.830 1.00
Dasyprocta Fruit�leaf 0.827 1.00
Myocastor Fruit�leaf 0.241 1.00
Coendou Fruit�leaf 0.868 1.00
P. guyannensis Fruit�seed 0.310 1.00
P. brevicauda Fruit�seed 0.235 1.00
P. cuvieri Fruit�seed 0.104 1.00
Thrichomys Fruit�seed 0.415 1.00
Hydrochoeris Grass�leaf 0.704 1.00
Lagidium Grass�leaf 0.730 1.00
Cavia Grass�leaf 0.249 1.00
Ctenomys Grass�leaf 0.832 1.00
Chinchilla Grass�leaf 0.947 1.00
Neoreomys Fruit�seed 0.000 1.00
742 JOURNAL OF MAMMALOGY Vol. 89, No. 3