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GROWTH, SEXUAL MATURATION AND BODYSIZE DIMORPHISM IN THE INDIAN BULLFROG,HOPLOBATRACHUS TIGERINUS (DAUD)Author(s): Narahari P. Gramapurohit, Bhagyashri A. Shanbhag, Srinivas K.SaidapurSource: Herpetologica, 60(4):414-419. 2004.Published By: The Herpetologists' LeagueDOI: http://dx.doi.org/10.1655/03-97URL: http://www.bioone.org/doi/full/10.1655/03-97

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Herpetologica, 60(4), 2004, 414–419� 2004 by The Herpetologists’ League, Inc.

GROWTH, SEXUAL MATURATION AND BODY SIZE DIMORPHISMIN THE INDIAN BULLFROG, HOPLOBATRACHUS TIGERINUS (DAUD.)

NARAHARI P. GRAMAPUROHIT, BHAGYASHRI A. SHANBHAG, AND SRINIVAS K. SAIDAPUR1

Department of Zoology, Karnatak University, Dharwad – 580 003, India

ABSTRACT: The pattern of growth, age and size at sexual maturity, and sexual size dimorphism weredetermined for 30 Indian bullfrogs, (Hoplobatrachus tigerinus) reared in out-door terraria, from metamorphosisto 3 years of age. Size and the growth rates for males and females were comparable throughout the study. ForSVL, annual mean growth rate was very high in the first year (7.31 cm) but declined in the 2nd (2.70 cm) and 3rd

(0.53 cm) years. For body mass, annual mean growth rate was similar (;68 g) for the 1st and 2nd years butdeclined (24.31 g) in the 3rd year. In general, growth was faster during summer than in winter months. Afterattaining maturity, growth rate declined for both sexes. Some males attained sexual maturity in the first year (10months old), while the remaining males and all females matured at the end of 2nd year. In naturally occurringbreeding populations, males tend to be smaller than females. This difference appears to be due to the presenceof males and females of different age groups rather than differences in early growth rates.

Key words: Anura; Frog; Growth; Hoplobatrachus tigerinus; Indian bullfrog; Sexual maturity; Sexual sizedimorphism

PATTERN of growth is considered to be one ofseveral factors responsible for sexual sizedimorphism (SSD) in adult anurans (Hayesand Licht, 1992). Sexual maturation is a criticaltransition in life history and the age and size atwhich maturity is reached can explain SSD inmany species of anurans (Marquez et al., 1997;Ryser, 1988). Sexual size dimorphism (SSD) iscommon in anurans with nearly 90% of thespecies studied known to exhibit SSD withlarger female size (Shine, 1979). Attemptshave been made to explain size dimorphismamong anurans based on sexual selection(Woolbright, 1983) or differences betweenreproductive selection gradients (Arak, 1988).Neither of these models considers selectivepressures affecting size, sex difference in age atfirst breeding and longevity, although Arak(1988) indirectly mentioned the importance ofdemographics in studying SSD. RecentlyMonnet and Cherry (2002) proposed that sizedimorphism in anurans is mainly due todifference in the age structure between thesexes in the breeding population.

The Indian bullfrog, Hoplobatrachus tiger-inus (formerly known as Rana tigrina, Ranatigerina) is a commercially important speciesthat grows to a comparatively large size (500–750 g) and hence was exploited for its meat. Itis a seasonal and explosive breeder that breeds

in temporary pools, ponds and small waterbodies formed soon after heavy pre-monsoonand monsoon showers (Saidapur, 1997). Thelarval duration is ;2 months. At naturalbreeding sites, males tend to be smaller thanfemales (Dutta and Mohanty-Hejmadi, 1981).Many aspects of its life history, includinggrowth, size at maturity and fecundity offemales at first reproduction are not known.In the present study we examine growth,sexual maturity, fecundity at first reproductionand seasonal variation in the growth rate incaptive frogs. These data may shed some lighton the cause of the apparent differences inbody size between males and females observedat breeding sites.

MATERIALS AND METHODS

Larval H. tigerinus at the end of meta-morphosis, stage 45 (Gosner, 1960) werecollected from a temporary pond (n 5 50)near the University campus, Dharwad city (158179 N and 758 39 E), in early July 1998. Theywere housed in an outdoor cement cistern (120cm L 3 120 cm W 3 80 cm H) with a slopedportion having water and the other end dry tofacilitate emergence of metamorphs. All indi-viduals metamorphosed within three days oftheir collection. In the first week of meta-morphosis 10 individuals died. The remainingfroglets were housed in two outdoor terraria (n5 20/terrarium; 300 cm L 3 100 cm W 3 100cm H) located behind the Zoology department1 CORRESPONDENCE: e-mail, saidapur@hotmail.com

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and exposed to natural temperature andphotoperiod. Each terrarium was providedwith a substrate of sand-mixed soil, smallherbs, shrubs and some retreat sites. At oneend, a small pool (100 cm L3 100 cm W3 12cm H) of water was provided. During the first3 months, metamorphs were fed with termites,small grasshoppers, and insects daily once adlibitum between 1100–1200 h. After 3 monthsof age, they were fed daily with a variety offoods including small fishes, grasshoppers,roaches, silk-moths, and other insect larvae(October 1998–June 2001). Ten individualsdied due to unknown reasons during first 6months after metamorphosis, while two indi-viduals died in the 2nd and 3rd years.

Snout–vent length (SVL, cm) and body mass(g) of the frogs were recorded at monthlyintervals. SVL was recorded to nearest 1 mmby gently pressing the body against a flatsurface. Body mass was recorded (afteremptying the bladder) on an electronic bal-ance for the first 6 months and later usinga spring balance. Sex of the frogs was identifiedfollowing development of vocal sacs in malesand spawning in females. At the end of the 1st

year, a biopsy of gonads was performed in twoindividuals (smallest and the largest) of eachsex to ascertain the progression of gonadaldevelopment by routine histological methods.After one year, all frogs were marked by toe-clipping so that changes in SVL and body massof individual frogs could be assessed. In June2000 (two years after their metamorphosis),seven amplexed pairs were observed and onepair spawned. We injected (ip) all otheramplexed and unpaired females with pro-gesterone (2 mg/ml saline) following whichthey also spawned. Daily changes in thetemperature (maximum and minimum) wererecorded in the terraria during the studyperiod to examine the relationship betweengrowth and seasonal fluctuation in tempera-ture. At the end of the study all frogs werereleased in their natural habitat.

Monthly growth rates for the 1st year werecalculated for the cohort, as the frogs wereunmarked. The growth rates for the 2nd and 3rd

years were calculated for each individual asthey were marked. For body mass, growth ratewas calculated by ln(Wt/W0), where Wt is themean mass of frogs at the end of each monthand W0 is the mean mass at the beginning of

the month. For SVL, growth rate was calcu-lated by ln(Lt /L0), where Lt is the SVL at theend of each month and L0, at the beginning ofeach month. Growth rates were used forregression analysis in relation to the age ofindividuals. Annual growth rates of SVL andbody mass for three years were computed bysubtracting the respective values at the start ofeach year from those at the end of the year.

To know the minimum and maximum size ofbreeding adults in a natural population, adultmale (n5 55) and female (n5 63) H. tigerinuswere collected at random from breeding sitesin different localities around the Universitycampus and the mean adult size was thencomputed.

To know the fluctuations in growth rate(SVL and body mass) with respect to seasonand age, slopes for SVL and body mass werederived using time as the independent vari-able. Slopes were derived by taking observa-tions of five months at a time moving from 1–5(July–November), 2–6 (August–December),3–7 (September–January) and so on up tothe 37th month to maintain the continuity.These slopes were compared by global F testand the significance level was set at 0.05. Dif-ferences in SVL and body mass between sexeswere analysed using Mann-Whitney U-tests atthree points (i.e., at the end of 1st, 2nd and 3rd

year respectively). Differences in growth ratesof SVL and body mass between the sexes for2nd and 3rd years were analysed using in-dependent sample t-tests. The relationshipbetween fecundity (number of eggs spawned)and body size (SVL and body mass) was ana-lysed using simple correlation. All the data inthe text are represented as mean 6 SE.

RESULTS

Growth Patterns

The size of males and females raised in theout-door terrarium did not differ significantly atthe end of 1st year (SVL:U5 84, P. 0.05; bodymass:U5 83, P. 0.05), the 2nd year (SVL:U560, P . 0.05; body mass: U 5 79, P . 0.05) orthe third year (SVL: U 5 62, P . 0.05; bodymass: U5 71, P. 0.05). Growth rates of malesand females in the 2nd and 3rd years did notdiffer significantly (SVL: t 5 1.34, df 5 1, P .0.05; body mass: t 5 0.10, df 5 1, P . 0.05).

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Therefore, the data were pooled for furtheranalysis.

Following metamorphosis in July 1998,initially growth was rapid but ceased fromDecember 1998 to February 1999 (Fig. 1).Growth was rapid between March and Sep-tember 1999, but ceased from October 1999 toJanuary 2000. In 2001 growth resumed inFebruary and continued until June, but ata much reduced level (Fig. 1). Changes ingrowth rates of SVL or body mass closelyfollowed the changes in ambient temperature(Fig. 2A,B). Thus, growth rate was high duringsummer and low during winter months. ForSVL, growth in the first year was 7.31 cm. Inthe second year, it lowered to 2.7 cm and in thethird year it plummeted further to 0.53 cm.

For frogs collected from natural habitats,mean adult SVL and body mass of male frogswere 13.49 cm 6 0.27 and 227.73 g 6 13.77respectively. Likewise, mean adult size (SVLand body mass) of female frogs was 15.34 cm6 0.30 and 348.43 g 6 16.58. Breeding malesof H. tigerinus were significantly smaller inboth SVL (t5 4.56, df 5 1 P, 0.05) and bodymass (t 5 5.60, df 5 1, P , 0.05) than females(Fig. 3). The frogs raised in out-door enclo-sures attained 50% of mean adult SVL of thenatural population at the end of the 1st yearfollowed by 66% and 72% in the 2nd and 3rd

years respectively.For body mass, overall growth was similar in

the first (67.58 g) and second (67.76 g) years,while in the third year it was lowest (24.10 g).

Age and Size at Sexual Maturity

Four out of 10 males developed vocal sacsand thumb pads 10 months after metamor-phosis. Testicular biopsies revealed formationof spermatozoa, indicating attainment of sex-ual maturity. Mean SVL and body mass ofthese frogs was 11.4 cm 6 0.4 and 119.12 g 614.4 (n 5 4) respectively. The range of SVLand body mass for these males was 10.3–12.2cm and 80–145 g, respectively. The remainingsix males developed vocal sacs and thumb pads22 months after metamorphosis. They mea-sured 12.2 cm 6 0.3 in length (range 11.5–13.2 cm) and weighed 135 g 6 11.4 (range100–175 g).

Ovaries of one year-old females containedfirst growth phase (FGP) oocytes havinga mean diameter of 354 mm 6 5.4. In females

aged 20–24 months, and measuring 12.2 cm 60.2 (SVL) and 134.1 g 6 8.0 (body mass n 518), the ovaries exhibited large ovulatory sizedfollicles. The SVL and body mass of thesefemales ranged from 11.5–14.2 cm and 115–200 g respectively. Fecundity was positivelycorrelated with both SVL (r5 0.856) and bodymass (r 5 0.967).

DISCUSSION

Anuran tadpoles of different species meta-morphose at sizes ranging from 20% of adultmass (Rana septentrionalis) to 0.1% of adultmass ([Bufo marinus] Werner, 1986). TheIndian bullfrog, H. tigerinus metamorphosedat 16% of its adult mean SVL, and theproportion of mass at transformation was thuslow (0.36% of adult mean mass). Therefore, inH. tigerinus post-metamorphic growth seemsto play a major role in attaining large adult sizeand most of the growth in mass occurs on landrather than in aquatic life. In Dharwad, thelarval period of H. tigerinus is ; 2 months andthe metamorphs exhibit a smaller size (;23mm SVL) in comparison to the metamorphs ofother two ranids, Euphlyctis cyanophlyctis(Gramapurohit et al., 2004a) and Rana cur-tipes (Gramapurohit et al., 2004b), both ofwhich attain sexual maturity very early in theirpostmetamorphic life. In H. tigerinus, growthrate is rapid during the first year and a fewmales even attain maturity in the first year.

FIG. 1.—Changes in mean 6 1 SE SVL (open circles)and body mass (solid circles) in H. tigerinus over a periodof 3 years starting from metamorphosis (m) in July 1998.Arrows show the attainment of sexual maturity in malesand females.

416 HERPETOLOGICA [Vol. 60, No. 4

FIG. 2.—Rate of change of growth in (A) SVL (m) and (B) body mass (m) in H. tigerinus following metamorphosis untilsexual maturity and thereafter derived using slopes (dy/dt), plotted with ambient air temperature (open circles).

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These observations suggest that rapid growthrate in the first year is essential to achievea critical minimum body size required to attainmaturity. A subsequent decline in growth ratemay be due to a trade-off in energy partition-ing between somatic growth and sexualmaturation as reported for other anurans suchas Rana catesbeiana, B. Marinus and Ranaclamitans (Jørgensen, 1992). In H. tigerinusgrowth rate declined progressively with in-crease in body size and it was very low aftersexual maturity. This extreme reduction ingrowth following maturity was also reportedfor the tropical frog Rana erythreae (Brownand Alcala, 1970).

A cyclical pattern of growth is often notedfor temperate species of anurans because theiractive feeding is restricted to summer months(Jørgensen, 1992). In winter they usuallyundergo hibernation. A similar cyclical patternof seasonal growth was also evident in H.tigerinus even though environmental fluctua-tions are not very harsh in southern India.However, in H. tigerinus fluctuation in growthrate closely followed changes in ambienttemperature, a pattern similar to that reportedfor R. clamitans (Martof, 1956) and Ranavirgatipes (Standaert, 1967). Seasonal changesin growth rates are not ubiquitous amonganurans. In Bufo fowleri and B. vallicepsgrowth rate was not affected by the seasons,and it was hypothesized that the toads emergeto feed about 30 times a year with emergencetimes spaced equally (Clarke, 1974; Pearson,1955). Despite constant supplies of food anda tropical climate, growth rate fluctuated inaccordance with the changing ambient tem-perature in H. tigerinus reared in out-doorterraria, indicating that growth patterns arespecies specific and seem to be fixed byendogenous rhythms in the Indian bullfrog.

Earlier maturation of H. tigerinus malesthan females is consistent with the generallyobserved pattern of early maturation of maleanurans (Collins, 1975; Duellman and Trueb,1986; Howard, 1978; Jørgensen, 1992; Mar-quez et al., 1997). Age of sexual maturity formale H. tigerinus was variable, with a fewmales attaining sexual maturity in the first year,while others reached maturity in the secondyear at the same time as the females. Thisvariability could be due either to geneticdifferences among males or to phenotypic

plasticity. For females, delaying maturity untilthe second year may provide a fecundityadvantage. Indeed, fecundity is highly corre-lated with body size in this species (Hoque andSaidapur, 1994).

For H. tigerinus collected from breedinggrounds, males are on an average smaller thanfemales (Dutta and Mohanty-Hejmadi, 1981;present study). This difference in size does notappear to be due to early differences in growthbetween the sexes. Instead, SSD observed inspecimens collected from nature appears to bedue to the presence of different age groups atthe collection sites. A difference in age atsexual maturity between the sexes in H.tigerinus may very well contribute to partici-pation of younger males in reproduction. Thepresence of more smaller males at breedingsites could also reflect a greater mortality ratein males than females, but this hypothesisneeds to be tested.

Acknowledgments.—The work was supported by a grantfrom the Department of Science and Technology (DST;SP/SO/C-022/95) and the University Grants Commission(SAP-II), New Delhi. NPG was a junior research fellow inthe DST project. The authors thank Prof. I. D. Shetty,Department of Statistics, K. U. Dharwad for his help instatistical analysis.

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Accepted: 3 June 2004Associate Editor: Peter Niewiarowski

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