I

Asiatic Herpetological Research

June 1995

Vol.

6, pp.

38-44

Size-gradation in Syntopic Frogs in South India

INDRANEIL DASAnimal Ecology Research Group, Department of Zoology, University of Oxford, South Parks Road, Oxford

0X1 3PS, England
Present address: Centre for Herpetology, Madras Crocodile Bank, Post Bag No. 4, Mamallapuram-603 104,Tamil nadu, S. India
Abstract. -An assemblage of eight metamorphosed anuran amphibians were studied at a seasonal locality inSouth India to examine patterns of body size-gradation. In general, body shape was found to be closelycorrelated to ecological characteristics of the species. The mean ratio of linear dimensions of the body inadjacent species, when arranged in a size -series was 1.3 (range 0.83-1.88), as predicted by Hutchinson (1959)
for closely-related sympatric species.

It

was concluded that competition may be at work in producing theseratios.

Key words: Amphibia, Anura, India, body size, competition.
Introduction

The body forms and sizes of organisms
have been considered, since the time of
Darwin (1859), to be determined by thepowerful forces of natural selection, andmay mirror a wide range of ecologicalinteractions often too complex tocomprehend in their entirety. The conceptof a form-function relation appears to havestood the test of time, and is central to the
problem of organismic evolution (Gans,1988). Animal size, for example, may be anevolutionary response to demands of theimmediate environment, related to key life
history traits, such as fecundity, foraging,locomotion, and reduction of predation,Somedesiccation, heating and cooling.
workers (e.g. Hutchinson, 1959; Schoener,1986a) have argued that smaller species are
generally more specialized, as reflected intheir restricted diets and geographical

Gould (1966), however,ranges.
considered both large and small species tobe viable strategists, optimal size, according
to Phillipson (1981) being

dependent on

Schoener (1965) and subsequent workers(reviewed by Schoener, 1974; 1986b),
differences in the comparative sizes of theorganisms are to be expected. In fact, suchapparent differences were reported muchearlier by Hutchinson (1959), who foundconstant differences in the ratios of linear
dimensions of the trophic (feeding)apparatus among closely-related sympatricspecies in his now controversial "Homage to

Santa Rosalia, or why are there so manykinds of animals?" paper.
Hutchinson
thatsimilar
could
coexist if
arguedspeciesthe ratios of linear dimensions and weights
of presumed competing species are around

While possible
1.3 and 2.0 respectively.
causal factors remain unclear, such ratios
have been found in a wide range of bothinvertebrate and vertebrate communities,
including beetles, spiders, frogs, lizards andbirds (reviewed by Schoener, 1986a).
I
report patterns of bodyand shape, and of size-gradation
observed in metamorphosed anurans from a
locality in South India.
Specifically, I

In this paper,size

abilities
and survival
probabilities of the various size and age
classes.
However, large species tend to

searched for patterns of size gradation
within the eight syntopic frog species.

appear later in a group's evolutionary
history, an exception being the Amphibia

Materials and Methods

competitive(Peters, 1983).

Within ecological communities, if
environmental resources are partitioned
according to dimension, as documented by© 1995 by Asiatic Herpetological Research

Eight species of anuran amphibians
occur in sympatry in the coastal scrublandsof Chengai Anna District, Tamil Nadu State,
South India. These, along with their mean
snout-vent lengths have been listed below::

Vol. 6, p. 39

Asiatic Herpetological Research

June 1995

To interpret shape changes, logarithmictransformations of the dimensions of theorgan of each species were used in thefunction log y = b log x + log a (where xTomopttma rolandat

Microhyla(1.566(2 82 patrubragm)and y are the morphological variates), which
has been considered to approximate shapechange in most organisms (see Gould,1966, for justification).

Observations
The assemblage of eight anuran species
Uperodon(13tystorna

Microhyla(0 362gm)

ornaiagm)

studied display a range of body forms
for(Figure 1) and sizes (Table 1). Except
theof
the diminutive species
genus
Microhyla (maximum SVL 17.4 and 26.5mm) and the large ranid, Rana hexadactyla(maximum SVL 132.2 mm), all species
were small to medium, with maximum SVL
between 43.2 to 93.1 mm (mean 68.2).

A general impression is that someand squat,
species are comparatively shortTothin.
andwhile others are longquantify
was divided by the cube ofdifferences,

WT

theRana htiadacryla(99 64 gm)

FIG. 1. Shapes and masses of the species studied.
Figures in brackets are mean weights.

Microhyla ornaia (4.1 mm), M. rubra (7.2mm), Tomopterna rolandae (10.9 mm),
Uperodon systoma (12.4 mm), Polypedates
maculatus (20.9 mm), /?a/za cyanophlyctisR.
(16.1 mm), fl. crassa (22.9 mm) andhexadactyla (33.4 mm).

Abbreviations used include SVL (snoutvent length),(head width at the angle ofthe jaws, perhaps better defined as theHW

distance fromgape), TBL (tibia length, thethe convex
tokneeofsurface
convextheand tarsus
tibia
bothwithofsurface
heel,flexed) andWT (wet body weight). Linear

measurements were taken to the nearest 0.1
mm with a Mitutoyo Dial Vernier Caliper,
weights were taken to the nearest 0.1 gm
with an Acculab Electronic Balance (Model333).

SVL for individuals of each species.

Three morphological groups along acontinuum were recognized based on thedata inpatterns of body form (length-weight
whichTable 2),comprise species withsimilar ecological preferences, including
bodiedheavy-bodied, terrestrial forms; lightofformarborealanandaquatic forms;
intermediate body mass.

The relationships of untransformed
3
values of WT/SVL (Figure 2.1), as well as
the arcsine-transformed data (Figure 2.2)
can be described as linear, the slope b being
0.38 and 1.89 respectively, not differingfrom isometry,significantly (r-test, P>0.05)
not likely to bearethat
large frogs
indicatingcomparatively lighter.

of differences in linear
dimensions (SVL, HW, ED and TBL) andweight (WT) of frogs from this study,arranged in a size series (mean dimensionsTheratios

by corresponding figures for
Microhyla ornata, the smallest member of
the assemblage) are shown in Figure 3. The
dividedrange of ratios of SVL (1.57-6.17) appear
Anolisgreater than shown by Neotropicalor
1.01-1.46
1978),(Duellman,lizards,Vol. 6, p. 40

TABLE 1

Asiatic Herpetological Research

June 1995

Morphometric data on the eight species of anurams studied. References to species: MO,Microhyla ornata; MR, Microhyla rubra; TR, Tomopterna rolandae; US, Uperodon systoma; RC, Ranacyanophlyctis; PM, Polypedates maculatus; RCR, Rana crassa; RH, Rana hexadactyla. References to bodyparts: SVL, snout-vent length; HW, head width; TBL, tibia length; WT, weight. Linear dimensions in cm;
wights in gm.

Species.

June 1995

Asiatic Herpetological Research

TABLE 2. Weight to cube of snout-vent length in the eight species of anurans studied.

SpeciesVol.6, p. 41Vol. 6, p. 42

Asiatic Herpetological Research

SVL1-June 1995

Vol. 6, p. 43

Asiatic Herpetological Research

June 1995

invertebrate and vertebrate groups (reviewedby Schoener,1984;1986a), whichissuspected to be indicative of interspecificcompetition. To complicate matters further,ratios have been shown to be sizedependent, a function of allometry, and may
thus change with growth (Roth, 1981).
These are also thought to vary
geographically, assemblages in the tropics
show smaller ratios than temperate ones

(Klopfer and Mac Arthur, 1961), suggestiveof greater niche overlap in tropicalassemblages. In the present study, HW,
considered a measure of the trophic
apparatus of frogs, showed negative
ontogenetic allometry, the slopes b of the(on log paper)
regressions of SVL andinalleight anuran
scaling allometricallythethatassemblage undercomprise
species

HW

investigation, indicating differences in therelative size and shape of the trophic
apparatus among the different size-classes

and sexes

within a
influenced by food size.

species

may beAcknowledgments
The staff of Madras Crocodile Bank
Trust helped with field work, and provided
laboratory space. In particular, I would liketo thank Romulus Whitaker, Harry Andrew,

Romaine Andrews, Manjula Tiwari andMy research on theecology of frogs in South India wassupported by Inlaks Foundation, Madras
Crocodile Bank Trust, Trinity College,
Oxford and an Overseas Research Student(ORS) Award. I thank R. Avery, M. Coe,and C. Perrins for comments.
Aurofilio Schiavina.

1987.

219.
1859.

HORN, H. S. AND R. M. MAY. 1977. Limits toNaturesimilarity among coexisting species.
270:660-661.

HUTCHINSON, G. E.

1959.

Homage to Santa

Rosalia, or why are there so many kinds of animals?
American Naturalist 93:145-159.

KLOPFER, P. H. AND R. H. MACARTHUR.

1961.

On the causes of tropical species diversity: Nicheoverlap.

American Naturalist 95:223-226.

LOSOS, J. B., S. NAEEM AND R. K. COLWELL.
1989.

Hutchinsonian ratios and statistical power.

Evolution 43:1820-1826.

MCALPINE, D. F. AND T. G. DILWORTH.

1989.

Microhabitat and prey size among three species ofRana (Anura: Ranidae) sympatric in eastern Canada.

Canadian Journal of Zoology 67:2244-2252.

MAIORANA, V. C. 1978. An explanation ofecological and developmental constants. Nature273:375-377.
PETERS, R. H. 1983. The ecological implicationsof body size.
Cambridge University Press,Cambridge.

PHILLIPSON, J. 1981. Bioenergenetic options and&P.
Pp. 20-45. In C.R. Townsend
An(Eds).
Physiological ecology:
evolutionary approach to resource use. Blackwell

phylogeny.

Calow

Scientific Publishers, Oxford.

ROTH, V. L.

1981. Constancy in the size ratios ofAmerican Zoologist

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404.

SCHOENER, T. W.

1965.

118:394-The evolution of billof birds. Evolution 19:189-213.

Resource partitioning in a community of Philippineskinks. Florida State Museum Bulletin 32:151-

DARWIN, C.

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size differences among sympatric congeneric species

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On the origins of species bymeans of natural selection. Murray, London.

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June 1995