Journal of the Royal Society of Western Australia, 87:97-99, 2004 
Operational sex ratio and mating behaviour of the myobatrachid frog 
Neobatrachus kunapalari 
R A Davis and J D Roberts 
School of Animal Biology M092, University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009. 
El droberts@cyllene.uwa.edu.au 
(Manuscript received May 2004 ; accepted December 2004) 
Abstract 
Matings involving two or more males and a single female (polyandry) are now widely reported 
in frogs. Polyandry may occur when the sex ratio at breeding sites (the operational sex ratio, OSR) 
is heavily biased towards males. We report here variation in OSR and the occurrence of 
polyandrous groupings in the frog, Neobatrachus kunapalari. When the OSR had a low male bias 
conventional amplexed pairs formed with one male and one female. When the OSR was more 
strongly male biased, groups containing one female and up to seven males formed. At a site with 
an OSR of 21 females:35 males, mated males were significantly smaller than unmated males but 
there was no correlation between male and female size in amplexed pairs. We argue that skew in 
the OSR may favour the facultative appearance of behaviours, e.g., multiple male matings, that 
raise the risk of sperm competition occurring. 
Keywords: operational sex ratio, frogs, amplexus, polyandry, mating behaviour 
Introduction 
Reports of multiple male frogs in amplexus with a 
single female occur sporadically in many frog families 
(Byrne & Roberts 2004). In some cases, these sorts of 
associations lead to polyandrous fertilisation of eggs 
from a single clutch (e.g., D'orgeix and Turner 1995; 
Roberts el al. 1999) and polyandry is reasonably 
suspected in others (e.g., Jennions and Passmore 1993). 
Many multiple-male, single-female associations in frogs 
may actually involve a real risk of sperm competition. 
Byrne et al. (2002) reported that the risk of sperm 
competition was associated with an increase in testis 
mass in Australian myobatrachid frogs. Their analysis of 
"risk of sperm competition" was based on a survey of the 
literature where they extracted data on the likelihood of 
males coming into close, physical proximity or contact 
with each other (and females), e.g., as in multiple male 
amplexus. They found that several Neobatrachus species 
had relatively high testes mass (N. kunapalari, testes 
approximately 0.26% of body mass, range in genus, 0.05 
to 1.23% across 10 species, Byrne et al. 2002). There are 
reports of both very dense breeding aggregations (N. 
pictus, Roberts 1978) and multiple-male, single-female 
aggregations in Neobatrachus species (e.g., cover photo of 
Main 1965 for N. pelobatoides) indicating a high risk of 
sperm competition. In Crinia georgiana, density, bias in 
the operational sex ratio (OSR, ratio of fertilisable females 
to available males, Emlen and Oring 1977) and the 
frequency of polyandrous matings are all positively 
correlated (Byrne & Roberts 2004) suggesting a possible 
general correlate of polyandry: strong male bias in the 
OSR leads to males using alternative mating tactics, e.g., 
group matings, to obtain fertilisations. 
Neobatrachus kunapalari is an explosive breeder that 
breeds after heavy summer and winter rains in the 
© Royal Society of Western Australia 2004 
semi-arid region of south-western Australia (Mahony & 
Roberts 1986; Roberts & Majors 1989). We report here 
observations of multiple male, single female 
aggregations in the myobatrachid frog Neobatrachus 
kunapalari, and relate this to variation in the operational 
sex ratio (OSR) in breeding ponds. We speculate about 
how variation in OSR might affect mating behaviour in 
N. kunapalari. 
Methods 
We observed breeding behaviour of N. kunapalari at 
two sites: 13 km north of Kellerberrin on the Trayning 
Road (16-19 April, 2002) and in a rubbish dump at 
Hedges, 13 km south south-west of Narembeen (21 
February, 1986), both in the semi-arid, grain-growing 
region of Western Australia. At Kellerberrin, the 
breeding site was a human-made, clay-lined drainage 
ditch on gently sloping pastureland, approximately 5x1 
m and 10 cm deep. Pond water was very turbid and 
opaque due to suspended clay. Observations were made 
between 19:00 hrs and 21:00 hrs from 16 th April to 19 th 
April, 2002. The Narembeen pond was formed after a 
heavy, late afternoon-evening thunderstorm. It was a 
roughly rectangular clay pond approximately 30 m x 5 m 
x 25-30 cm deep with turbid water. Headtorches with 
white light were used for making all observations. 
Amplexed pairs were placed in small plastic containers 
to collect eggs as they were deposited. Snout-vent length 
of all frogs collected at Narembeen were measured. At 
both sites all frogs collected were considered mature. For 
males this was based on the presence of nuptial pads 
and/or calling activity. Frogs considered female were 
carrying mature eggs, lacked nuptial pads, were 
depositing eggs or they were in amplexus and from their 
size they appeared gravid. We have never observed 
small, immature frogs of any Neobatrachus species in 
ponds where breeding was taking place despite a 
97 
