Davis & Roberts: Operational sex ratio and behaviour 
males may leave the mating assemblages before eggs are 
released or never release sperm. Demonstrations that 
additional males release sperm would require direct 
collection of sperm, e.g., using condoms (Hettyey & 
Torok 2004), or genetic determination of extra-pair 
paternity. 
Apart from attempting to obtain fertilisations, what 
else could attendant males be doing? Males may be 
trying to displace other males and gain exclusive 
fertilisation but the pairing success of smaller males at 
Narembeen suggests this is unlikely as we would expect 
larger males to win fights leading to a large male 
advantage. In Bufo bufo, fighting and displacement of 
amplexed males over several days shifts the population 
from initially randomly paired by size to positive, size 
assortative mating, particularly because large males 
displace smaller males amplexed with larger females (cf. 
Davies & Halliday 1977). The lack of a significant 
correlation between the size of mated males and females 
in N. kunapalari also argues against displacement leading 
to more effective fertilisation in pairs with better matched 
body sizes (cf. Davies & Halliday 1977), as some pairs 
were likely to already be depositing eggs. 
Assuming males release sperm sometime during egg 
deposition, the frequency of multiple male matings in N. 
kunapalari, 21.5% over two sites, is much lower than in C. 
georgiana where 43.5 % of matings involve two or more 
males (Roberts et al. 1999). Our data suggest that attempts 
at multiple male amplexus may be a facultative response 
to male bias in the operational sex ratios as multiple male 
single females aggregations only occurred when the OSR 
was strongly male biased (Table 1). This also occurs in C. 
georgiana where the frequency of multiple-male matings 
and the number of males per mating both rise with 
increasing male bias in the OSR (Byrne & Roberts 2004). 
Sullivan et al. (1995) describe variations in mating 
behaviour with increasing density in several frog species. 
At higher densities males are more mobile and less likely 
to call to attract mates but they did not report multiple- 
male, single female amplexes but in these examples it 
was not clear how density and OSR co-vary. In C. 
georgiana, male bias in OSR and density are positively 
correlated (Byrne & Roberts 2004). 
The comparative analyses of testes mass reported by 
Byrne et al. (2002) suggested the real possibility of sperm 
competition increasing testes mass in many species. 
However, the mechanism of sperm competition was not 
clear: it may be direct competition as in C. georgiana 
(Roberts et al. 1999) or by more diffuse competition as 
suggested for Rana temporaria (Laurila and Seppa 1998) 
and Bufo americanus (Kaminsky 1997). In N. kunapalari, 
males amplexed in unconventional positions may release 
sperm that are sufficiently mobile to achieve at least some 
fertilisations. 
Our data for N. kunapalari are preliminary but are 
consistent with studies on other frogs where increasing 
chorus density and increasing male bias in the OSR are 
correlated with shifts in male mating tactics (Byrne & 
Roberts 2004). Our data also are quantified, natural 
history observations (rather than anecdotes), coupled 
with specific data on sperm release or multiple paternity, 
needed to determine whether sperm competition is 
relatively common in anurans (as suggested by several 
recent authors, e.g., Halliday & Verrel 1984; Halliday & 
Tejedo 1995, Roberts et al. 1999, Byrne et al. 2002). Ideally, 
future studies will experimentally manipulate OSR and 
follow amplexed pairs until egg deposition is complete 
to test our interpretations. 
References 
Byme P G & Roberts J D 2004 Intrasexual selection and group 
spawning in quacking frogs (Crinia georgiana). Behavioral 
Ecology (in press). 
Byrne P G Roberts J D & Simmons L W 2002 Sperm competition 
selects for increased testes mass in Australian frogs. Journal 
of Evolutionary Biology 15:347-355. 
Davies N B & Halliday T 1977 Optimal mate selection in the 
common toad Bufo bufo. Nature 269:56-58. 
D'Orgeix C A & Turner B 1995 Multiple paternity in the red¬ 
eyed treefrog Agalychnis callidryas (Cope). Molecular Ecology 
4:505-508. 
Emlen S T & Oring, L W 1977 Ecology, sexual selection and the 
evolution of mating systems. Science 197:215-223. 
Halliday T R & Tejedo M 1995 Intrasexual selection and 
alternative mating behaviour. In: Amphibian Biology Volume 
2: Social Behaviour (eds H Heatwole & B Sullivan). Surrey 
Beatty and Sons, Chipping Norton, Australia, 419-468. 
Halliday T R & Verrel P A 1984 Sperm competition in 
amphibians. In: Sperm competition and the evolution of 
animal mating systems (ed R L Smith). Academic Press, 
Orlando, USA, 487-508. 
Hettyey A. & Torok J (2004): 'In situ' prevention of anuran 
fertilization - a simple method for the detection of sperm 
competition with potential for other applications. 
Herpetological Review (in press). 
Jennions M D & Passmore N 1 1993 Sperm Competition in frogs: 
testis size and a 'sterile'male experiment on Chiromantis 
xerampelina (Rhacophoridae). Biological Journal of the 
Linnean Society 50:211-220. 
Kaminsky S F 1997 Bufo americanus reproduction. Herpetological 
Review 28: 84. 
Laurila A & Seppa P 1998 Multiple paternity in the common 
frog (Rana temporaria ): genetic evidence from tadpole kin 
groups. Biological Journal of the Linnean Society 63:221-232. 
Mahony M & Roberts J D 1986 Two new species of desert 
burrowing frogs of the genus Neobatrachus (Anura: 
Myobatrachidae) from Western Australia. Records of the 
Western Australian Museum 13:155-170. 
Main A R 1965 Frogs of Southern Western Australia. Western 
Australian Naturalists' Club, Perth, Australia. 
Roberts J D 1978 Redefinition of the Australian leptodactylid 
frog Neobatrachus pictus Peters. Transactions of the Royal 
Society of South Australia 102:97-105. 
Roberts J D 1997. Geographic variation in calls of males and 
determination of species boundaries in tetraploid frogs of the 
Australian genus Neobatrachus (Myobatrachidae). Australian 
Journal of Zoology 45,95-112. 
Roberts J D 1997. Call evolution in Neobatrachus (Anura: 
Myobatrachidae): speculations on tetraploid origins. Copeia 
1997, 791-801. 
Roberts J & Majors C M M 1993 Range extensions, range 
definitions and call structures for frogs from Western 
Australia. Records of the Western Australian Museum 16:315 
-322. 
Roberts J D Standish R J Byrne P G & Doughty P 1999 
Synchronous polyandry and multiple paternity in the frog 
Crinia georgiana (Anura: Myobatrachidae). Animal Behaviour 
57:721-726. 
Sullivan B K Ryan M J & Verrell P A 1995 Female choice and 
mating system structure. In: Amphibian Biology Volume 2: 
Social Behaviour (eds H Heatwole & B Sullivan). Surrey 
Beatty and Sons, Chipping Norton, Australia, 469-518. 
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