Operational sex ratio and mating behaviour of the myobatrachid frog Neobatrachus kunapalari

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. Royal Society of Western Australia 2004.