Intrasexual selection and group spawning in quacking frogs (Crinia georgiana)

In the Australian frog Crinia georgiana, matings frequently involve a single female and multiple males (group spawning). The aim of the present study was to demonstrate a connection between variation in the intensity of intrasexual competition, measured by using male density and operational sex ratio (OSR), and the incidence of group spawning. Over a 3-month breeding period, male density and OSR varied substantially and were significantly influenced by climatic conditions. The frequency of agonistic interactions between males was higher in denser choruses. Fights were typically over the possession of sites used to broadcast advertisement calls and were almost always won by larger males. At higher densities, males allocated significantly less time to calling to attract females and spent more time as nonmoving satellites or roaming through an aggregation (searching). However, large males always called more than did small males. The number of males involved in a spawning correlated positively with variation in both male density and OSR. Observation of group spawnings revealed that they generally arose when a satellite male joined a mating pair after a female chose to mate with a calling male or when a female was seized by a searching male and the pair was joined by other searching males. These findings, coupled with past research documenting costs but no benefits of multiple paternity to females, suggest that competitively inferior males force group spawns.