Female mate selection in the natterjack toad: active choice or passive atraction?

Abstract

Variation in seasonal mating success among male natterjack toads (Bufo calamita) was influenced by the number of nights that males spent at the pond and by male body size. Large males produced louder and lower frequency calls than small males, and maintained larger acoustic territories. After arriving at the pond, one half of all observed females mated with the first male they encountered. The remainder visited several males before initiating amplexus, but no criteria could be identified that females might have used when deciding whether to accept or reject a male. Movements between several males seem to be best explained by low female responsiveness to male advertisement calls on cold nights which were nonoptimal for oviposition. Females attempted to reject non-calling males both before and after amplexus, but this may be a mechanism to avoid mismating with males of the common toad (Bufo bufo), an explosive breeder that utilised the same pond. In two-choice playback experiments using synthetic advertisement calls, females showed no preference for calls based on their frequency. Females preferred calls of intermediate pulse repetition rate equivalent to those produced by a male at the same body temperature. Pulse rate is thus potentially subject to stabilishing selection and may be an important character for species-recognition. Females preferred fast rather than slow call rates, but only when the alternative rates were extreme. They also preferred calls which they perceived at the highest sound pressure level, but did not discriminate between absolute sound pressure levels of alternative stimuli at different distances. Since females that delay mating and oviposition may suffer predation, it is suggested that female preference for loud, rapidly repeated calls may be adaptive in the sense of minimizing the costs of locating conspecific males, rather than maximizing the probability of obtaining a high quality mate. Competition between males to maintain large acoustic territories and produce calls that can be easily detected by females would seem to be a sufficient mechanism to explain the evolution of the striking calls produced by male natterjacks.