Why do male and female cane toads, Rhinella marina, respond differently to advertisement calls?

Abstract

The main function of male sexual advertisement calls is to attract females, and therefore, most studies of responses to calls focus on females. Males may, however, respond to the calls of other males in some species, for one of two reasons: (1) to parasitize the calling of an attractive male by sitting close by and mating with attracted females, or (2) to locate breeding aggregations. We hypothesized that if males were parasitizing the calls of other males, they would select calls with characteristics that attracted females. On the other hand, if they showed no preferences for call characteristics it would suggest that they used calls to locate breeding aggregations or other resources. We used as our model organism invasive cane toads, Rhinella marina, a species in which males and females both respond to conspecific vocalizations with phonotaxis. To determine preferences in call choice experiments, we used the natural calls of five males, with extreme and median call characteristics (dominant frequency and pulse rate). Female cane toads tended to prefer calls with low pulse rate and dominant frequency, which are indicative of large body size in males, compared to a call with median characteristics. Overall, females preferred the calls of larger males, and selected calls of the largest male that was smaller than themselves, possibly because mating between similar-sized individuals facilitates fertilization in anurans. Male cane toads exhibited strong and rapid phonotaxis, but showed no preferences for specific calls. We therefore concluded that male toads probably use acoustic signals as an indicator of the location of water, or of breeding sites, and thus potentially of females. Sexual differences in call preferences apparently reflect differences in the use of advertisement calls between the sexes in cane toads.