Sexual communication in cane toads, Chaunus marinus: what cues influence the duration of amplexus?

Successful reproduction often involves complex communication, but signalling capabilities frequently differ between the sexes. Research on sexual communication in anuran amphibians has focused heavily on the advertisement call made by reproductive males, with less attention paid to the release signal that males (and some females) give in response to misdirected attempts at amplexus. In the cane toad the release signal consists of a mechanical component (vibrations of the body wall, given by both sexes) as well as an auditory call (apparently given by males alone). We investigated the cane toad release signal to ask: (1) do females call in a manner similar to that of males but at a frequency inaudible to human ears? and (2) what stimuli (acoustic or otherwise) induce males to terminate amplexus? Analysis of calls over a wide (4–100 000 Hz) frequency range confirmed that female toads are indeed mute. Acoustic playback experiments revealed that males were less likely to dismount if we prevented an otherwise vocal target animal from making a release call. However, adding a release call to an otherwise mute target did not initiate dismount. Instead, the cue eliciting male dismount appears to involve synchrony between the release call and some other factor (probably, vibration of the clasped animal). Sexual dimorphism in skin rugosity also may play a role in amplexus termination.