Competitively mediated changes in male toad calls can depend on call structure

Many species of males aggregate in large groups where they signal to attract females. These large aggregations create intense competition for mates, and the simultaneous signaling by many individuals can impair any given male’s ability to attract females. In response to this situation, male signals can be modified, either evolutionarily or facultatively, such that the detectability of the signal is enhanced. The way in which signals are modified varies among even closely related species, yet few studies have evaluated what causes such variation. Here, we address this issue using male spadefoot toads (Spea multiplicata and S. bombifrons), which call to attract females. Using data from natural populations, we examined if, and how, male calls of three different call types (S. multiplicata with a slow call, S. bombifrons with a slow call, and S. bombifrons with a fast call) varied depending on competition with other males. We found that, in both call types consisting of slow calls, call pul...