Territorial male bullfrogs (Rana catesbeiana) do not assess fighting ability based on size-related variation in acoustic signals

Some animals use communication signals to assess their opponent's size and fighting ability during aggressive conflicts. Male frogs assess their opponent's size based on the fundamental frequency (pitch) of advertisement calls, which is negatively correlated with body size, an important determinant of fighting ability in frogs. I conducted a field playback experiment to investigate whether territorial male bullfrogs assess the size of opponents based solely on size-related variation in fundamental frequency. I repeatedly broadcast synthetic bullfrog advertisement calls to three groups of males. Playback stimuli simulated a large male (n = 24), a small male (n = 24), or an acoustically size-matched male (n = 34). Neither the simulated size of the opponent, the subject's own size, nor the degree of size asymmetry between the subject and simulated intruder had significant effects on the magnitude of responses during the playback test or on the rate of habituation that occurred with repeated stimulation. Post-hoc analyses of effect sizes and statistical power indicated that the effects in this study were quite small compared to previous studies in other frogs. More important, power analyses indicated that this study had high power (1 - beta > 0.90) to detect the magnitude of effect sizes observed in previous studies. Thus, territorial male bullfrogs do not appear to assess an opponent's fighting ability based solely on the fundamental frequency of acoustic signals. These results contrast starkly with theoretical predictions and previous empirical work with frogs.