Male green frogs lower the pitch of acoustic signals in defense of territories: a possible dishonest signal of size?

In responses to broadcasts of conspecific advertisement calls, male green frogs (Rana clamitans) lower the dominant frequency of their calls. Because dominant frequency is negatively correlated with male body size in green frogs, frequency alteration provides a means of potentially exaggerating size during territorial contests. In field playback experiments, we broadcast synthetic stimuli representing small, medium, or large intruders to territorial residents. We tested the hypotheses that males use frequency alteration to provide honest signals of their size or their size-independent fighting ability, or to dishonestly signal size. Dominant frequency did not better predict male size in response calls than in unsolicited calls. The magnitude of frequency alteration was not related to body size, general condition, or an indirect measure of fighting ability. Thus, males did not use frequency alteration to provide honest information about body size or size-independent fighting ability. However, males significantly increased their apparent size by producing lower frequency calls. Small males produced relatively lower frequency calls in response to the large-male stimulus (compared to the small-male and medium-male stimuli), but large males did not. Further, the magnitudes of frequency alteration were significantly greater in responses to the large-male stimulus, primarily because small males responded with a greater decrease in frequency to the large-male stimulus than to the small-male and medium-male stimuli. These results support several predictions of the dishonest signal hypothesis and suggest that dishonesty may be a conditional strategy used by small males.