BEHAVIORAL PLASTICITY MEDIATES AGGRESSION IN CHORUSES OF THE PACIFIC TREEFROG

Male Pacific treefrogs, Hyla regilla, produce advertisement and encounter calls in breeding choruses. Both types of call contribute to inter-male spacing. The aggressive threshold of males to the advertisement call, defined as the lowest amplitude of playback that elicits encounter calls, correlates positively with the maximum amplitude of the advertisement calls of neighbours, as measured at the focal male's position. Results of two studies support the hypothesis that this correlation derives from plasticity of a frog's aggressive threshold to the advertisement call, as opposed to the hypothesis that a frog has a fixed threshold and adjusts his spacing from his neighbours so that their call amplitudes fall below his threshold. First, aggressive thresholds decreased following removal of a male's closest calling neighbours. Second, playback of the advertisement call at superthreshold amplitudes elevated a male's aggressive threshold for this call. A third study tested the hypothesis that the mechanism underlying this threshold plasticity was sensory adaptation in the peripheral auditory system. The aggressive thresholds of frogs to both the encounter and advertisement calls, which have similar frequencies but different temporal structures, were measured before and after playback of the advertisement call at a superthreshold amplitude. Contrary to the prediction of the sensory adaptation hypothesis, the playback elevated the aggressive threshold to the advertisement call without significantly increasing the threshold to the encounter call. This result suggests that the plasticity of the aggressive threshold to the advertisement call occurs at higher, more central levels of the auditory system. © 1993 The Association for the Study of Animal Behaviour.