PLASTICITY OF AGGRESSIVE THRESHOLDS IN HYLA-REGILLA - DISCRETE ACCOMMODATION TO ENCOUNTER CALLS

Male Pacific treefrogs, Hyla regilla, use advertisement and encounter calls to regulate inter-male spacing within breeding choruses. Encount er calls are produced when the advertisement calls of a neighbour exce ed a threshold amplitude. These 'aggressive thresholds' are plastic: m ales resume advertisement calling (accommodate) to repeated presentati on of advertisement calls at amplitudes 4 dB above their aggressive th reshold. A male's aggressive threshold for the advertisement call is p ositively correlated with the maximum amplitude of advertisement calls of neighbours, measured at the focal male's position. The aggressive thresholds of males for the encounter call, however, are not significa ntly correlated with the maximum call amplitude of their neighbours. T he present study tested whether this disjunction results from males no t having recently experienced supra-threshold encounter calls, or from fu;ed aggressive thresholds for this call. Playback of encounter call s to males in the field showed that males generally accommodated withi n 2 min when calls were 4 dB above their aggressive thresholds for thi s call, but accommodated more slowly, and sometimes moved, at 8 dB abo ve threshold. A second study tested whether accommodation to the encou nter call elicits parallel elevations of a male's aggressive threshold to; the advertisement call. Playback of encounter calls at supra-thre shold levels caused the aggressive threshold of males to increase 15 d B for the encounter call but only 2 dB for the advertisement call. The se two call types are spectrally highly similar; therefore, selective accommodation cannot result from sensory adaptation in the periphery. Plasticity of aggressive thresholds, like plasticity of aggressive sig nalling, may be important in balancing the costs and benefits of aggre ssive behaviour. (C) 1997 The Association for the Study of Animal Beha viour.