Plasticity of aggressive thresholds inHyla regilladiscrete accommodation to encounter calls

Abstract

Male Pacific treefrogs,Hyla regillause advertisement and encounter calls to regulate inter-male spacing within breeding choruses. Encounter calls are produced when the advertisement calls of a neighbour exceed a threshold amplitude. These ‘aggressive thresholds’ are plastic: males resume advertisement calling (accommodate) to repeated presentation of advertisement calls at amplitudes 4dB above their aggressive threshold. A male's aggressive threshold for the advertisement call is positively 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 significantly correlated with the maximum call amplitude of their neighbours. The present study tested whether this disjunction results from males not having recently experienced supra-threshold encounter calls, or from fixed aggressive thresholds for this call. Playback of encounter calls to males in the field showed that males generally accommodated within 2min when calls were 4dB above their aggressive thresholds for this call, but accommodated more slowly, and sometimes moved, at 8dB above threshold. A second study tested whether accommodation to the encounter call elicits parallel elevations of a male's aggressive threshold to the advertisement call. Playback of encounter calls at supra-threshold levels caused the aggressive threshold of males to increase 15dB for the encounter call but only 2dB for the advertisement call. These 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 signalling, may be important in balancing the costs and benefits of aggressive behaviour.