Design for conspicuousness in a movement-based signal

The structure of animal signals reflects selection to modify the behaviour of others. To convey information effectively animals must maximise the chance of being detected by increasing the signal to noise ratio. This can be accomplished by signalling at particular times of the day, when receivers are most sensitive, when other signals are absent, or by maximizing detectability is the use of alerting signals. These introductory components are often structurally simple and of higher intensity, but shorter duration, than the signal that follows. Alerting components are likely to be particularly important in visual signals because these can only be perceived if the intended receiver is appropriately oriented. Here we investigate whether the complex threat display of the Jacky dragon (Amphibolurus muricatus) has properties predicted by theoretical models. This signal consists of a rapid series of stereotyped motor patterns delivered in an obligatory sequence: introductory tail-flicking is followed by backward and forward arm-waves, then a push-up and body-rock. The application of a new approach for the description of biological movement is used to elucidate differences in structure between components of the display and model the way in which these are processed by the lizard visual system. Results explain aspects of signal design and illustrate the importance of sensory processes for understanding signal evolution.