Abstract

Visual systems are typically selective in their response to movement. This attribute facilitates the identification of functionally important motion events. Here we show that the complex push-up display produced by male Jacky dragons (Amphibolurus muricatus) is likely to have been shaped by an interaction between typical signalling conditions and the sensory properties of receivers. We use novel techniques to define the structure of the signal and of a range of typical moving backgrounds in terms of direction, speed, acceleration and sweep area. Results allow us to estimate the relative conspicuousness of each motor pattern in the stereotyped sequence of which displays are composed. The introductory tail-flick sweeps a large region of the visual field, is sustained for much longer than other components, and has velocity characteristics that ensure it will not be filtered in the same way as wind-blown vegetation. These findings are consistent with the idea that the tail-flick has an alerting function. Quantitative analyses of movement-based signals can hence provide insights into sensory processes, which should facilitate identification of the selective forces responsible for structure. Results will complement the detailed models now available to account for the design of static visual signals.

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