Active space of a movement-based signal: response to the Jacky dragon (Amphibolurus muricatus) display is sensitive to distance, but independent of orientation.

The efficacy of any animal signal is constrained by the range over which it remains above the sensory threshold of potential receivers. The spatial area in which reliable detection occurs defines active space; this is influenced by signal structure, the signalling environment and the sensory characteristics of receivers. Identification of the factors influencing active space has provided valuable insights into signal design, particularly in bioacoustics, in which signal distortion and degradation can be easily quantified. In the present study, we consider whether active space can similarly help to explain the design of a movement-based visual signal. The Jacky dragon (Amphibolurus muricatus) threat display is composed of five distinct motor patterns delivered in an obligatory sequence: tail-flicks, backward and forward foreleg waves, a push-up and a 'body-rock'. In contrast to other communication systems, the introductory element is characterized by reduced intensity (average speed) but greater duration than subsequent motor patterns. Furthermore, the tail-flick sweeps a three-dimensional (3D) space around the lizard, whereas the motor patterns that follow are largely restricted to a single plane. Structural properties thus suggest that the active space of the tail-flick might be greater than that of the other motor patterns in the display, which would provide a parsimonious explanation for its use as an alerting component. We tested this prediction in a playback experiment incorporating 3D animations of lizard displays, comparing response probabilities to the factorial combination of three motor patterns, three viewing angles and three distances. Results suggest that the tail-flick does not have a greater active space than other display motor patterns, but that each degrades predictably with distance, thereby providing potential ranging cues. In addition, display components are remarkably robust to variation in receiver orientation, so that efficacy should be maximized in most potential signalling situations. These findings are consistent with the hypothesis that duration is the principal determinant of signal efficacy in this system.