13.1 Introduction

Consideration of the design and use of animal signals is of fundamental importance for our understanding of the social organisation and the perceptual and cognitive abilities of animals (e.g. Endler & Basolo, 1998). Movement-based visual signals have proven particularly difficult to understand because (in contrast to colour and auditory signals) perception, environmental conditions at the time of signalling and information content of motion signals cannot be easily modelled. Image motion has to be computed by the brain from the temporal and spatial correlations of photoreceptor signals. Although the computational structure of motion perception is well understood, in most situations it is still practically impossible to accurately quantify image motion signals under natural conditions from the animal's perspective. This undermines our ability to understand the perceptual constraints on movement-based signal design.

Extrapolating from other signalling systems, the diversity of movement-based signals between species is likely to be a function of the characteristics of competing, irrelevant sensory stimulation, or ‘noise’, and sensory system capabilities. The extent to which the spatiotemporal properties of signal and noise overlap remains unclear, however, and indeed, the motion characteristics that reliably lead to segmentation of the signal from noise are largely unresolved. It is therefore difficult to know the circumstances in which signal detection is compromised. In this chapter, I begin to generate the kind of data that will help explain movement-based signal evolution by modelling the changing perceptual task facing the Australian lizard Amphibolurus muricatus in detecting conspecific communicative displays.