Quantifying the structure of dynamic visual signs

Our understanding of the role that perceptual processes have played in the evolution of animal signals can be attributed largely to the development of efficient methods for quantifying signal structure. However, when suitable methods are lacking our knowledge is much more superficial. This is particularly true in the case of dynamic visual signals such as threat displays. Traditional ethological techniques are inadequate for describing complex movement sequences. We introduce a new approach for quantifying the structure of such signals. This involves calculation of optic flow fields to provide estimates of the direction and speed of motion (velocity), which can be used to identify the spatial location and characteristics of movement. Artificial sensory units tuned to particular velocities then provide a summary output suitable for statistical analysis. To illustrate the application of this approach, we examine the aggressive display of the Jacky lizard (Amphibolurus murcatus) and demonstrate that the introductory tail flick is conspicuous againstt the background of wind-blown vegetation against which it must be perceived by conspecifics.