Visual Sensitivity to Speed and direction of motion in the Jacky dragon (Amphibolurus muricatus): an ecological approach

Signal evolution is constrained by sensory properties. Recent work demonstrates that the design of both calls and ornaments can be explained in part by the sensitivity of receivers. Much less is known about dynamic signals such as threat displays, which are defined by movement. We used a novel technique to measure the motion sensitivity of Jacky dragons, a native Australian lizard that uses complex visual signals for opponent assessment. Subjects were initially trained to respond drifting random dot patterns moving at three speeds (5 deg., 20 deg., 80 deg. /s) with 100% directional coherence (i.e. all dots moving either right or left). These predicted the appearance of an animated invertebrate in the corresponding direction. Responsiveness was maintained by periodic reinforcement with live mealworms. Once reliable performance had been achieved, we conducted experimental trials to map variation in performance with all possible combinations of eight levels of dot speed (0.5 deg. - 160 deg. /s) and coherence (0 -100%). Results revealed a main effect for both speed and coherence, together with an interaction between these factors. Jacky dragons are very sensitive to fast movements, but have difficulty perceiving relatively slow-moving targets (values below 10 deg. /s). The visual system of these lizards is hence acutely sensitive to speeds characteristic of the motor patterns used for visual communication, particularly the tail-flick and push-up components of aggressive displays. In contrast, Jacky dragons may have difficulty in detecting the slow movement of predators such as snakes, or sedentary invertebrate prey.1 page(s