Detection of a looming stimulus by the Jacky dragon: selective sensitivity to characteristics of an aerial predator

Rapidly looming objects are highly salient to most animal visual systems. The sensory processing of such stimuli is now well understood in birds and insects. We conducted the first analogous study in lizards, concentrating on the ecologically realistic challenge posed by an approaching aerial predator. In an initial experiment, video footage of a trained raptor flying towards the camera produced flight responses in Jacky dragons, Amphibolurus muricatus, but control sequences showing a retreating or stationary stimulus were ineffectual. Two additional experiments then explored the processing of motion and of morphological attributes separately. We presented a series of expanding disks, systematically manipulating area/time characteristics to test looming sensitivity in the absence of other cues. Lizards oriented significantly more frequently to a sequence matching the area change of the approaching predator than to any other. Comparisons show that this response was specific to an exponential increase following a period of slow change, a pattern remarkably similar to those described in other taxa. In the final experiment, we presented a range of stimulus shapes, all with identical area and movement. Lizards were most responsive to a realistic raptor silhouette. Controls allowed us to exclude the possibility that this result was attributable to looming rate, size or the axis of asymmetric expansion. We conclude that response to an approaching aerial predator depends upon a hierarchical series of cues, including area/time profile, edge length, shape and orientation. The integration of this information will be an important problem for future work.