Design of a deceptive visual signal: Death adders attract lizards by mimicking the movement of invertebrate prey

Sensory exploitation occurs when signals address biases in the nervous system of potential receivers, thereby providing a fitness benefit to the signaller. The Death adder (Acanthopis antarcticus) is thought to attract prey by wiggling the distal portion of its tail (caudal luring). To understand the design of this signal, we explored the sensory processes in Jacky dragons (Amphibolurus muricatus), a representative prey species. Jackys have superb motion sensitivity, which likely reflects selection for the efficient detection of conspecific displays and insect prey. We hypothesised the caudal luring might reproduce salient aspects of prey movement. To test this we used 3D modelling software to create an animated 'cybersnake' based on digital video footage of Death adders engaged in caudal luring. We then presented a range of stimuli to lizards using video playback. Sequences differed systematically in their motion characteristics (no motion, slow and fast) and in conspicuousness against the background (high and low). Moving stimuli reliably elicited predatory responses, and this effect was most pronounced when the lure was conspicuous. To identify putative models for caudal luring, we collected a range of prey items from the areas adjacent to lizard perch sites and filmed these moving across a sand substrate. Digitised sequences were analyzed with Matlab routines which extract velocity and acceleration values using an optic flow algorithm. The motion characteristics of caudal lures fall precisely within the range typical of invertebrate prey, which is consistent with the idea that sensory exploitation has shaped the design of this deceptive signal.