Receiver psychology and the design of the deceptive caudal luring signal of the death adder

Signal design can reflect the sensory properties of receivers. The death adder, Acanthophis antarcticus, attracts prey by wriggling the distal portion of its tail (caudal luring). To understand the design of this deceptive signal, we explored perceptual processes in a representative prey species: the Jacky dragon, Amphibolurus muricatus. We used 3D animations of fast and slow death adder luring movements against different backgrounds, to test the hypothesis that caudal luring mimics salient aspects of invertebrate prey. Moving stimuli elicited predatory responses, especially against a conspicuous background. To identify putative models for caudal luring, we used an optic flow algorithm to extract velocity values from video sequences of 61 moving invertebrates caught in lizard territories, and compared these to the velocity values of death adder movements. Caudal lures had motion characteristics that matched common invertebrate prey speeds, each corresponding to a peak in a bimodal distribution. Subsequent video playback tests using animations of crickets showed that significantly more attacks were evoked by stimuli moving at common than at rare invertebrate speeds. Overall, these results suggest that biases in the nervous system of the receiver, originally selected for prey recognition, might have been exploited by the design of the caudal luring signal. We suggest that viewing caudal luring from this perspective, rather than thinking of it as aggressive mimicry per se, may help us understand the function and evolutionary origin of this behaviour.