Deceptive digits: the functional significance of toe waving by cannibalistic cane toads, Chaunus marinus

Many ambush foraging predators possess specialized structures and behaviours that plausibly function to attract prey, but this hypothesis has rarely been subject to direct empirical tests. If luring evolved to attract specific prey types then we predict that it will be manifested only if that prey type is present, and only by predators of the size class that feed on that prey type. Also, luring should induce closer approach by prey; and aspects of the behaviour (e.g. frequency of movement of the lure) should have been fine tuned by selection to induce maximal response from prey. We describe a novel luring system: small- and medium-sized (but not metamorph and large) cane toads, Chaunus marinus, wave the long middle toe of the hindfoot up and down in an obvious display. In keeping with the functional hypothesis, toe waving is elicited by moving edible-sized objects such as crickets or metamorphic toads. Metamorphic toads are attracted to this stimulus, and trials with a mechanical model show that both the colour and the vibrational frequency of the toe correspond closely with those most effective at attracting smaller conspecifics towards the lure. The independent evolution of visual luring systems in many animal lineages provides a powerful opportunity for robust empirical tests of adaptive hypotheses about signal design.