Eyespots interact with body colour to protect caterpillar-like prey from avian predators

A wide range of animals possess conspicuous eye-like markings on their bodies, commonly called ‘eyespots’. These markings are generally thought to serve an antipredator function, particularly in lepidopteran caterpillars where eyespots may mimic the eyes of their enemies' predators. Despite this, the protective value of caterpillar eyespots remains unclear and has yet to receive empirical study. Moreover, most eyespot caterpillars are otherwise cryptically coloured, suggesting that the efficacy of eyespots may depend on additional aspects of prey coloration. We assessed the protective value of caterpillar eyespots with and without a form of countershading by pinning artificial caterpillars to tree branches in the field and monitoring their survival over 90 h. Eyespot and countershading treatments were arranged in a 2×2 factorial design, allowing us to quantify both main and interactive effects. We also photographed attacked prey to determine whether eyespots affected the area of the prey's body that was attacked (i.e. pecked by an avian predator). We observed no main effect of eyespots or countershading on survival; instead, countershading and eyespots acted synergistically to reduce predation, although whether this interactive effect reflects a countershading mechanism or different background matching when prey are viewed from above is unclear. Importantly, bird pecks were directed both towards eyespots and towards the body section furthest from the eyespots. This is the first field experiment to examine the protective value of eyespots on caterpillar-like prey and demonstrates that the extent of protection depends on additional factors, including body colour and possibly behaviour.

► We used pastry caterpillar prey to examine the protective value of eyespots and countershading.
► Treatments followed a 2×2 factorial design and were deployed in the field for 90 h.
► There was no significant main effect of the eyespots or countershading treatment.
► Eyespots and countershading acted synergistically to reduce predation.
► Pecks were directed both towards eyespots and towards the body section furthest from the eyespots.