Significance Eyespots are a widespread form of antipredator defense that have long captured the imagination of evolutionary biologists, geneticists, psychologists, and artists. These markings are particularly common within Lepidoptera, and eyespots on caterpillars have been shown to deter avian predators; however, why eyespots have evolved in particular caterpillar species, and why they are not even more widespread, remain unclear. Here we answer this question using a powerful three-pronged approach. Our phylogenetically controlled analysis of hawkmoths demonstrates that eyespots are typically restricted to large caterpillars, and our field and laboratory experiments provide an explanation for this. Eyespots are costly to small caterpillars because they enhance detectability without providing a protective advantage, but they are beneficial to large caterpillars because they deter predators.
Animals warn off potential predators through conspicuous, distinctive colors and patterns, called visual warning signals. These visual patterns have well understood effects on predators: they deliver unlearnt wariness and enhanced memorability. But it is not clear what specific image characteristics of such patterns cause these behavioural effects. Here we tackle the question of if, and why, specific spatial frequencies are pervasive in warning signals. We hypothesized that the dominant frequencies involved in animal warning signals may specifically stimulate predator visual systems at the typical distance at which they decide whether to attack or not. We first developed a database of hyperspectral images of Lepidoptera (butterfly and moth) wing patterns, from both species that utlise warning signal patterns and species that do not. We also built a generic mathematical model of the bird visual system, including model ‘neurons’ sensitive to different spatial orientations and spatial scales. We found that warning signals trigger a higher model activity, and do so maximally for a subpopulation of the model units sensitive to specific spatial frequencies. We found that at 8-12cm (the typical decision distance for some birds), the maximum model sensitivity corresponds to the peak of the bird contrast sensitivity function, namely 1 cpd. When viewing distance is taken into account, our findings show that patterns dominated by 1cpd spatial frequencies maximally stimulate a generic model of the early bird visual system. This suggests that such patterns might be specifically deterrent to birds predators Thus, some characteristics of animal warning signals may have evolved to deter predators at the distance where a decision is made.
The question, "Why should prey advertise their presence to predators using warning coloration?" has been asked for over 150 years. It is now widely acknowledged that defended prey use conspicuous or distinctive colors to advertise their toxicity to would-be predators: a defensive strategy known as aposematism. One of the main approaches to understanding the ecology and evolution of aposematism and mimicry (where species share the same color pattern) has been to study how naive predators learn to associate prey's visual signals with the noxious effects of their toxins. However, learning to associate a warning signal with a defense is only one aspect of what predators need to do to enable them to make adaptive foraging decisions when faced with aposematic prey and their mimics. The aim of our review is to promote the view that predators do not simply learn to avoid aposematic prey, but rather make adaptive decisions about both when to gather information about defended prey and when to include them in their diets. In doing so, we reveal what surprisingly little we know about what predators learn about aposematic prey and how they use that information when foraging. We highlight how a better understanding of predator cognition could advance theoretical and empirical work in the field.
Abstract Camouflage is the most common form of antipredator defense, and is a textbook example of natural selection. How animals’ appearances prevent detection or recognition is well studied, but the role of prey behavior has received much less attention. Here we report a series of experiments with twig-mimicking larvae of the American peppered moth Biston betularia that test the long-held view that prey have evolved postures that enhance their camouflage, and establish how food availability and ambient temperature affect these postures. We found that predators took longer to attack larvae that were resting in a twig-like posture than larvae resting flat against a branch. Larvae that were chilled or food restricted (manipulations intended to energetically stress larvae) adopted a less twig-like posture than larvae that were fed ad libitum. Our findings provide clear evidence that animals gain antipredator benefits from postural camouflage, and suggest that benefits may come at an energetic cost that animals are unwilling or unable to pay under some conditions.
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