“Mimicry among unequally defended prey should be mutualistic when predators sample optimally”

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Thomas G. Aubier, Mathieu Joron, and Thomas N. Sherratt

Controversial quasi-Batesian mimicry among unequally defended prey should be rare if predators sample optimally

The unpalatable Heliconius numata butterfly, here shown sitting on a Heliconia flower, once considered a quasi-Batesian mimic because it occurs in several different cooccurring forms.
(Credit: Mathieu Joron)

Open almost any textbook on evolutionary biology and you will read about two forms of mimicry that students and professors continue to confuse. Batesian mimics are sheep in wolves clothing: palatable species that have evolved a resemblance to an unpalatable or otherwise defended species (the “model”) to gain protection from predators. By contrast, Müllerian mimics are wolves in wolves clothing: unpalatable species that have evolved a resemblance to other unpalatable species (“co-models”) to reduce the cost of educating naïve predators to avoid them. Batesian mimics are widely regarded as parasites, eroding the effectiveness of their model’s signals. Indeed, this parasitism can undermine the deterrent effect of the model’s appearance to such an extent that the mimic species sometimes evolve multiple morphs, with each morph resembling a different model. By contrast, Müllerian mimics are as mutualistic and reinforcing – the more unpalatable species with a given appearance, the more effective the signal.

Unfortunately however, life is not quite as simple as textbooks imply. In particular, species show wide variation in their level of defenses, so when a moderately unpalatable species resembles a highly unpalatable species, is it a “quasi-Batesian” mimic, eroding the effectiveness of the shared signal, or is it a classical Müllerian mimic? This question has been hotly debated for over a century, because it has long been recognized that Batesian and Müllerian mimicry may be on something of a continuum. Its answer is important because if moderately defended prey were parasites then it could help explain the puzzling cases of polymorphism observed in unpalatable species, such as that seen in Heliconius numata (left).

In this paper Thomas Aubier and his colleagues tackled the question head on, by identifying from first principles what a naïve predator should do if it encountered models and mimics with these characteristics and the predator acted in a way that maximized its payoff. After all, you would expect natural selection to come up with good solutions. Despite a great deal of analysis, their answer was relatively clear-cut. The more individuals of a given appearance there are, the more optimally sampling predators will be motivated to find out their properties. However, attacking a weakly defended mimic still represents a vote in favor of avoiding the prey type in the future. The net result is that moderately defended mimics should act as mutualists, decreasing the overall mortality of prey with this appearance, rather than parasites. While this means that researchers will have to look elsewhere to explain the puzzling examples of polymorphism in unpalatable species, it suggests that mimicry among prey with unequal defenses is generally mutualistic. Read the Article