6533b832fe1ef96bd129a5d1

RESEARCH PRODUCT

Co-mimics have a mutualistic relationship despite unequal defences

subject

description

Defensive mimicry, where species have evolved to resemble others in order to evade predators, is quite common in the animal kingdom. The two extremes of the mimicry spectrum are known as 'batesian' and 'mullerian'. Batesian mimics develop signals — visual cues for instance — that are similar to those of species being mimicked, but stop short of adopting the attribute that makes it unprofitable prey to predators. Mullerian mimics both resemble the model species and share the anti-predation attribute — by being dangerous or unpalatable. These different types of mimic were identified a century ago, but the dynamics of mimicry between unequally defended prey remain unresolved. In an experiment designed as a direct test of the various competing theories, Rowland et al. demonstrate that both types of mimicry can be beneficial for both parties. The lab-based 'novel world' experiment pitted artificial prey (pieces of almond wrapped in paper) against real predators (great tits). Unequally defended mimics — including those that are edible — gained survival benefits from their association with one another. This goes against the commonly accepted idea that a moderately defended species might dilute the protection of a better defended one, so changes to the textbooks and a few Wikipedia entries may be in order. Although batesian and mullerian mimicry were identified 100 years ago, the dynamics of mimicry between unequally defended prey remain unresolved. This paper experimentally tests the contrasting theories, demonstrating that unequally defended (even edible) mimics gain survival benefits from their association with one another. In the first clear mathematical treatment of natural selection, Muller1 proposed that a shared warning signal (mimicry) would benefit defended prey species by sharing out the per capita mortality incurred during predator education. Although mimicry is a mainstay of adaptationist thinking, there has been repeated debate on whether there is a mutualistic or a parasitic relationship between unequally defended co-mimic species2,3,4,5. Here we show that the relationship between unequally defended species is mutualistic. We examined this in a ‘novel world’6 of artificial prey with wild predators (great tit, Parus major). We kept the abundance of a highly defended prey (‘model’) constant and increased the density of a moderately defended prey (‘defended mimic’) of either perfect or imperfect mimetic resemblance to the model. Both model and defended mimic showed a net benefit from a density-dependent decrease in their per capita mortality. Even when the effect of dilution through density was controlled for, defended mimics did not induce additional attacks on the model, but we found selection for accurate signal mimicry. In comparison, the addition of fully edible (batesian) mimics did increase additional attacks on the model, but as a result of dilution this resulted in no overall increase in per capita mortality. By ignoring the effects of density, current theories may have overestimated the parasitic costs imposed by less defended mimics on highly defended models.