THE COST OF BAD TASTE

Abstract

Some butterflies have evolved a special defence against predators –they taste bad. And they want predators to recognize them as bad-tasting, so they're brightly coloured and fly slowly and steadily to show off their colours. Between the colouration and slow flight, predators can spot them easily, so they're easy to catch. But once predators learn to associate bright colours and slow flight with bad taste, they avoid these butterflies. In evolutionary terms, the colouration and flight pattern is an `honest signal':it has both costs and benefits. Unpalatable butterflies pay the cost of being easy to spot but gain the benefit of predators learning to avoid them.It seems that there might be a loophole in this arrangement. If good-tasting butterflies mimic the same bright coloration and flight pattern as bad-tasting ones, then the mimics get the benefits without the costs. For some reason, however, there are very few of these brightly coloured tasty butterflies, called Batesian mimics, and Robert Srygley explores possible reasons why in his recent paper.Srygley wondered whether flight pattern might help explain the lack of Batesian mimics. He had a hunch that the slow steady flight used by both the mimics and the bad-tasting butterflies might take a lot of energy, which would make it less beneficial to bother being a mimic. The quick and erratic way normal palatable butterflies fly might be easier, possibly explaining why so few butterflies have gone down the Batesian route. And as they are so rare, it wasn't clear before he started his study whether they fly erratically, like the good-tasting butterflies they are, or slowly, like the bad-tasting butterflies they mimic.Srygley filmed the flight patterns of three orange tiger-striped unpalatable butterflies, three Batesian mimics modelled on the orange tiger-striped butterflies, and two species of tasty green butterflies closely related to the mimics. Would the Batesian mimics fly slowly like the foul orange tiger-striped butterflies or erratically like the tasty green butterflies? For the first time, he demonstrated that Batesian mimics do fly in the same way as the unpalatable butterflies that they are modelled on: not only did they both have similar wing beat speeds but they also both flew significantly slower than the green palatable butterflies.From these films, he also estimated the power each butterfly uses to fly at ordinary speeds of ∼1 m s–1 using a relatively simple mathematical theory. Because of the flight pattern differences, both the mimics and the distasteful butterflies that they mimic used significantly more power to fly than the tasty green ones. Interestingly, the mimics also used substantially more power than the conspicuous orange tiger-striped butterflies. Thus, it was most costly to be a mimic, least costly to fly in a`normal' fast way and in-between to fly slowly like the bad-tasting conspicuous butterflies.Srygley hypothesized that Batesian mimics try to do too much – they fly slowly like unpleasant tasting butterflies when they aren't threatened but fly erratically like palatable butterflies when they're pursued. As a result,they aren't very good at either and don't get much benefit from their mimicry,which may be why there aren't many Batesian mimics.