Butterfly 'blinks' scare predatory birds

18 June 2005

BLUFFING, the oldest trick in the poker-player's book, comes in rather handy in the natural world, too.

The peacock butterfly, a perfectly tasty snack for a bird, often cheats death with what is nothing more than a bluff. Suddenly exposing the prominent eyespots on the butterfly's wings is enough to startle a foraging bird and save the insect's life, says Adrian Vallin, a zoologist at Stockholm University, Sweden.

Though butterfly eyespots have long been assumed to be an anti-predator defence, there has been little experimental evidence to support this. Vallin and colleagues tested the idea by blacking out the eyespots of peacock butterflies (Inachis io) with a marker pen, and putting them at the mercy of blue tit predators. They found that while 13 out of 20 butterflies treated this way were eaten by the birds, only one out of 34 butterflies with intact eyespots succumbed (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2004.3034).

The big question is why birds don't learn to recognise a harmless little butterfly. It is likely that the consequences of calling its bluff are too dire, says Graeme Ruxton, an ecologist at the University of Glasgow, UK. It might mean being eaten by a predator such as an owl. "Blue tits are generalists, and tend to err on the side of caution," he says.

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