Thursday, 20 August 2009

Scary peacock butterflies

Butterflies have high predation rates by birds. I am sure most of you have come across a butterfly with damaged wings suggestive of a beak 'bite' mark. Long-lived butterfly species often rely on camouflage (crypsis) to avoid being attacked in the first place. The Comma, the Small Tortoiseshell and the Peacock are some of these, mimicking shriveled leaves. They remain immobile if resting, with their wings closed. The Peacock (Inachis io) has a second defense mechanism. If discovered, they suddenly flash their wings open, exposing four large eyespots they also may flick their wings repeatedly and at the same time they make a hissing noise and a series of inaudible clicks by rubbing two wing veins together, during this display, they continually adjust the tilt of their bodies to face the potential attacker.
This Peacock was feeding upside down on a Buddleia showing its eyespots
 This intimidating display was described over a century ago and its effect on predators noted, but experimental support for its effects on the survival of the butterflies themselves was lacking until very recently. Adrian Vallin and collaborators tested the effect of eyespots, stridulation and both combined by modifying captive-reared peacock butterflies experimentally. They removed the eyespots by painting over them with a black marker pen and removed the stridulation ability of the butterflies by cutting out the veins responsible for making the hissing noise. They also tested the effect of the combined factors, that is removing both the eyespots and the noise-making wing veins. Unmodified controls, and controls in which a similar area of wing was painted black without touching the eyespots or cut without affecting the ability to make noise were also tested (bottom row below).

The researchers recorded the effect these various treatments had on survival of peacocks upon exposure to a potential predator, blue tits (Cyanistes caeruleus), a small insectivorous bird. The results show conclusively that butterflies without eyespots are more likely to be predated by the blue tits. All except one butterfly with intact eyespots survived by scaring the blue tit away, showing how effective the eyespots are in intimidating the bird. The effects of the sound or the combined effect were not significant. The predated butterflies were readily eaten by the birds, indicating that the Peacock is not distasteful and supporting the view that, in the words of the researchers, 'a harmless prey can increase its fitness by survival through the adoption of intimidation by bluffing'.
Vallin, A., S. Jakobsson, J. Lind & C. Wiklund (2005) Prey survival by predator intimidation: an experimental study of peacock butterfly defence against blue tits. Proc. R. Soc. B. 272:1203-1207.

1 comment:

Antje said...

That's fascinating. :-) Thank you for sharing, that's always a very interesting read.