Why do butterflies have marginal eyespots?

Speckled Wood butterfly (Pararge aegeria) with eyespots. Credit, sannse.Scientists and artists alike have long been intrigued by the eyespots of butterfly wings. In the past few decades, we have learned that birds, too, are attracted to such decorations, often to the detriment of their predation success.

A recent study published in the Public Library of Science has deciphered the context-dependent message that the woodland brown butterfly, Lopinga achine, sends using its marginal eyespots.

Previous studies have confirmed that large dorsal eyespots can successfully deter bird predators through intimidation, but smaller, marginal eyespots have yet to demonstrate this ability.

Instead, it has been hypothesized that these marginal eyespots are intended to mimic the butterfly’s true head, focusing the predator’s attack on a less vital body part and allowing the butterfly a brief opportunity to escape.
 
Martin Olofsson and researchers from Stockholm University examined these small eyespots from an avian predator’s sensory point of view, rather than a human perspective alone.

Birds are able to see the world through short-wavelength and ultraviolet light, often foraging in the early morning. In this feeding window, butterflies are unable to escape by flight due to low ambient temperatures, and may capitalize on the increased brightness contrast of the eyespot pupils to deflect attackers.

To test this light-dependent defense tactic, the researchers presented a generalist bird predator, the blue tit, Cynanistes caeruleus, with woodland brown butterflies under varying light and UV conditions.

In high light intensities with UV present, all birds lethally attacked the head; however, holding UV intensity constant, lower light intensities instead provoked nearly all birds to misdirect their attacks at the eyespots.

Because birds are the major predator of butterflies, it is likely that these eyespots have evolved as a specific response to avian attack strategies.

Despite verifying the environmental context necessary for anti-predator signals in the woodland butterfly, the ecological dynamics of predator-prey have yet to be determined in the field.

Furthermore, if these highly coevolved signals are dependent on conditions such as light intensity, it will be important to consider how artificial light from urban areas may influence the fitness of such small but bright eyespots.

by Alexis Mychajliw

Olofsson, M., Vallin, A., Jakobsson, S., & Wiklund, C. (2010). Marginal Eyespots on Butterfly Wings Deflect Bird Attacks Under Low Light Intensities with UV Wavelengths PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010798