For centuries, researchers have thought that butterflies with eye-like patterns on their wings mimic the menacing gaze of predators in order to scare away insect-eaters. However, this idea has been untested this entire time. Maybe these eyespots ward off would-be attackers by simply being conspicuous and startling? Now, researchers studying tropical butterflies reveal that the eyespots do, in fact, successfully scare off small songbirds looking for a tasty butterfly meal. The findings were published in Proceedings of the Royal Society B this week.
In what’s called Batesian mimicry, harmless species acquire some protection from enemies and predators by resembling more threatening animals. For example, there’s a group known as owl butterflies (genus Caligo) who have eyespots resembling owl eyes, at least to people. Similar visual markings can be found on fish, mollusks, amphibians, and birds as well, though to a lesser extent.
To test the eye-mimicry hypothesis against the conspicuousness one, a team led by Johanna Mappes from the University of Jyväskylä recreated the encounters between predator and prey in a controlled setting. For their avian model, they used small, wild-caught songbirds called great tits (Parus major) who have never been exposed to these butterfly eyespot patterns before, and hence have no bias.
The team used a computer monitor to show the great tits images of Caligo martia butterflies—with naturally contrasting pairs of black “pupils” and yellow “irises”—as well as butterflies with digitally removed eyespots and butterflies with eyespots that had been manipulated to display equal contrast but reduced resemblance to an eye. That is, they reversed the colors. As a control, the researchers also intimidated the birds using images of the face of a pygmy owl (Glaucidium passerinum), both with and without eyes. These common owls are important predators known to elicit strong aversion in great tits.
In the image above, you can see the five different treatments: owl without eyes (OW), owl with eyes (OE), butterfly with real eyespots (BR), butterfly with modified eyespots (BM), and butterfly without eyespots (BW). The team recorded the birds’ reactions, which were classified as no response, stare, explore, startle, or flee.
After analyzing 179 observations, the team discovered that the natural, mimetic eyespots were as effective at scaring the birds as the real eyes of owls. These birds flew away or chirped a warning call. Furthermore, natural eyespots were far more efficient in eliciting an aversive response than manipulated, less mimetic eyespots. Butterflies with their eyespots digitally removed hardly startled the birds at all.
The great tits were all released back to the wilds of central Finland after the experiment.
Images: shutterstock.com (top), S. De Bona et al., Royal Society 2015 (middle)