For two hundred years, scientists have believed that the eyespots of butterflies and moths evolved to look like large eyes in order to frighten off predators. A bird might think that the bright eyespots are the eyes of a concealed cat, for example.
It sounds logical, but there is a hidden assumption: We are assuming that a predator such as a bird pays attention to the same features that we would. But does it?
Cambridge behavioral ecologist Martin Stevens and his team decided to test the longstanding assumption: They nailed paper moths to trees in Cambridgeshire, with a mealworm stuck to each one, to attract birds.
Some of the paper moths also had bright spots that looked like eyes, but others had bright spots such as bars and squares that did not look like eyes.
The researchers reasoned that if the longstanding assumption was correct, then birds such as blackbirds, and house sparrows would avoid the moths whose spots looked most like big eyes.
But that is not what happened.
Birds came to eat the "moths" at the same rate, whether their spots looked like eyes (at least to a human) or not.
However, paper moths that had lots of spots were attacked 30% less often than others. Also large spots were more effective than small ones.
The researchers concluded that the theory that eyespots evolved to look like eyes has no experimental support. Rather, the spots deter birds just by being colourful and conspicuous.
Dr. Stevens offers a suggestion as to why conspicuous spots deter predators: They suggest that the insect might be poisonous. He told New Scientist, "Predators tend to stay away from highly conspicuous prey, possibly because most conspicuous objects in nature are toxic," says Stevens. "We think this is the primary eyespot effect."
He does not rule out the idea that some eyespots evolved to look like eyes. He offers the hawkmoth caterpillar, whose eyes may look like snakes, as an example.
See also
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Butterfly "stare"
doesn't intimidate birds (
New Scientist, March 8, 2008)
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Insect "eyespots"
don't mimic eyes, study says" by Anne Casselman (
National Geographic News, February 22, 2008)
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"Zoologists
Challenge Longstanding Theory That 'Eyespots' Mimic The Eyes Of Predators' Enemies" ScienceDaily (Feb. 28, 2008)
Journal reference: Conspicuousness, not eye mimicry, makes ‘‘eyespots’’ effective antipredator signals (Martin Stevens, Chloe J. Hardman, and Claire L. Stubbins) Behavioral Ecology doi:10.1093/beheco/arm162
Abstract: Many animals bear colors and patterns to reduce the risk of predation from visually hunting predators, including warning colors, camouflage, and mimicry. In addition, various species possess paired circular features often called "eyespots," which may intimidate or startle predators preventing or postponing an attack. Most explanations for how eyespots work assert that they mimic the eyes of the predators own enemies. However, recent work has indicated that spots may reduce the risk of predation based purely on how conspicuous they are to a predator's visual system. Here, we use a field technique involving artificial prey marked with stimuli of various shapes, numbers, and sizes, presented to avian predators in the field, to distinguish between the eye mimicry and conspicuousness theories. In 3 experiments, we find that the features which make effective antipredator wing markings are large size and higher numbers of spots. Stimuli with circles survived no better than those marked with other conspicuous shapes such as bars, and changing the spatial construction of the spots to increase the level of eye mimicry had no effect on the protective value of the spots. These experiments support other recent work indicating that conspicuousness, and not eye mimicry, is important in promoting avoidance behavior in predators and that eyespots on real animals need not necessarily, as most accounts claim, mimic the eyes of other animals.
Key words: antipredator, conspicuousness, eyespots, mimicry, predation, vision.
(Note: The image of a, common Buckeye found at Toronto Island, is from the Government of Canada's online listing of the butterflies of Canada. The insect is pictured with both wing faces.)