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An Eye for Camouflage

9 January 2007 (All day)
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Monkey see?
This capuchin may not see every color in the rainbow, but he knows a good meal when he spots one. Susan Perry

Being colorblind can be a good thing. Researchers studying capuchin monkeys in the forests of Costa Rica have shown that colorblind individuals are better at detecting camouflaged insects than are those that see a wider spectrum of colors. The finding is the first evidence from the wild that colorblindness confers advantages during foraging.

Capuchin monkeys and other New World monkeys of Central and South America vary in their ability to see color. Some capuchins, for example, have dichromatic vision--or are red-green colorblind--and see the world in shades of blue and yellow, whereas others have trichromatic vision similar to that of humans, allowing them to distinguish red, orange, yellow, and green. Biologists have long thought that better color vision is, well, better, especially because primates ostensibly use color to determine the ripeness of fruit, for example. So why has colorblindness persisted in these populations?

The answer may lie in tasks that don't depend on seeing in color. Experimental evidence from both humans and captive monkeys shows that being colorblind is useful when trying to identify a camouflaged object, so Amanda Melin, a graduate student at the University of Calgary in Canada, and her colleagues set out to test this in wild monkeys. They spent 9 months following two groups of capuchins in Santa Rosa National Park in Costa Rica, observing the dietary habits of 34 monkeys.

Colorblind capuchins, Melin and her colleagues report in the January issue of Animal Behaviour, made approximately four more attempts per hour to capture camouflaged insects than trichromatic monkeys did. Presumably, they were spotting bugs that the others had overlooked. And this isn't just snacking; insects account for about a quarter of the capuchin diet, and almost half are camouflaged.

The findings make sense, says Nathaniel Dominy, a primatologist at the University of California, Santa Cruz: "If you reduce the amount of color information coming into the brain, your brain may be better able to detect shapes, contours, and contrasts." That may be why these color-challenged monkeys have managed to stick around in the gene pool, he says.

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