Monkeys and Humans See Differently, Experts Say

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Back in Britain, Smith devised his experiment to give the tamarins an eye-test. At the Belfast Zoological Garden in Ireland, he simulated the forest canopy on a wire scaffold decked with large green paper leaves that roughly matched the color of Abuta.

Among the leaves he scattered small cardboard boxes with lids whose color matched the shades of Abuta fruit from a raw green to a ripe orange. The "ripe" boxes concealed chunks of fudge; the less ripe, smaller chunks. "Unripe" boxes were empty.

Then the researchers guided male and female saddleback and red-bellied tamarins (Saguinus fuscicollis and Saguinus labiatus) into the enclosure, one by one, to forage for the "ripe fruit."

Trichromat tamarins, as it turned out, were 50 percent more adept at choosing the ripe fruit than their dichromat fellows.

Advantages of Two-Color Vision?

"(Smith's) findings are very strong," said Nathaniel Dominy, a primatologist at the University of Chicago in Illinois. "He is the first person to show that there is an advantage to trichromacy in New World monkeys—they are clearly better at finding and distinguishing ripe fruit."

In 2001 Dominy published his study of four species of Old World apes and monkeys in Uganda—chimps (Pan troglodytes), black-and-white colobus (Colobus guereza), red colobus (Piliocolobus badius), and red-tailed monkeys (Cercopithecus ascanius). Trichromatic color vision, Dominy determined, was essential for choosing tender young red leaves—more digestible and nutritious than the more mature green ones.

"Smith's work shows that it is definitely better to be a trichromat," said Gerald Jacobs, an expert on mammalian color vision at the University of California, Santa Barbara. "Dichromats have a real visual deficit and clearly don't do as well in the world."

For example, some forms of dichromacy entail less sensitivity to light, making the world appear dimmer.

A mystery for the scientists is why, if trichromacy is so advantageous, it hasn't replaced dichromacy altogether in the evolution of primates.

"There may be some unidentified advantages to being a dichromat," Dominy said.

Smith's preliminary results suggest that dichromats may be better at "breaking the camouflage of predators and prey," he said. New World monkeys, in addition to fruit, also consume large quantities of prey—katydids, frogs, and lizards.

"Perhaps dichromats are not as distracted by colors and better at seeing shapes and forms," Smith said.

Nature endows each way of seeing. Trichromats may be better at finding fruit; dichromats, at catching prey.

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