"As humans, we tend to think all creatures perceive the world the way we do, but that isn't the case," said Andrew Smith, a primatologist at the University of Stirling in Scotland.

For nearly a decade, Smith and his colleagues have ventured into the Peruvian Amazon to study how different types of sight affected the foraging behavior of New World monkeys called tamarins.

Humans have so-called trichromatic, or three-color, vision. So do Old World species such as chimpanzees, gorillas and orangutans. Trichromats have three types of light sensitive cells in the retina, fine-tuned to wavelengths that appear blue, green and red.

But New World monkeys have a broad range of vision types. Every howler monkey, for example, is trichromatic. The owl monkey is monochromatic, seeing only in black and white. Among tamarins and spider monkeys, all males are dichromats-they can't perceive reds or greens. But females split 60-40 between three- and two-color vision.

"You can have six individuals from the same species, even the same family, who see the world in six different ways," Smith said.

Tracking Tamarins

Like the one in 12 men who are colorblind, many New World monkeys have trouble discriminating between red and green, which can hamper the animals' ability to tell ripe fruit from raw.

Smith and his colleagues prowled the forests to follow the tamarins as they jumped from tree to tree high in the canopy. With a spectrometer, Smith measured the color of the fruit and the leaves on which the tamarins feed.

Tamarins eat the fruit of more than 833 plants from 167 different species. A favorite is the Abuta fluminum plant.

Ripe Abuta is orange, like other fruits that the tamarins like. But orange is hard to detect without red-green perception.

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.