National Geographic On Assignment
Monkeys and their human cousins don't necessarily see the world the same way, according to new research from the Peruvian Amazon and a clever experiment from a lab in Scotland.
In fact, some monkeys, even within the same species, see things differently from one another. The research suggests that various forms of sight may confer a range of survival advantages.
"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 dichromatsthey 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.
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.
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