Old world monkeys have trichromatic, full color vision, while new world monkeys have dichromatic, colorblind vision. Research suggests that the evolution of a trichromatic trait may have become fixed in old world monkey species because it provides an advantage in detecting ripe fruit in trees. Dichromats can have difficulties seeing differences in ripe and unripe fruit, especially when it requires the ability to distinguish between the red and green. The ability of trichromats to see three colors makes the palatable ripe fruit easier to pick out against a backdrop of green leaves.
The Food Selection Hypothesis: A Test
Do trichromats gain a selective advantage through increased foraging efficiency? If so, then we would predict that they would perform better in color-related tasks than their dichromatic counterparts. Conversely, in tasks where color discrimination is not important, we would predict that dichromats would perform equally as well, or better, than trichromats.
To see whether or not color-based selection was occurring, Smith et al (2003) tested trichromat and dichromat foraging ability when presented ripe and unripe fruit hidden amongst green leaves. In their experiment, trichromatic monkeys selected the ripe fruit about 53% of the time whereas dichromatic monkeys did so about 37% of the time. This study supports the hypothesis that, in some situations, trichromatic vision gives an advantage for ripe-fruit foraging.
In another experiment, Caine and Mundy (2000) had trichromat and dichromat monkeys forage for colored pieces of cereal scattered and hidden in different colors of shrubbery and grass.
They found that trichromats found significantly more orange pieces of cereal in a green environment where color distinction was difficult for dichromats. However, they also found that dichromats found more green pieces of cereal in the green environment. Although trichromats had an advantage in finding food in some cases, the effect was somewhat dependent on the environment. When the color of the food matched the surroundings, dichromats actually performed better than trichromats.
Some experiments suggest that there are times when trichromatic vision may be disadvantageous to foraging efficiency. In a study by Saito et al (2005) dichromat monkeys were significantly more efficient than trichromat monkeys in picking out shapes that were camouflaged in red and green to match a red and green background. Trichromat monkeys were distracted by the contrasting colors in the camouflage pattern, but to dichromats the camouflage looked like one single color. This apparently allowed the dichromats to pick out the shapes more easily.
The evidence seems to indicate that trichromat monkeys have a foraging advantage when it comes to choosing ripe fruit over unripe fruit. They also seem more able to pick out non-green fruit amongst green leaves. Dichromat monkeys seem to have a foraging advantage when it comes to choosing green food amongst green leaves. They also tend to have more rod cells than trichromat monkeys that may make them more efficient foragers in low light conditions.