The Scientist has a nice article about the evolution of trichromatic vision in primates: “The Rainbow Connection”. Trichromacy in anthropoid primates is a great example of many evolutionary mechanisms, and I discuss it every semester in my introductory course. New World monkeys have a different mechanism (allelic trichromacy) than catarrhines, and the appearance of the two kinds of trichromacy is a clear example of evolutionary convergence.
The linked article discusses some interesting consequences of allelic trichromacy, which leaves most New World monkey populations polymorphic for color vision – a mix of heterozygous individuals who are trichromats with homozygous individuals who are dichromats:
Amanda Melin of Washington University in St. Louis has spent years traveling to the forests of Costa Rica to observe capuchin monkeys in their natural habitat. Like squirrel monkeys, these New World primates have dichromatic males and either dichromatic or trichromatic females. Melin spends days at a time watching the animals forage, walking kilometers through the forest as the monkeys move from tree to tree, and collecting DNA from fecal samples, in an effort to determine which colors each animal can see. Much to her surprise, she’s found that fruit feeding rates between dichromats and trichromats are the same, initially suggesting that color vision doesn’t offer an advantage for foraging.
Digging deeper into the data, however, Melin uncovered a subtler effect. “Where we see the difference is in accuracy,” she says. “Trichromats are making way fewer mistakes, but foraging at a more leisurely pace.” Dichromats, on the other hand, appear more frantic, touching, sniffing, and biting more fruits, including unripe or inedible ones. The question Melin is trying to answer now is whether that sloppier foraging behavior has any nutritional impact on the animals.
I also love the accompanying infographic, which emphasizes that many kinds of vertebrates have even better vision than anthropoid primates. Everything from goldfish to chickens see four colors, with sensitivity into the ultraviolet!