Monkey Vision
Old World monkeys are trichromats, as are female New World monkeys. Most male New World monkeys are red-green colorblind.
Howler monkeys are an exception. Thanks to a duplicated gene on their X chromosomes, both female and male howlers see red. The reason is leaves. Howlers graze on leaves when fruit cannot be found. The monkeys prefer younger, more nutritious leaves. The reddish hue of new leaves makes them more pronounced with trichromatic vision: hence the adaptation, and trichromacy in Old World monkeys, where leaves are common fare.
Other New World monkeys usually go for insects instead of leaves when there’s no fruit, so dichromatic vision is a better lifestyle fit.
Color can impede ability to see patterns, borders, and textures. Insects hide and camouflage. ~ Canadian biological anthropologist Amanda Melin
