Faces betray people's most secret feelings: scarlet with rage, pink with arousal, red with embarrassment. That's no accident, Caltech neurobiologists say. They suspect that the sophisticated color vision of primates might have evolved to reveal these emotional hues. They also contend that the trait could help explain why humans became the naked ape.
Mark Changizi and his research team found that the eyes of Old World primates (including baboons, gorillas, and humans) are fine-tuned for detecting increased blood-oxygen levels in the skin—a blush. This visual sensitivity helps the animals read the moods of their kin and their enemies, he speculates.
"But there's no good in having this special power to read emotions on primates' skin if you can't see the skin," Changizi says. Sure enough, he and his colleagues find that barefaced or bare-rumped monkeys have detailed color vision, while completely fur-covered primates do not. "There's a tremendous amount of rump signaling among Old World monkeys," he says. Changizi also speculates that humans may have lost their hairy hide in part because it makes more of the body useful for signaling emotions.