Species that are more social and thus live in larger groups have more complex facial patterns, UCLA biologists reported, because identification may be a more difficult task in larger groups, while species that have smaller group sizes tend to have simpler faces with fewer colors.
"Humans are crazy for Facebook, but our research suggests that primates have been relying on the face to tell friends from competitors for the last 50 million years and that social pressures have guided the evolution of the enormous diversity of faces we see across the group today," ecology and evolutionary biology Professor Michael Alfaro said.
The researchers studied faces of 139 Old World African and Asian primate species that have been diversifying over about 25 million years.
Some species have many different facial colors -- black, blue, red, orange and white -- in all kinds of combinations and often striking patterns while other primate faces are quite plain, they found.
"Faces are really important to how monkeys and apes can tell one another apart," Alfaro said. "We think the color patterns have to do both with the importance of telling individuals of your own species apart from closely related species and for social communication among members of the same species.
"Our research shows that being more or less social is a key explanation for the facial diversity that we see."