'Monkey Facebook': More Diversity Discovered in Faces of Social Primates

Primates can be surprisingly different in appearance--especially when it comes to their faces. Black, blue, red and yellow are mixed in different combinations. Noses and face-shape itself also shift between different species. Now, scientists have taken a closer look at primate faces, revealing a little bit more about the evolution of these species.

Last year, biologists reported on the evolution of 129 primate faces in species from Central and South America. But that excluded Old World African and Asian primate species. Now, scientists have taken a closer look at 139 of these species, which have been diversifying over 25 million years.

"Humans are crazy for Facebook, but our research suggests that primates have been relying on the fact 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," said Michael Alfaro, one of the researchers, in a news release. "Faces are really important to how monkeys and apes can tell one another apart. 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."

In fact, the scientists found that species that have smaller group sizes tend to have simpler faces with fewer colors. This is possibly due to the fact that more color patches in the face results in great potential for facial variation across individuals within species. This variation, in turn, could aid in identification, which may be a more difficult task in larger groups.

Most Old World monkeys and apes are social. In fact, some species, like the mandrills, can live in groups with up to 800 members. At the other end of this spectrum are orangutans; adult males often travel and sleep alone while the females are accompanied by their young.

"Our research suggests increasing group size puts more pressure on the evolution of coloration across different sub-regions of the face," said Alfaro in a news release.

The findings reveal a little bit more about the evolutionary pressures which may have led to the startling diversity in primate species. More specifically, they show exactly how these animals manage to select for particular features.

"Our research shows that being more or less social is a key explanation for the facial diversity that we see," said Alfaro in a news release. "Ecology is also important, such as camouflage and thermal regulation, but our research suggests that faces have evolved along with the diversity of social behaviors in primates, and that is the big cause of facial diversity."

The findings are published in the journal Nature Communications.