Forget Facebook, Primates Use Facial Variety to Distinguish Among Themselves

University of California - Los Angeles researchers report Old World African and Asian primate species are more social and have more complex facial patterns than their counterparts in Central and South America. The primates also exhibit greater variety in facial features and coloration, which the researchers suggest is something the primates have been relying on to tell one another apart for the past 50 million years.

The researchers also observed that the more social the primates species is, the more complex the group's facial patterns will be. Species with smaller group size tended to have more plain facial features, perhaps because the presence of color patches on the face leads to a greater potential for facial variation across individuals within a species.

"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," said Michael Alfaro, an associate professor of ecology and evolutionary biology in the UCLA College of Letters and Science and senior author of the research, which is published in the journal Nature Communications.

"Faces are really important to how monkeys and apes can tell one another apart," he 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."

Having unique, identifiable facial features can be especially important for primates that live among large groups, such as the mandrill, an Old World monkey that lives in troops of as many as 800.

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

Interestingly, the habitat where the primates live may also play a role in their facial coloration.

"Our map shows clearly the geographic trend in Africa of primate faces getting darker nearer to the equator and lighter as we move farther away from the equator," said co-author Jessica Lynch Alfaro, an adjunct assistant professor in the UCLA Department of Anthropology. "This is the same trend we see on an intra-species level for human skin pigmentation around the globe."

Primates living in more tropical and more densely-forested areas tended to have darker, more pigmented, faces. But the researchers note that the complexity of facial pigmentation is not related to habitat type.

"We found that for African primates, faces tend to be light or dark depending on how open or closed the habitat is and on how much light the habitat receives," Alfaro said. "We also found that no matter where you live, if your species has a large social group, then your face tends to be more complex. It will tend to be darker and more complex if you're in a closed habitat in a large social group, and it will tend to be lighter and more complex if you're in an open habitat with a large social group. Darkness or lightness is explained by geography and habitat type. Facial complexity is better explained by the size of your social group."

Lynch Alfaro said that the great apes they studied had significantly lower facial complexity compared to monkeys.

"This may be because apes are using their faces for highly complex facial expressions and these expressions would be obscured by more complex facial color patterns. There may be competing pressures for and against facial pattern complexity in large groups, and different lineages may solve this problem in different ways," she said.

Alfaro suggested there is a strong correlation between facial diversity and how social the primates are.

"Our research shows that being more or less social is a key explanation for the facial diversity that we see," Alfaro said. "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."