The Most Social Monkeys Have The Most Distinct Facial Features
November 20, 2013

The Most Social Monkeys Have The Most Distinct Facial Features

Brett Smith for - Your Universe Online

A new study of Old World monkeys, published in the journal Nature Communications, has suggested that they rely on facial features to recognize each other, particularly for those primates living in larger groups.

The new report comes from the same UCLA biologists that released a similar analysis of the faces of nearly 130 New World monkeys from Central and South America in 2012.

"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 study author Michael Alfaro, an associate professor of ecology and evolutionary biology at UCLA.

In the study, the researchers found that Old World monkeys and apes, from various regions of Africa and Asia, are more social and have more complex facial features than their New World counterparts. Primates that live in smaller group sizes tended to have less complex facial features with fewer colors than those that tend to congregate in bigger groups, the researchers found.

The study team said more color patches in the face results in greater possibilities for facial variation across individuals within species and this level of variation could assist in identification among members of a population. They added that species living near other closely-related species also tend to have more complex facial patterns, an indication that complex faces may also assist in species recognition.

"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."

Some Old World species, like the mandrills, can live in groups with up to 800 members. Conversely, adult male orangutans travel and sleep alone, with females accompanied only by their young. Other Old World populations have more complex social structures that can include sub-groups split up part of the time and coming together at others.

To reach their conclusion, the team analyzed photographs of the complex patterns on primate faces. Each face was divided into several regions – each of which was classified based on color. Each primate face was scored according to the total number of different colors across the different facial regions. The team then compared the complexity scores of primate faces in relation to the animals' social structure.

The researchers analyzed environmental variables for each primate, such as geographic location, canopy density and temperature. They also considered the evolutionary history and relationships among the different groups.

In addition to discovering that facial complexity was related to social variables, the researchers also found that facial pigmentation was related to ecological and spatial factors.

"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 study 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."

The team also found that Old World primates living in more tropical and more densely forested habitats tended to have darker faces.

"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 explained. "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.”