New research suggests the most social monkeys also have the most colorful faces, and the different patterns and hues are used as a "primate Facebook."
A research team looked at the facial evolution of 139 Old World African and Asian primate species; the species are believed to have been diversifying for about 25 million years, a UCLA news release reported.
The team noticed the monkeys that socialized in smaller groups had simpler faces than primates that lived in larger communities. The researchers wondered in more facial colors and patterns could help the monkeys distinguish each other within their own groups.
"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," Michael Alfaro, an associate professor of ecology and evolutionary biology in the UCLA College of Letters and Science and senior author of the study, said.
"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."
The social habits of monkeys vary greatly across different species. Mandrills often live in large communities of up to 800 individuals but orangutans often venture through life alone. Hamadryas baboons have extremely advanced societies that include "harems, clans, bands and troops," according to Jessica Lynch Alfaro, an adjunct assistant professor in the UCLA Department of Anthropology and UCLA's Institute for Society and Genetics.
"Our research suggests increasing group size puts more pressure on the evolution of coloration across different sub-regions of the face," Michael Alfaro said. "This allows members of a species to have "more communication avenues, a greater repertoire of facial vocabulary, which is advantageous if you're interacting with many members of your species."
The researchers analyzed photographs of primate faces and gave each one a "facial complexity" score which took a number of factors into account including patches of color. They then compared these scores to the primates' social patterns.
The team also looked at the species' geographic location, evolution history, and climate, to see how it related to their findings.
"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," Lynch Alfaro said. "This is the same trend we see on an intra-species level for human skin pigmentation around the globe."
Primates living in densely forested regions also tended to be darker.
"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," Michael 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."
The team was surprised to find monkeys from Central and South America that lived in larger groups actually had simpler facial patterns.
"We expected to find similar trends across all primate radiations - that is, that the faces of highly social species would have more complex patterning," Lead study author Sharlene Santana, who conducted the research as a postdoctoral fellow with the UCLA Department of Ecology and Evolutionary Biology and UCLA's Institute for Society and Genetics and who is now an assistant professor at the University of Washington and curator of mammals at the Burke Museum of Natural History and Culture, said. "We were surprised by the results in our original study on neotropical (Central and South American) primates."
The researchers wondered if the apes were possible using their faces differently than primates from other regions.
"In the present study, great apes had significantly lower facial complexity compared to monkeys," Lynch Alfaro said. "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."
"Our research shows that being more or less social is a key explanation for the facial diversity that we see," Michael 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."
