A curious thing happens to white-faced capuchin monkeys when they anoint their bodies with mud and plant matter, a natural insect repellent: With their heads and faces slathered in goop, these highly social primates lose their ability to recognize each other. Previously friendly monkeys can become fighting foes.
This abrupt change in behavior hints at the importance of facial expressions for recognition, University of Washington evolutionary biologist Sharlene Santana says, and could help to explain why primate faces are so wildly divergent: Some species, like white-faced capuchins, have monotone hair and skin color; others, like the northern owl monkey, sport a dramatic mix of fur and flesh tones.
Biologists have long seen primates’ facial expressions during social interactions as clues that factors like group size drove the stark differences in their facial evolution. But there was little direct evidence to support the theories, so Santana decided to study a large number of monkey species, in a wide variety of social group sizes and environments, to see how their faces had evolved.
Santana found that the complexity of a species’ facial color patterns is tightly linked to certain social systems. Species that live in larger groups tend to have plainer faces than those living in smaller groups. Primates in large social groups most likely benefit from plain faces that allow for a greater range of expressions, she explains.
But among primates in smaller groups (and those that live near many different species, regardless of group size), more complex patterning makes it easier to recognize members of their own species, so they can focus their mating efforts and territorial disputes where it counts.
The connection between group size and facial evolution tells us something about our own ancient history too, since human faces are about as plain as it gets. “We can predict that the ancestors of humans were living in large groups, where facial expressions were really important,” Santana says.
She is currently investigating whether her results hold up among the Old World primates of Africa and Asia, a group that includes monkeys like baboons and mandrills and apes such as orangutans and chimps.