American Scientist

It’s hard not to stare at another primate’s face. Those expressive eyes, so often like our own, demand attention. But so do the striking differences, whether it is tomato-red skin, a snout-like nose or thick, long fur. Biologists at the University of California, Los Angeles, have concluded that sociality along with ecological pressures can be read in primate faces. In an article published in February in the Proceedings of the Royal Society B, they reported significant differences between Neotropical primates that live with many of their kind compared to those that live in relative isolation. In a written exchange, associate professor Michael Alfaro and postdoctoral scientist Sharlene E. Santana described their investigation with American Scientist senior editor Catherine Clabby.

What inspired your research into primate faces?

When you see the faces of primates, you see an extraordinary diversity of shapes, colors and patterns, so we wondered what were the factors behind this diversity. Social behaviors seemed to be a very likely candidate underlying the diversity of primate faces, so that drew us in to further explore how behaviors can shape the evolution of anatomy in these mammals. Neotropical primates were ideal to start our studies of facial diversity because they are a single evolutionary radiation spanning a wide variety of habitats and social systems, and they have an extraordinary variation in their facial features.

Are the faces of primates really that different from the faces of other mammals?

What is particular about primates is their high reliance on facial cues to interact socially, more so than many other mammals. Primates use characteristics of their faces and facial expressions to recognize individuals in their groups and to assess each other’s behaviors. Related to this, primates have evolved a very well-developed visual system and neural centers for facial recognition. Such an important role in communication has shaped the evolution of primate faces, along with ecological and physiological functions.

What did you observe regarding the ties between the appearance of the Neotropical animals and their social behavior?

Species that live in smaller groups have evolved more complex patterns of facial color. The more complex faces have also evolved in species that share their habitat with many other closely related species. Both of these findings indicate that Neotropical primates use facial color patterns to identify species.

And what did you observe about those that live among more of their own species?

We initially thought that more social species might have the most complex faces, in that more differently colored parts of the face would allow for discrimination among individuals. It turns out, however, that the most social species have the least complex faces. We think this may be because facial expressions are more easily conveyed on simpler faces.

What role does ecology play here?

The evolution of pigmentation and hair length across several areas of the face is linked to ecological factors. Some parts of the face have evolved to be darker in species that live in more humid and forested habitats, and the area around the eyes has evolved to be darker in species that live in more sunny environments. This darker area around the eyes may help reduce glare in sunny habitats, in a similar way as when baseball players put black paint under their eyes. Also, the hair around the face has evolved to be longer as species live farther from the equator, possibly to keep them warmer.

You based the analysis on photos of male monkeys. Why? And what did you compare?

For most species of Neotropical primates, there is no difference in the facial colors of male versus female monkeys, but we decided to describe the color patterns of adult males to standardize for sex and age across species. We compared what we call facial color pattern complexity, which is the number of different colors in the face, the darkness and hair length of various facial regions, including the crown of the head, the nose, the mouth, the eye mask and the face margin across 129 species of primates.

Do these findings help you understand human faces too?

Since Neotropical primates are distantly related to humans, all we can do for now is speculate about the conditions under which the faces of our ancestors could have evolved. From the perspective of our study, human faces are relatively bare and simple, so we would predict that these faces evolved in highly social groups while sharing the environment with few closely related species. It is also interesting to think about how humans across cultures have developed a great diversity of facial paintings to identify themselves as part of particular groups or tribes, which seems to parallel what other primates are doing by evolving complex facial color patterns when they share their habitat with many closely related species.

What research in this area are you pursuing now?

We are expanding our study on color patterns to the rest of primates and to carnivores. We want to see how their facial and coat patterns have evolved and what social and ecological factors could be driving their evolution.