“If you look at New World primates, you're immediately struck by the rich diversity of faces,” said Michael Alfaro, a UCLA associate professor of ecology and evolutionary biology and the senior author of the study. “You see bright red faces, moustaches, hair tufts and much more. There are unanswered questions about how faces evolve and what factors explain the evolution of facial features. We're very visually oriented, and we get a lot of information from the face.” Some of the primate species studied are solitary, while others live in groups that can include dozens or even hundreds of others.
The life scientists divided each face into 14 regions; coded the color of each part, including the hair and skin; studied the patterns and anatomy of the faces; and gave each a “facial complexity” score. They studied how the complexity of primate faces evolved over time and examined the primates' social systems. To assess how facial colors are related to physical environments, they analyzed environmental variables, using the longitude and latitude of primates' habitats as a proxy for sun exposure and temperature. They also used statistical methods to analyze the evolutionary history of the primate groups and when they diverged from one another. “We found very strong support for the idea that as species live in larger groups, their faces become more simple, more plain,” said lead author Sharlene Santana, a UCLA postdoctoral scholar in ecology and evolutionary biology and a postdoctoral fellow with UCLA's Institute for Society and Genetics. “We think that is related to their ability to communicate using facial expressions. A face that is more plain could allow the primate to convey expressions more easily.