Evolution Is Written All Over Your Face
ScienceDaily (Jan. 11, 2012) — Why are the faces of primates so dramatically different from one another?
Primates from Central and South America. (Credit: Image courtesy of University of California - Los Angeles)
UCLA biologists working as "evolutionary detectives" studied the faces of 129 adult male primates from Central and South America, and they offer some answers in research published Jan. 11, in the early online edition of the journal Proceedings of the Royal Society B. The faces they studied evolved over at least 24 million years, they report.
"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.
"Humans have pretty bare faces, which may allow us to see facial expressions more easily than if, for example, we had many colors in our faces."
The researchers' finding that faces are more simple in larger groups came as a surprise.
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Adaptive evolution of facial colour patterns in Neotropical primates
Sharlene E. Santana1,2,*, Jessica Lynch Alfaro1 and Michael E. Alfaro2
1UCLA Center for Society and Genetics, University of California,Los Angeles, CA 90095, USA
2Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095, USA
*Author for correspondence (firstname.lastname@example.org).
The rich diversity of primate faces has interested naturalists for over a century. Researchers have long proposed that social behaviours have shaped the evolution of primate facial diversity. However, the primate face constitutes a unique structure where the diverse and potentially competing functions of communication, ecology and physiology intersect, and the major determinants of facial diversity remain poorly understood. Here, we provide the first evidence for an adaptive role of facial colour patterns and pigmentation within Neotropical primates. Consistent with the hypothesis that facial patterns function in communication and species recognition, we find that species living in smaller groups and in sympatry with a higher number of congener species have evolved more complex patterns of facial colour. The evolution of facial pigmentation and hair length is linked to ecological factors, and ecogeographical rules related to UV radiation and thermoregulation are met by some facial regions. Our results demonstrate the interaction of behavioural and ecological factors in shaping one of the most outstanding facial diversities of any mammalian lineage.
Received November 4, 2011.
Accepted December 20, 2011.
This journal is © 2012 The Royal Society