Biologists studied the faces of 129 adult male primates from Central and South America to answer the question: why are the faces of primates so dramatically different from one another? In the picture (left) faces of male primates from Central and South America. Faces of adult male primates from Central and South America. Warmer colors indicate higher complexity in facial color patterns. Species shown are: (1) Cacajao calvus, (2) Callicebus hoffmansi, (3) Ateles belzebuth, (4) Alouatta caraya, (5) Aotus trivirgatus, (6) Cebus nigritus, (7) Saimiri boliviensis, (8) Leontopithecus rosalia, (9) Callithrix kuhli, (10) Saguinus martinsi, and (11) Saguinus imperator. Some answers in a research published Jan. 11, in the early online edition of the journal Proceedings of the Royal Society Biological Sciences: Adaptive evolution of facial colour patterns in Neotropical primates written by Sharlene E. Santana (UCLA Center for Society and Genetics, University of California, Los Angeles), Jessica Lynch Alfaro (UCLA Center for Society and Genetics, University of California, Los Angeles), Michael E. Alfaro (Department of Ecology and Evolutionary Biology, University of California, Los Angeles).
The faces they studied evolved over at least 24 million years. 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.
Facial expressions, is the result, are increasingly important in large groups, then highly social,
The evolutionary biologists also found that when primates live in environment with more species that are closely related, their faces are more complex, regardless of their group size. This finding is consistent with their need to recognize individuals of other closely related species that live in the same habitat to avoid interbreeding, Santana said.
This work is the first quantitative evidence linking social behavior to the evolution of facial diversity and complexity in primates, and they also show that ecology controls aspects of facial patterns.
In the future, Santana, Lynch Alfaro and Alfaro may use computer facial-recognition software to help quantify the faces in a more sophisticated way. They also plan to study the faces of carnivores, including big cats.
Does the study have implications for the evolution of human faces? The findings do suggest - as UCLA Newsroom wrote (January 15, 2012: Evolution is written all over your face) - Alfaro said, that an important factor in shaping human faces is the premium on making unambiguous facial expressions.
"Humans don't have all these elaborate facial ornamentations - Alfaro said - but we do have the ability to communicate visually with facial expressions. Does reduced coloration complexity create a blank palate for visual expressions that can be conveyed more easily? That is an idea we are testing."
Santana's research is funded by fellowships from the National Science Foundation and UCLA's Institute for Society and Genetics.