November 15, 2013

Monkeys: Not just a pretty face!

The face of a vervet monkey illustrating the procedure
used to characterize facial color pattern complexity.
Illustration by Sharlene E. Santana.
Monkeys have the most colorful faces of all mammals, and new research suggests that’s not just a pretty face.

Burke Curator of Mammals Dr. Sharlene Santana recently led a team of researchers to investigate the colorful facial variation in 139 species of Catharrines—Old World monkeys and apes from Africa and Asia.

The team analyzed hundreds of monkey "head-shots," photos from online databases, to compare the facial coloring of all 139 species. Researchers looked for similarities and differences across species, habitat, and social groups.

They found an interesting connection between monkeys’ facial color patterns and pigmentation and the social structure and environment where they live.

Primates living in larger (more social) groups and with other closely-related species have faces with more complex color patterns than those that live in smaller groups, possibly making it easier to tell each other apart.

They also found that the coloring of primate faces is related to their geographic location and environment. For example, species that live closer to the equator in dense, humid forests had darker faces than those who live in less-vegetated and dry areas further away from the equator. When living in a dark forest environment, darker face colors potentially help camouflage these primates so they go unnoticed by predators. Interestingly, only primate species living in Africa show these trends, potentially because environmental gradients are more accentuated in this continent.

A Red-eared guenon (Cerocopithecus erythrotis camerunensis).
This species lives in tropical forests of Nigeria and Cameroon and it
uses its colorful face during displays that allow individuals to recognize
each other and defend territories. Copyright: N Rowe.
This study builds on previous research by Dr. Santana and colleagues into the facial colors and complexities of New World monkeys. In that study they found that monkeys living in dark forests had darker colors on their faces compared to those that live in different environments, similar to the results for Old World monkeys and apes.

“Our study helps explain the evolution of two major aspects of primate facial diversity, color patterning and darkness. We demonstrate that a combination of behavioral and ecological pressures may underlie the evolution of outstanding anatomical diversity,” Dr. Santana said. “These types of complex interactions are likely operating in many other mammal groups.”

In an interesting twist, one aspect of the Old World primate study results is the opposite of previous findings about New World monkeys; New World monkeys that lived in smaller groups tended to have more complex color patterns than those who lived in larger groups.

The researchers are looking forward to further investigation into this topic, and hope to address what factors could be underlying these differences between New and Old World primates.

To read the full study, visit Nature Communications' website.