Why Monkeys and Apes Have Colorful Faces
Species that live in larger social groups have complex, colorful facial patterns.
One face, two face, red face, blue face -- the palette of primate faces is rich and varied, and a new study explains why.
For Old World monkeys and apes, species that live in larger social groups have complex, colorful facial patterns, whereas those that live in smaller groups have simpler, plainer faces, the study researchers found. Facial diversity might make it easier to identify individuals in bigger groups, scientists say.
"Faces are really important to how monkeys and apes can tell one another apart," study researcher Michael Alfaro, an evolutionary biologist at UCLA, said in a statement, adding, "We think the color patterns have to do both with the importance of telling individuals of your own species apart from closely related species and for social communication among members of the same species."
Old World monkeys and apes, which are native to Africa and Asia today, have diverse social structures. Mandrills, for example, live in groups of up to 800 individuals. Other species are much more solitary - orangutan males travel and sleep alone, and females live only with their young. Still others, such as chimpanzees, have "fission-fusion" societies, living in small groups and occasionally convening in very large groups. And hamadryas baboons have complex hierarchies involving harems, clans, bands and troops. [Image Gallery: Snapshots of Unique Ape Faces]
In the study, Sharlene Santana, a postdoctoral fellow in Alfaro's lab, developed a way to quantify the complexity of primate faces in photographs. Santana divided the face into different parts, classified the color of each part (including the hair and skin), and scored the face based on the total number of different colors.
To see whether the animals' habitat influenced facial complexity, the researchers considered geographic location, tree canopy thickness, rainfall and temperature. The team also took into account evolutionary relationships among the primate species.
Facial complexity corresponded to the group size and number of closely related species in the same habitat, whereas facial pigmentation - how light or dark the face is - was better explained by geographic factors, the results showed. The findings were detailed online Nov. 11 in the journal Nature Communications.
In human society, Facebook provides a way of keeping track of one's friends. "Humans are crazy for Facebook, but our research suggests that primates have been relying on the face to tell friends from competitors for the last 50 million years and that social pressures have guided the evolution of the enormous diversity of faces we see across the group today," Alfaro said.
The team also found that in Africa, Old World monkeys and apes with darker faces lived nearest to the equator and those with lighter faces lived farther away. Primate species living in more tropical, forested areas also had darker faces, they said.
But facial complexity was not related to geographic location or habitat. Instead, complexity appeared to depend on the size of the social group: Species that formed larger groups had more diverse faces.
In a previous study, the researchers found the opposite pattern among primates from Central and South America: New World monkeys that lived in larger groups had simpler facial patterns.
"Our research suggests that faces have evolved along with the diversity of social behaviors in primates, and that is the big cause of facial diversity," Alfaro said.
Original article on LiveScience.
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Nonhuman primates living in large social groups may use facial complexity to tell each other apart.
The most comprehensive guide to primates, released this week, names the "five funniest faces" in the monkey world. The massive guide, "Handbook of the Mammals of the World" (Lynx Edicions, 2013), contains information on the 16 families, 77 genera, 479 species and 681 taxa of primates.
Number one on the book's list of funniest faced monkeys is the emperor tamarin. Editors Russell Mittermeier, Anthony Rylands and Don Wilson mention that this monkey was "named after Emperor Franz Joseph of Austria." Members of the species "have a white curved 'mustache' hanging down as far as their chest." The mustache is thought to aid in visual communication.
"Proboscis" is the scientific term for certain mammals' noses. The shnoz on the proboscis monkey is certainly hard to miss. Noses on males can exceed 4 inches in length, hanging lower than the mouth.
Mittermeier, Rylands and Wilson explained that "their long nose is used as a resonating chamber for its loud honking calls."
"Tarsiers must have been the inspiration for Yoda in Star Wars," according to the editors. "They have huge forward facing eyes, each larger than their brain."
These nocturnal, insect-eating primates need such eyes to locate prey and other things in the dark. They live on islands of Southeast Asia.
The researchers mention that bearded sakis "display an extraordinary beard and bouffant hairstyle." Five species have this unique look, all classified in the genus Chiropotes. The one shown here is Uta Hick's bearded saki, native to Brazil.
Black-and-white colobus monkeys "always look disgruntled," according to the editors. That's just the way they look, although they have plenty to be perturbed about.
These plant-eaters are prey for many forest predators, including humans. They have also suffered due to logging practices and habitat destruction. This individual appears to have been captured in a particularly non-cranky moment, nibbling on a leaf.