Monkey Mustaches Reveal Evolution of Facial Hair

Mustaches and eyebrows help certain monkeys recognize each other, research finds.

Distinctive facial hair, like mustaches and eyebrows, are not unique to people -- those traits also help certain monkeys identify each other.

Mustaches, beards, bushy eyebrows and other can't-miss facial hair might have first evolved in primates to help them easily recognize each other in forest environments, according to new research in the latest issue of the Proceedings of the Royal Society B.

The study shows how much primates rely on vision in their social networks.

"Primates are a visual group that have lots and lots of face-to-face interactions," co-author James Higham told Discovery News. "The evolutionary history of primates is one of increasing reliance on vision at the expense of olfaction, which is the sense that is used more ancestrally among mammals."

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For the study, Higham, an assistant professor of anthropology at New York University, and co-author William Allen studied guenon monkeys, which sport all sorts of quirky facial hair, from ear tufts to beards and bushy eyebrows. Both male and female guenon monkeys have these features, to the point that humans looking at them cannot always tell which monkey is a male and which is a female.

The researchers designed a computer algorithm that could assess 500 photographs of 12 species of guenons. The images were collected in various settings, including at U.S. and U.K. zoos and at a wildlife sanctuary in Nigeria. The photos were of both male and female guenons.

"We sought to test a computer's ability to do something close to what a guenon viewing other guenons' faces would do," explained Allen, who is now a post-doctoral researcher at the University of Hull.

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The computer succeeded in correctly identifying individuals and their species, but like humans looking at the images, it too was stumped at trying to figure out if the photos were of males or females. It also could not categorize the photos by the age of the monkeys.

There seems to have been selection for a role for facial appearance in species recognition, Higham said. For age and sex, he speculated that "perhaps if you live in a stable social group, and you have individual recognition, then you can just learn those other individuals and whether they are old or young, or male or female, and so do not need this to be encoded in their facial appearance."

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This makes sense for guenons, which often live high in rainforest canopies with many closely related species. For breeding and other social purposes, they need to quickly identify their own kind.

The distant ancestors of humans were also tree dwellers that likely lived close to similar species. A growing body of evidence even supports that more recent members of our genus co-existed, such as Neanderthals and early Homo sapiens sharing some of the same territories.

While guenon monkeys, owl monkeys and some other primates have males and females that look nearly identical, it is usually very easy to distinguish a human man from a woman with just a glance. Facial hair in our case takes on a sexier function.

"In humans, mustaches and beards are what we call a 'sexually dimorphic' trait, i.e. they are present in one sex and not the other," Higham said. "This suggests that they are a sexually-selected trait, and perhaps play a role in mate choice or in competition between individuals of the same sex."

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Facial hair might also serve other functions, such as offering extra protection to sensitive areas of the face (around the lips and above the eyes, for example), but that aspect has yet to be fully confirmed.

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Martin Stevens, an associate professor of sensory and evolutionary ecology at the University of Exeter, described the new study as "terrific," adding that "the methods used are highly innovative and novel."

Allen and Higham believe that the computer technique could be used to analyze the facial traits of all sorts of other animals, helping to determine the function of these traits and why and how they evolved in the first place.