Why are monkeys' faces all so bizarrely different?

New World Monkeys are the strangest-looking primates on Earth — and they all look nothing like each other, from the bald-headed, demon-like Uakari to the lion-maned golden marmoset to the massively mustachioed emperor tamarin up top. What's behind this insane variety?

That's the question UCLA researcher Michael Alfaro set out to answer that question. The monkeys of Central and South America represent a truly staggering amount of facial diversity, with many species like the emperor tamarin sporting truly epic facial hair. But it's unlikely that all these monkeys evolved such bizarre appearances just to amuse us — so what's really going on here?

Why are monkeys' faces all so bizarrely different?

Alfaro and his team realized the monkeys' faces weren't the only thing that had unusually strong variation. The social structure of the different species also varied greatly, with some living almost completely solitary existences while others lived in huge populations of a hundred or more. The team created a facial complexity score for each of the different monkey species and examined how all the different evolutionary trees of 129 separate species fit together, as you can see on the left.

They discovered that the monkeys with the most complex faces tended to live by themselves, while those who lived in groups tended to have plain faces. Another factor behind facial diversity seems to be the proximity of other species. When lots of different monkey species live in close quarters, they will tend to have much more complicated faces than more isolated species.

The researchers suspect plain faces evolved in the more social monkeys because they need to be able to communicate a wider range of concepts with their faces, and an overly ornate appearance would make that unnecessarily difficult. As lead author Sharlene Santana explains in a press release, this wasn't the result they expected, but they realize now how much sense it makes:

"Initially, we thought it might be the opposite. You might expect that in larger groups, faces would vary more and have more complex parts that would allow one individual to identify any member of that group. That is not what we found. Species that live in larger groups live in closer proximity to one another and tend to use facial expressions more than species in smaller groups that are more spread out. Being in closer proximity puts a stronger pressure on using facial expressions."

This finding also reveals some intriguing hints about human evolution. Our species, generally speaking, has quite simple, bare faces, and of course we've also evolved what is arguably the most sophisticated system of communication in our planet's history. Language itself might never have emerged if we were lion-maned or hugely-mustached or even polka-dotted — basically, anything that would have kept our ancestors from producing crisp, clear facial expressions.

Via Proceedings of the Royal Society B. Image of Emperor Tamarin by Richard G. Hawley on Flickr. Chart by Stephen Nash.