Image: Rollie, an Emperor Tamarin monkey
Greg Neise  /  AP / Lincoln Park Zoo
Researchers say the characteristic white mustache of an emperor tamarin monkey may help members of the same species recognize each other.
By Senior writer
updated 1/11/2012 12:30:01 AM ET 2012-01-11T05:30:01

Monkeys in Central and South America have amazingly diverse faces, from the white-mustached visage of the emperor tamarin to the boiled-lobster-red skin of the bald uakari. Now, research reveals that New World monkeys rely on facial coloration mostly to recognize their own species.

"We found that species that live in smaller groups have evolved more complex patterns of facial color," study researcher Sharlene Santana wrote in an email to LiveScience. "This (more complex faces) is also true for species that share their habitat with many other closely related species."

In other words, the less likely a monkey is to run into one of its own, the more complicated its facial patterns. For example, bald uakaris have a striking, but simple, pure red face. These monkeys live in large communities that can number up to 100 individuals. In contrast, the three-striped night monkey, or owl monkey, lives in small family groups with mates and offspring only. These nocturnal monkeys sport masklike white-eye markings against a background of black fur, a much more complicated pattern.

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For monkeys who live in small groups, complex patterns may offer more ways to differentiate between species, which is important given how rare it is for one of these monkeys to meet one of its own. For monkeys living in larger groups, simple facial patterns might allow for better recognition of individuals and better communication via facial expressions — an important skill in a big community. [Gallery: Monkey Mug Shots]

The evolution of a face
While biologists have long suspected that facial colors arise due to some evolutionary pressures, Santana's research is the first to pin down the specific culprits.

She and her colleagues gathered monkey "mug shots" of 129 New World species (monkeys native to Central and South America) and ranked them on color pattern complexity, skin pigmentation, and length and color of facial hair. They then analyzed the patterns in terms of the social life and ecology of each species, accounting for evolutionary relationships that would lead to similarities in patterns between close relatives.

They found that habitat puts pressure on facial evolution, such that species that live in the dark, humid forests of the Amazon have darker "beards" and darker hair near the crowns of their heads, a feature that likely helps them blend in with their surroundings and elude predators. Species that live in areas with lots of ultraviolet radiation from the sun have darker eye masks, perhaps to keep down glare. Monkeys that make their homes far from the equator, in chillier climates, have longer hair on the faces, probably for warmth.

Craziest faces
The researchers also unraveled the evolutionary pressures of social interactions, finding that species made up of more isolated individuals had the most elaborate facial patterns. Some particularly fancy monkeys included the black-capped squirrel monkey, with its black muzzle and white eye mask, and the Kuhl's marmoset, which features long tufts of black hair emanating from its ears.

These quirky facial-color patterns make these monkeys stand out from other species, and in that way they allow their kin to easily spot them in the wild. Species that live in larger groups of 100 or more monkeys likely have a greater need to recognize individual monkeys; the researchers speculate that perhaps rather than color patterns, the monkeys check out facial expressions or the sizes of various facial features to particular individuals. That might be easier to do with simpler facial features.

The researchers plan to extend the facial complexity study to other primates and other mammals to see if the connection between social behavior and faces holds up. They also hope to learn more about how primates recognize faces within their own species.

"Our results shed some light about the cues primates may use to recognize the species an individual belongs to," Santana said, "but more research is needed to further link this finding to how primates use facial characteristics to recognize particular individuals."

You can follow LiveScience  senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience  and on Facebook.

© 2012 LiveScience.com. All rights reserved.

Explainer: Top 10 oddballs of the animal world

  • Image: Platypus
    Healesville Sanctuary

    From the outside, the platypus looks like a grade-school art project assembled by a kid too busy making spitballs to pay attention in class. The creature, which is classified as a mammal, has a duck's bill and webbed feet, lays eggs like a reptile, but has fur and rears its young on milk.

    Researchers say the platypus genome is equally cobbled together from bird, reptile and mammalian lineages. One more oddity: Males can deliver venom from tiny spurs on each hind limb. Click on the "Next" arrow above to learn about nine more oddballs in the animal world.

    More info: Mixed-up platypus genome unscrambled

    — By John Roach, msnbc.com contributor

  • Colossal squid has plate-sized eye

    AP

    In April 2008, scientists in New Zealand looked a thawing colossal squid in the eye and discovered that the eye is, well, colossal — about the size of a dinner plate. That makes it the largest animal eye on Earth. Fishermen caught the 1,000-pound creature last year in Antarctic waters and froze it intact for scientific study. Colossal squids can reach 46 feet in length and have tentacles equipped with suckers and hooks. Scientists believe the creatures can descend to 6,500 feet and are active, aggressive hunters.

    More info: Huge squid caught, could be biggest ever

  • Aye-aye gives grubs the finger

    Duke University Lemur Center

    The aye-aye is a bushy-tailed primate from Madagascar with big eyes and bat ears. But call it funny-looking and it just might extend its extra-long middle finger in your general direction. The member of the lemur family otherwise uses the extended digit to fish out grubs from the crevices of trees. Captive aye-ayes such as the one shown here from Duke University are teaching scientists about the evolution of color vision.

    Learn more about the brainiac of all lemurs

  • Star-nosed mole sniffs out food, fast

    Image: Star-nosed mole
    Kenneth C. Catania / Vanderbilt

    The fleshy appendages that ring the snout of the star-nosed mole, shown here, make it one strange-looking creature. But when it comes to eating, those 22 tentacles help the mole detect and devour food faster than the human eye can follow — in a fraction of a second. Researchers say the speedy feeding allows the mole to prey on small insect larvae that would otherwise be too energetically costly to eat. The creature lives and forages under the cover of marshes and wetlands along the east coast of North America.

  • Burrowing toad is genetically different

    Robert Puschendorf

    For an amphibian, the stocky and squat Mexican burrowing toad doesn't look all that strange, but it's actually unique. A global conservation program called EDGE of Existence ranks the toad as the most "evolutionarily distinct" amphibian in the world. A fruit bat, polar bear, killer whale, kangaroo and human are all more closely related to one another than the toad is to any other species, according to the program. The Mexican burrowing toad, as its name suggests, spends most of the year underground, coming out only after particularly heavy rains to breed in pools of water.

    Learn about other bizarre amphibians under threat

  • Yeti crab lurks on the ocean bottom

    A Fifis  /  AP

    Named after the legendary shaggy man-beast that tromps through the snows in some of the world's tallest mountains, the Yeti crab blindly scurries about hydrothermal vents along a ridge at the bottom of the Pacific Ocean. First observed in 2005, the crab, officially named Kiwa hirsute, sports a carpet of pale yellow hairs on its arms. Scientists suspect the crab uses those hairs either to farm bacteria or to feel its way around the seafloor for food and potential mates.

    More info: Scientists to list all species on Web

  • Narwhals, the 'unicorn' whales

    Glenn Williams / Harvard Medical School

    Unicorns are purely mythical creatures, but the myths may have been inspired by narwhals. Most males and some females among the 2,200- to 3,500-pound whales sport an 8-foot-long appendage that emerges from the left side of their upper jaw. Scientists recently discovered that the elongated tooth is packed with nerve endings, making it extraordinarily sensitive. The whales may use it to determine the salinity of water and search for food. Male narwhals are also known to rub their tusks together, presumably because it gives off a unique sensation.

    More info: Mystery of 'unicorn' whale solved

  • Sucker-footed bats stick to Madagascar

    Field Museum (left); Bat Conservation Int'l (right)

    In January 2007, scientists announced the discovery of a new species of bat that uses suckers on its thumbs and hind feet to stick to broad-leafed plants such as the traveler's palm. The new species, Myzopoda schliemanni (left image), was found on the dry, western side of the African island nation of Madagascar and is closely related to another sucker-footed bat called Myzopoda aurita (right image) that lives in the humid eastern forests. Conservationists were heartened by the discovery because it suggests the bats can adapt to pioneering broad-leafed plants in deforested areas. Only about 8 percent of the island's original forest cover remains.

    Watch NBC video: What's killing all the bats?

  • Long-eared jerboa hops onto the screen

    Image: Long-eared jerboa
    ZSL

    In December 2007, conservationists released the first known footage of an endangered rodent they've nicknamed the "Mickey Mouse of the Desert." Known more formally as the long-eared jerboa, the little critter has ears about one-third larger than its head, and legs that allow for hopping like a kangaroo. The International Union for Conservation of Nature lists the species as endangered. One threat: the domestic cat.

    More info: Mongolian 'Mickey Mouse' caught on tape

  • Ligers, wholphins and grolar bears, oh my!

    Image: Liger
    TODAY show

    Every now and again, trysts between two different species result in oddball offspring that capture the public's fascination. Ligers, which are a cross between a male lion and a female tiger, were immortalized in the 2004 cult movie "Napoleon Dynamite": The main character of the 2004 cult movie, played by Jon Heder, describes it as "pretty much my favorite animal." (A real one is shown above.) Other popular hybrids include wholphins, which are a cross between false killer whales and Atlantic bottlenose dolphins; and the "grolar bear," a cross between a grizzly bear and polar bear.

    Watch NBC video of a liger
    Whale-dolphin hybrid has baby wholphin
    Hairy hybrid: Half grizzly, half polar bear

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