Butts, Faces Help Chimps Identify Friends

Brian Handwerk
for National Geographic News
September 30, 2008
Chimpanzees may not forget a familiar face—or a behind, a new study says.

In a recent experiment, captive primates were able to identify photos of their acquaintances' rears and match them with the right faces.

The ability suggests that the animals possess mental "whole body" representations of other chimps they know.

Each participating chimp was flashed a picture of another's bum, with visible genitals, then shown the face of the derriere's owner and another face of the same gender.

Both males and females were successful in this anatomical match game, pairing faces and posteriors with much greater frequency than chance alone—but only if the photos showed chimps they already knew.

"Many animals look at parts of the body, the voice, the hands, as separate entities and don't wholly integrate them," said study co-author Frans de Waal of the Yerkes National Primate Research Center at Atlanta's Emory University.

"This study shows that they have whole body integration [because], at least if they know the individuals, they can match the faces and the behinds."

The study appeared recently in the journal Advanced Science Letters.

Nudist Advantage?

Had the chimps also matched strangers, the primates could have been merely picking up on genetic or physical clues to link faces and rear ends, de Waal explained.

"This means that their matching is based on their experience with the individual, not some kind of guesswork that they may do," he added.

No convincing evidence exists that other primates, including humans, could duplicate the feat.

"Of course humans' behinds are normally clothed," he said.

"I think the clothes interfere with things a bit. Maybe in a tribe in which people walk around naked all day, or a nudist colony … might be able to do this."

(Video: Watch chimps beat humans at a memory game.)

Gender Familiarity

The experiment also analyzed how well chimps could determine the sex of a chimp face seen in a photo.

Primates were again shown pictures of rear ends, with genitals, and then instructed to choose either a male or female face to match the sex of each posterior.

As before, success rates were markedly higher when chimps knew the subjects, which suggests that gender may be a big part of how chimps know one another.

"We interpret [this to mean] that a familiar individual is more gendered to them than an unfamiliar individual," de Waal said.

(Related: "Chimps Can Be Team Players, Selfless Helpers, Studies Show" [March 2, 2006].)

This phenomenon has also been seen in humans who were flashed pictures of faces stripped of obvious gender clues, like hair.

The human subjects guessed gender more quickly when shown with familiar individuals.

"That's based in humans on the fact that someone you know has already established their gender in your mind … ," said de Waal.

The Rump Says it All

Sarah Brosnan, a primatologist at Georgia State University in Atlanta, was not involved with the research, but has worked with de Waal in the past.

She explains that rear end recognition in chimps is highly visual because of swollen, pink, and hairless tissues that are uniquely shaped in individual animals.

Since swellings become even more prominent during ovulation, female rear ends are of extra-special note to interested males, she added.

"With dogs, derrieres are all about the scent," she said.

"There is also scent on a chimp, but sometimes it's easier to tell chimps apart by their swellings than by their faces—so in this case it makes a lot of sense that they'd be able to recognize each other visually by this signal."

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