Q&A: Why Do Chimps Recognize Each Other's Rumps?
Researchers tested chimps' ability to pair chimp rears with the correct chimp faces.
Credit: Frans de Waal & Jennifer Pokorny

Chimps never forget a familiar face — or butt.

The discovery that chimpanzees can easily match photos of their groupmates' rear ends to the correct faces on a computer screen won Emory University primatologists Frans de Waal and Jennifer Pokorny this year's Ig Nobel Prize in anatomy. 

The Ig Nobel Prize Ceremony, a spoof of the Nobels held each year at Harvard University, honors scientists who have made genuine, but also offbeat, scientific discoveries. Life's Little Mysteries caught up with de Waal at the 22nd annual ceremony yesterday (Sept. 20) to discuss chimp butts, and what they might say about human brains.

Life's Little Mysteries: How did you discover that chimps can recognize their friends' rear ends?

Frans de Waal: So we did a study on face recognition in chimpanzees, which is a very important area to see if they recognize faces and emotions and stuff like that, and in that context, we wanted to know if they recognized the sex of an individual like humans can. If you show [humans] pictures, even if the hair is cropped away and there's no ornaments, we can immediately see if they are male or female. But with chimps, it's very hard to ask them "Is this a male or a female?" and so we thought to be smart by having the behind of a chimp in the computer setup. The behind of a male is totally different from the behind of a female, because the females have sort of a big pink balloon on their behind.

So, we did that, and in that context, we discovered — which was not the goal of the research — that they can match the face with the behind, if they know the individual. If they don't know the individual, they cannot do it. But it is based on them having "whole body knowledge" of their groupmates. Because they know the whole bodies, you can show them parts, and they know which face belongs with which behind. And so the title of the paper became "Faces and Behinds," and that's how we got the award, I guess, even though it was a serious study!

LLM: Yes, it is! Could the chimps recognize the sex of chimps they didn't know based on their faces alone, or did they need to be familiar with a chimp's behind to know what sex it was?

FdW: They were capable of determining the sex [of the face] with individuals that they know. With ones they don't know, they have trouble. If they know the individuals, they can match a picture of their face to a generic behind that is the right sex. So for behinds that don't match with the face, they can still make the connection [between the individual and the behind of the correct sex]. They can still tell us which one is male and which one is female. But they aren't so good at doing this with individuals who they don't know.

LLM: So they can't always discern a strangers' sex just by looking at his or her face?

FdW: That's right. We think that when they do the test, they recognize which individual they see; they have a whole sort of image of that individual based on behavior and interaction and that makes it easier for them to determine the sex. And for humans, the same is true. Humans are quicker at guessing the sex of someone that they know. So if you see pictures of celebrities, for example, you are quicker to say which sex it is than individuals who you don't know. 

LLM: We know who they are so we know what sex they are.

FdW: Yes, so it's a quicker decision. 

LLM: But humans are much better than chimps at discerning the sex of strangers based on their faces, even with strangers who are quite androgynous-looking. Why is that?

FdW: Maybe because we humans, we need to rely more on the face. Because the chimps, when they look at each other, they see the whole body, and it's not dressed up or something. It's naked. With humans, everything is covered over, and so maybe we rely more on faces, possibly. [Why We Wear Pants]

But although chimps are not as good as humans at discriminating the sex of the face, face recognition itself is very highly developed in chimps. We've done tests in the past where, for example, you can show them a female's face, and then you show two juveniles — individuals that they don't know at all, so they've never seen these pictures before — and they can pick which juvenile is the offspring of the female. So their face recognition is so good that they can see the kinship connections.

LLM: What do you think is the takeaway message of your findings?

FdW: Well, face recognition is a very important area in human research and autism research and stuff like that, so faces and emotions are important, and we have special areas in the brain that are involved in that that we share with the primates. In primates, the same area of the brain is involved: the fusiform area. So it's an important capacity and it's important to test how primates do it. Twenty years ago, people said, "Face recognition is a uniquely human capacity." And that's not true anymore.

LLM: Do other non-primate species have this face recognition capacity?

FdW: Yeah, so people have done face recognition in sheep, for example. And found it [laughs]. So I don't think it's even uniquely primate, you know?

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