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Chimpanzees recognize each other’s butts the way we recognize faces

I hate to see you go, but I love to watch you leave

Animals Celebrate Christmas At Taronga Zoo Photo by Matt King/Getty Images

For chimpanzees, it’s all about the butt. You won’t find a female chimp going “My eyes are up here,” as it turns out that chimps often tell each other apart based on their behinds. Their brains even process butts in the same way that we process faces.

For a study published this week in the journal PLoS One (delightfully titled “Getting to the Bottom of Face Processing”), Dutch researchers recruited both humans and chimpanzees to match photos of various body parts and compared how they did.

Humans need to tell each other apart to survive, and we have specific brain areas for recognizing faces. We recognize people’s faces based on evaluating the whole face, not by putting together the individual parts. That’s why we get worse at recognizing faces when they’re upside down; the upside-down face has all the same parts, but it’s harder for our brains to put together. This is called the inversion effect.

Chimps are a different story. We know from previous research that chimps rely on butts for recognition. This is probably because the butts of female chimps swell when they’re ovulating, which is an important sexual cue for male chimps. But until now, we didn’t know whether chimps processed butts holistically in the same way that humans do. The researchers theorized that if this was the case, there would be a “behind inversion effect” — that is, they would have more trouble identifying inverted butts than recognizing right-side-up butts.

So, the researchers ran a series of experiments. First, they recruited about 100 lucky humans to look at pictures of faces, butts, and feet (of both humans and chimps) and match them. For example, you’d see a picture of someone’s foot and then later some other photos, and then would be asked to pick if that was the one you saw. Some of these images were right-side-up, some were inverted. Then, they got chimps to do the same thing.

Humans, as predicted, had the inversion effect for faces but not for butts. The chimps, on the other hand, had more trouble matching butt pictures when they were upside-down, but not face photos. The more you know.