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You’ll Never Guess How Chimps Recognize Each Other (Hint: It’s Not Their Faces)

As humans, we are able to distinctly recognize each other’s faces.

Yes, we sometimes gather clues from a person’s body type or style of hair, but what really makes the difference is looking at someone’s facial features and putting the sum of the parts together in our minds. Even the individual parts, such as the lips or eyes, don’t give away the whole person on their own—we rely on the distinct arrangement of these features all together to identity each other. This is known as configural processing. It’s this need to see all the features together in an expected order (for example: eyes, nose, mouth) that causes us to have trouble identifying someone who is upside down, for instance.

Since we are so closely related to chimpanzees, you would think they, too, rely on facial recognition to identify one another. But, it turns out, their clues are a little different. They, too, use configural processing to identify each other, but rather than identifying the sum of each other’s facial features—they come to recognize each other’s behinds.

That’s right, chimps rely on the distinguishing features of their bottoms to recognize one chimp from another. Researchers from the Netherlands and Japan tested this theory by observing the apes as they examined photographs of primate buttocks. The study, published in the journal PLOS One, had chimps play a version of “the matching game,” where faces, butts and feet were shown on a computer screen, both right side up and upside down. They were then shown a number of images following the first round, and they were asked to correctly identify the image that matched the original one.

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In the testing, the chimps were able to correctly identify the right-side up butts more quickly than faces, feet, or upside-down butts. The study supports the idea that just as we use faces to spot someone familiar in the crowd, chimps use their backsides.

To make matters more interesting, the researchers think this suggests that the evolution of our faces and butts are actually closely linked. According to the researchers, the human face shares important features with the primate behind. For instance, both a human face and chimp butt are always on show, they both have symmetrical features, and both can change color to give off social signals. But more importantly, as we now know, they both help members of their species to help recognize each other.