What Human Faces And Chimpanzee Butts Have In Common

December 28, 2016 by Justin Lehmiller

Sometimes, science makes us laugh before it makes us think–and this is precisely why the Ig Nobel awards were founded back in 1991. The goal of these awards is to recognize scientific achievements that might sound silly or absurd at first, but that ultimately yield useful knowledge and challenge us to think differently about the world.

If ever there was an Ig Nobel worthy contender, it would be a paper published in PLOS ONE that explores what the faces of humans and the rear ends of chimpanzees have in common.

Yep, that’s really what they studied. But please bear with me–I promise, it’s a funny story that tells us something fascinating about both sex and psychology.

The genesis for this study came from research into the “face inversion effect.” The basic idea behind it is that we evolved to pay particular attention to other people’s faces because they reveal a lot of important information. For one thing, they assist us in helping to identify others and to quickly differentiate friend from foe. However, faces can also reveal information about our age and attractiveness, our emotional states, as well as our health and genetic fitness. We can learn a lot from a quick glance at someone’s face, which is probably why our ability to recognize faces begins during infancy.

To take fullest advantage of all the information faces have to offer, we processes faces configurally—that is, in a holistic fashion. While faces are made up of a bunch of different parts, those parts tend to appear in a pretty similar pattern from one person to the next. It appears that we evolved to pay attention to the spatial configuration of facial features because when we see faces that are inverted (i.e., presented upside down), we don’t recognize them as quickly as when we see faces that are presented right side up. When a face is upside down, the usual configuration is thrown off and we have to instead process it in a piecemeal way by looking at specific features (eyes, nose, mouth, etc.) in order to figure out what it is.

You might be wondering whether we have impaired recognition for anything that’s presented upside down, but that’s not the case. For example, when we see an image of an object, such as a house, people recognize it just as quickly regardless of whether the image is presented upside down or right side up. In other words, the way we process faces seems to be very different from how we process other kinds of information.

OK, so what does this have to do with chimpanzee butts? Well the scientists leading this study proposed that, like humans, it would also make sense for chimps to have the ability to recognize members of their own species, but that perhaps recognition for butts would be more valuable than recognition for faces. Why? It could help males to more easily identify ovulating females, given that their rear ends swell and become redder when they’re most fertile. In addition, “rump recognition” could be useful for avoiding inbreeding, as well as for helping female chimps to identify competing females.

In order to test this idea, researchers exposed both humans and chimps to images of faces, butts, and feet (feet were shown as a control) and basically had them play a matching game. They would be shown one image and then asked if the second image was a match. However, the comparison image was upright sometimes and inverted other times.

What they found was that, as expected, humans showed a face inversion effect—it took longer for them to recognize faces that had been turned upside down. However, no inversion effect was found for rear ends or for feet among the human participants.

By contrast, chimps showed a “behind inversion effect”—it took them longer to recognize images of butts than had been flipped upside down. However, chimps showed no such inversion effects for faces or feet.

On a side note, the researchers also tested whether these effects occurred for both greyscale images and full-color images. For humans, whether the images were in full color or not didn’t affect the results; however, for chimps, the behind inversion effect was only there when the images were in color. This makes sense, given the prominent color changes that take place on female chimps’ rear ends during ovulation.

>Human face or chimpanzee butt? Source: Kret, M. E., & Tomonaga, M. (2016). Getting to the Bottom of Face Processing. Species-Specific Inversion Effects for Faces and Behinds in Humans and Chimpanzees (Pan Troglodytes).  PLOS ONE ,  11 (11), e0165357.

In light of these results, the researchers theorize that perhaps humans used to recognize one another based on their rear ends, too, but that we stopped doing this once we began walking upright. At that point, they think the focus of our attention shifted from the buttocks to the face and maybe—just maybe—this led “the human face to become more behind-like” (the authors’ words, not mine–they even created a handy image showing just how similar the human face is to a chimp’s butt).

I don’t know about you, but this study made me laugh, then think. And then it made me laugh again. Can we just give it an Ig Nobel already?

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To learn more about this research, see: Kret, M. E., & Tomonaga, M. (2016). Getting to the Bottom of Face Processing. Species-Specific Inversion Effects for Faces and Behinds in Humans and Chimpanzees (Pan Troglodytes). PLOS ONE, 11(11), e0165357.

Image Source: 123RF/Abeselom Zerit

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Written by
Dr. Justin Lehmiller
Founder & Owner of Sex and Psychology

Dr. Justin Lehmiller is a social psychologist and Research Fellow at The Kinsey Institute. He runs the Sex and Psychology blog and podcast and is author of the popular book Tell Me What You Want. Dr. Lehmiller is an award-winning educator, and a prolific researcher who has published more than 50 academic works.

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