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Humans are experts at recognising faces. Chimps are experts at recognising butts.

In a paper titled ‘Getting to the bottom of face processing’, researchers from Leiden University in the Netherlands found that chimpanzees process and recognise individual buttocks in the same way humans process and remember individual faces.


In humans, differentiating one face from another requires a holistic understanding of the whole face, from eyebrows to eye-whites, red lips and cheeks. Parallels, the researchers note, are easy to draw: both faces and buttocks are “hairless, symmetrical and attractive body parts” making it easy to distinguish between individuals based on stand-out features. Flip a familiar face upside down though and suddenly it becomes much harder to recognise. The same is true, it turns out, with chimpanzees and buttocks. The findings were published in the journal PLOS One.

To test their theory the researchers recruited 100 human volunteers and five chimpanzees. Each volunteer (or chimp) was shown images of the faces, buttocks and feet of three female humans and three female chimps. A sample image was presented for two seconds, followed immediately by two new photographs on the left and right side of the screen. The individual chimp or human then had to match one of the new images to the original image.

The humans had more difficulty matching upside down faces than they did upside down buttocks. In chimpanzees the opposite was true, with the so-called inversion effect having a greater impact on their ability to recognise buttocks.

A chimpanzee is asked to match upright and inverted images of faces, buttocks and feet. The image in the top-right shows distinguishing features of chimpanzee buttocks (left) and human faces (right) Mariska E. Kret


While the importance of buttocks in chimpanzee society is well-known – the anogenital region of chimpanzee buttocks swells and reddens around the time of ovulation, for example – it wasn’t known if they processed characteristics of individual buttocks in the same way as humans process faces. The so-called inversion effect means that humans process information about faces in a totally different way to how they process information about other objects.

The results, the researchers speculate, suggest that our reliance on recognising faces to know who’s who could have evolved from an earlier reliance on buttocks. “The present study demonstrates that chimpanzees, unlike humans, show a ‘behind inversion effect’ and suggests that identity recognition ‘moved up’ from the bottom to the face in our uprightly walking species,” the team concluded.

This evolution, they continued, suggests that the face took over important properties shared with primate buttocks and became the go-to area for “socio-sexual signalling”. At a stretch, they speculate, this evolution may have caused the human face “to become more behind-like”.