To most people, the term “butthead” represents nothing more than a ubiquitous elementary-school slur. For others, it may be a nostalgic reminder of one hilariously crass ‘90s cartoon. To the pair of researchers who just published their work in the journal PLoS ONE, however, it represents something much more serious: Buttheads are the culmination of millions of years of primate evolution, making our friends easier to pick out in a crowd.

Their research, partially entitled “Getting to the Bottom of Face Processing,” was a study of the curious ways in which chimpanzees recognize each other in social settings and their parallels to human interactions. When we look for acquaintances in a crowd, we say we are looking for a “familiar face.” In the social world of chimpanzees, the researchers report, it’s much more effective to look for a familiar butt. The behaviors are not all that different.

Chimp butts (left) and human faces (right): Not that different.
Chimp butts (left) and human faces (right): Not that different.

The researchers, who hail from Leiden University in the Netherlands and Kyoto University’s Primate Research Institute, begin by explaining that humans process the identity of a face by looking at its component parts, in particular the whites of the eyes, the furry eyebrows and the red-tinged lips and cheeks. They argue that these parts of the face, which may have developed into increasingly distinct features because of their role in signaling “emotions, intentions, health and sexual attraction,” also share “important features with the primate behind.”

In other words, they are alleging that we are all, in a way, literal buttheads, and just as chimps recognize each other by their butts, we identify each other using facial features. As crazy as this might sound, they make a pretty strong case. When chimp females ovulate, they explain, their anogenital region swells and reddens, which is thought to attract and signal to males that they’re ready to mate. Humans, in parallel, value full red lips and cheeks and cite them often as markers of beauty. In addition, the chimp butt is one of the least hairy parts of its body — just like the human face (at least, in most cases/). It’s not the first time scientists have alleged that our sexual desires may be rooted in primates’ interest in each other’s butts: Many researchers contend that natural selection shaped the development of women’s breasts to make them look more like butts.

But back to butts. As we can see here, a chimp does have a fairly red, swollen butt that, if you squint, sort of resembles red lips and cheeks.

Zoo 03 - Chimp Butt
Chimpanzees can identify each other by their butts.

With this assumption in mind, they carried out a series of face and butt recognition experiments with both human and chimpanzee participants. In particular, they were testing the long-studied “face inversion effect,” which is the idea that humans aren’t as good as identifying inverted faces, and, by extension, that chimps shouldn’t be as good at identifying inverted butts. After testing with images like the ones below, their hypothesis — “that chimpanzees process behinds configurally in a way humans process faces” — proved to be correct. “[Humans] demonstrate a face, but not a behind inversion effect and. . .chimpanzees show a behind, but no clear face inversion effect,” they explain.

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In this figure from the study, inverted human faces are paired with similar-looking chimpanzee butts.
In this figure from the study, inverted human faces are paired with similar-looking chimpanzee butts.

Their findings lead them to conclude that human faces and chimp butts serve the same purpose and to conjecture that, because humans evolved from lower primates, that the former evolved to resemble the latter:

The findings suggest an evolutionary shift in socio-sexual signalling function from behinds to faces, two hairless, symmetrical and attractive body parts, which might have attuned the human brain to process faces, and the human face to become more behind-like.

We now know that human faces, like chimp butts, can help us identify each other, but the inversion of that question remains: Can we distinguish acquaintances by their butts? The authors note that “it is not known how behinds as compared to faces are recognized in humans and their closest relatives,” but studying that further will tell us more about how Homo sapiens’ socio-sexual focus shifted from butts to faces — if at all.

Regardless of what future research brings, at least the current findings make one thing clear: Being called a butthead isn’t an insult — it’s the scientific truth.

Photos via Mariska E. Kret andMasaki Tomonaga, Flickr / Greg_e, Mariska E. Kret Masaki Tomonaga