This is called the “inversion effect,”and the authors of a new study in PLOS One found that chimpanzees have it when it comes to buttocks.
It was already known that chimps sometimes demonstrate an inversion effect with faces and bodies. But the researchers, based in the Netherlands and Japan, spotted a gap in the literature: “Previous studies included almost all body parts, except the most obvious one, which is the behind.”
Why would this be obvious? Because rear ends serve a big purpose in the chimp world. Female chimps’ buttocks grow redder and swollen when they are ovulating, signaling to males that it’s business time. And it’s important to know whose bottom it is, in part to prevent inbreeding. The buttocks have, in scientific parlance, a “high socio-sexual signaling function.”
But when we began walking upright, our bottoms became fleshier and no longer broadcast our ovulation status, possibly to discourage casual hookups in favor of pairing up and sticking together for the children’s sake. On the other hand, humans — “especially females,” the researchers write — developed ruddier and thicker lips, as well as fattier faces.
Bottoms and faces are both symmetrical, they add, and interpreting what a butt is saying is crucial to chimpanzees’ reproductive success, just as interpreting facial messaging is important to human mating.
“Thus,” the authors write, “human faces share important features with the ancient primate behind.”
So their hypothesis was that the inversion effect would hit chimpanzees harder when it came to buttocks. And in tests — which involved 100 people and five chimpanzees who matched images and inverted images of human and primate faces, buttocks and feet — they found this to be the case. The humans struggled more to match upside-down faces than rear ends. The chimpanzees had the opposite problem, which suggests they process images of buttocks the way we do faces.
The researchers say it also suggests that when we began walking, our recognition abilities moved from the “bottom up” — that is, that our facial identifying skills evolved out of an ancient ability to recognize specific buttocks.
“The findings suggest an evolutionary shift in socio-sexual signaling function from behinds to faces, two hairless, symmetrical and attractive body parts,” the authors wrote, “which might have attuned the human brain to process faces, and the human face to become more behind-like.”