Chimps recognize their friends from the shape of their buttocks! From National Geographic:
Each participating chimp was flashed a picture of another’s bum, with visible genitals, then shown the face of the derriere’s owner and another face of the same gender.
Both males and females were successful in this anatomical match game, pairing faces and posteriors with much greater frequency than chance alone—but only if the photos showed chimps they already knew.
Can humans do this? One of the study’s co-authors helpfully explains:
“Of course humans’ behinds are normally clothed,” he said. “I think the clothes interfere with things a bit. Maybe in a tribe in which people walk around naked all day, or a nudist colony … might be able to do this.”