LEIDEN, Netherlands, Dec. 2 (UPI) -- Chimpanzees can recognize one another from behind. Apes remember faces, too, but new research suggests chimps recognize buttocks just as well as humans recall familiar faces.
Just as faces serve as an epicenter of social information, so too do chimp rumps. More than just an identity cue, a chimpanzee's buttocks offer clues to the his or her attractiveness and health. The area around the vagina and anus of a female chimp who is fertile swell and take on a dark pink color.
The importance of face recognition among humans is demonstrated by a phenomenon called the "face inversion effect." When scanning an image featuring multiple objects, humans invariably recognize a face first. When a face is inverted, however, humans are slower to recognize it. The same is not true for other objects, like a car or house, which humans recognize in the same amount of time regardless of its orientation.
"We see faces so often and almost always upright that our brains have created a shortcut so that this category of images is recognized more efficiently and faster, but this only works if they are upright," Mariska Kret, a neuropsychologist at Leiden University, said in a news release.
Researchers discovered a similar phenomenon among chimps, who were quicker to recognize buttocks on a touchscreen when they were upright and slower when they were inverted.
"This is a good indication that this category has priority over other categories of objects," Kret said.
The importance of rump can be seen in chimpanzee physiology. Female apes have evolved hairless buttocks, so not to interfere with the body part's communication abilities. Chimps also evolved eyes capable of seeing many tints of red.
The new research was published this week in the journal PLoS ONE.