Recognizing one's tribe and mixing with them is part of the social life and existence. Humans recognizing each other through faces is a known fact. However, when it comes to chimpanzees, they recognize one another from buttocks.

These revelations came from a new study conducted by neuropsychologist Mariska Kret.

Published in PLOS ONE, the study offers insights on how humans and chimpanzees deftly process information about faces and buttocks to recognize their kin.

The study Getting to the Bottom of Face Processing had Dutch researchers undertaking tests on humans and chimpanzees by mixing and matching photos of various body parts and recording their response.

Kret's study with chimpanzees took place in a primate institute in Japan, where the author worked for a year.

The study noted that like humans, chimpanzees seek information on the identity, attractiveness and health of the peers, though the source is buttocks.

It describes how the monthly ovulation period of female chimps sends out sexual cues to other chimps when the swelled private parts such as anus and vagina acquire a dark pink color. That is more than enough for fellow chimpanzees to recognize with a mere glance of the buttocks.

Brain's Shortcut For Face Identification

Humans recognize faces in upright position faster than in inverted positions. Also, test on humans proved that they recognized buttocks quickly whether the photos were upright or inverted.

"We see faces so often and almost always upright that our brains have created a shortcut so that this category of images is recognised more efficiently and faster, but this only works if they are upright," Kret explained.

In chimps, brains process butt identification in the same way humans process faces.

During the researcher's test, chimpanzees were found faster in clicking on the buttocks when they were upright than inverted.

"This is a good indication that this category has priority over other categories of objects," Kret said.

It surmised that brains of chimpanzees process butts as fast as the humans' process faces.

For humans, easy recognition of the face comes from the use of specific brain areas. That is done by identifying the whole face and not by mentally joining the individual parts.

At the same time, humans have a problem in recognizing faces when they are upside down despite the parts being the same. That shows inversion makes it harder for brains to put together the information.

Inversion Effect In Chimps

This also was found true in the case of chimps as far as butt recognition was concerned. There was no evidence until now whether chimps processed butts holistically as humans did. The researchers thought if that was the case, Chimps must have a "behind inversion effect" in identifying inverted butts than recognizing them in normal position, as was shown in the test.