"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."
The experiment also analyzed how well chimps could determine the sex of a chimp face seen in a photo.
Primates were again shown pictures of rear ends, with genitals, and then instructed to choose either a male or female face to match the sex of each posterior.
As before, success rates were markedly higher when chimps knew the subjects, which suggests that gender may be a big part of how chimps know one another.
"We interpret [this to mean] that a familiar individual is more gendered to them than an unfamiliar individual," de Waal said.
(Related: "Chimps Can Be Team Players, Selfless Helpers, Studies Show" [March 2, 2006].)
This phenomenon has also been seen in humans who were flashed pictures of faces stripped of obvious gender clues, like hair.
The human subjects guessed gender more quickly when shown with familiar individuals.
"That's based in humans on the fact that someone you know has already established their gender in your mind ," said de Waal.
The Rump Says it All
Sarah Brosnan, a primatologist at Georgia State University in Atlanta, was not involved with the research, but has worked with de Waal in the past.
She explains that rear end recognition in chimps is highly visual because of swollen, pink, and hairless tissues that are uniquely shaped in individual animals.
Since swellings become even more prominent during ovulation, female rear ends are of extra-special note to interested males, she added.
"With dogs, derrieres are all about the scent," she said.
"There is also scent on a chimp, but sometimes it's easier to tell chimps apart by their swellings than by their faces—so in this case it makes a lot of sense that they'd be able to recognize each other visually by this signal."
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