Butts, Faces Help Chimps Identify Friends

2008-10-02T07:28:00.000-07:00

Chimpanzees may not forget a familiar face—or a behind, a new study says.

In a recent experiment, captive primates were able to identify photos of their acquaintances' rears and match them with the right faces.

The ability suggests that the animals possess mental "whole body" representations of other chimps they know.

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.

"Many animals look at parts of the body, the voice, the hands, as separate entities and don't wholly integrate them," said study co-author Frans de Waal of the Yerkes National Primate Research Center at Atlanta's Emory University.

"This study shows that they have whole body integration [because], at least if they know the individuals, they can match the faces and the behinds."

The study appeared recently in the journal Advanced Science Letters.

Nudist Advantage?

Had the chimps also matched strangers, the primates could have been merely picking up on genetic or physical clues to link faces and rear ends, de Waal explained.

"This means that their matching is based on their experience with the individual, not some kind of guesswork that they may do," he added.

No convincing evidence exists that other primates, including humans, could duplicate the feat.

"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."

Gender Familiarity

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."

Ancient Tsunami Carried Giant Boulders to Tonga

2008-10-02T07:19:00.000-07:00

Huge coral boulders on Tonga's main island of Tongatapu were possibly tossed hundreds of feet inland by one of the largest tsunamis ever triggered by a volcano.

Researchers believe the ancient wave may have hit the island's shore sometime within the past 7,000 years, after the melting of the most recent ice age brought sea levels to roughly where they are today.

The largest of the seven boulders is 50 feet (15 meters) wide and estimated to weigh 1,600 tons. It currently sits more than 300 feet (100 meters) from the sea and 30 feet (10 meters) above sea level, an anomaly on the South Pacific island's flat landscape.

"We suspect that this may be the largest [object] moved uphill by a tsunami," said Cliff Frohlich, a senior scientist at the Institute for Geophysics at the University of Texas at Austin.

"It wasn't like we found these rocks everywhere in Tonga," Frohlich noted. "We found them just in one place on one island."

By comparison, the 130-foot (40-meter) waves triggered by the 1883 eruption of the Indonesian volcano Krakatau—the most powerful explosion in recorded history—are known to have moved a boulder only about a third that size the same distance, Frohlich said.

It's possible there simply were no larger boulders for that tsunami to displace, he added. But he and his colleagues believe the Tonga tsunami may have been as big or larger than Krakatau's.

The team will be presenting some of its findings at a geology conference in Houston on October 5. Their study is currently under peer review.

Striking Distance

The sea was likely close to present-day levels when it carried the boulders, which Frohlich believes were formed in the reefs surrounding the island.

The coral boulders were alive and growing about 122,000 to 130,000 years ago. It's possible that this is when the wave occurred, but the researchers favor a more recent date.

They estimate it struck within the last 7,000 years, because the surrounding area lacks the erosion and other signs of weathering one would expect after more than 100,000 years.

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Butts, Faces Help Chimps Identify Friends

Ancient Tsunami Carried Giant Boulders to Tonga