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Primate Planet: The Bond of Brothers
26 APRIL 2007

Primate planet: The bond of brothers
After the killing spree in Virginia and the bombing sprees in Baghdad, it's hard to recall that humans cooperate, not just compete. What are the roots of cooperation? One place to look is our closest relative -- the chimpanzee. Chimps -- especially males -- spend a lot of time together, and could be the second most cooperative primate, after the primate that invented handguns and car bombs, says Kevin Langergraber, an anthropology Ph.D. student at the University of Michigan.

Cooperating improves the chance for survival and reproduction, and it could give mammals two types of evolutionary advantage. If you cooperate with a brother, who shares many of your genes, both of you may have more offspring.

But unrelated men also cooperate, as we can see from Gilbert and Sullivan, or Peyton Manning and Marvin Harrison. These unrelated pairs also get evolutionary benefits: When you pass the football and I catch it, we both get to endorse shoes, feed our families, and pass our own genes along.

Three chimps sit in a row, picking things unseen from chimp before them.

You scratch my back, and I'll scratch yours: Three adult male chimps in Kibale National Park, Uganda, during a grooming session. Grooming is a key social behavior in many primates.
Photo: John Mitani.

To tease apart the issue, Langergraber and colleagues John Mitani and Linda Vigilant studied 41 male chimps in a group of about 150 animals in Uganda's hilly Kibale National Park. Unlike most primates, males hang out with mom for about a dozen years before leaving to try to get a foot on the breeding ladder.

Large lake cuts through southeastern corner of country dominated by small lakes and connecting rivers.
Arrow marks Kibale National Park, Uganda. CIA

The question was this: Were pairs of guys that hung tight related, and if so, how? Female chimps mate with several guys, so a male can have two types of brothers: paternal (related through dad) and maternal (related through mom).

Bond of non-brothers
The results were intriguing. Guys related through a father did not have unusually close relationships, possibly because they could not recognize each other. (In contrast, chimps easily recognize maternal siblings because they grow up together.) But in six categories of behavior, maternal brothers did spend more time with each other.

Even so, most pairs of close-associating guy were not related. "Around 90 percent of the pairs who frequently affiliated and cooperated are unrelated; this shows extensive cooperation among non-relatives," says Langergraber. "The total impact of kinship on the pattern of affiliation and cooperation wasn't so widespread. Many individuals don't have maternal brothers, yet they still affiliate and cooperate with certain other individuals."

But isn't it possible that guys just hang out because they like each other? Yes, says Langergraber, "but this works at two levels of analysis... . Biologists talk about proximate and ultimate causes." The proximate cause may be a friendship, he indicates, while the ultimate cause may be the evolutionary advantage that results from hanging out with pals.

Smaller male chimp sits on forest floor littered with leaves.   Branforde, an adolescent male chimp. What evolutionary advantage could he get from cooperating with unrelated males? Photo: Kevin Langergraber.

One reason to study chimpanzees is to understand ourselves, Langergraber says. "Humans are the most cooperative species on the planet, that is obvious to anyone, and one reason for that is the extensive cooperation among non-related individuals. It has often been assumed that most cooperation in most non-human animals can be explained by kin selection," the benefit an animal derives from propagating the genes of its close kin -- like brothers. "It's always been thought that kin selection explains the high rate of cooperation among chimps," Langergraber continues. But we found that although kin selection does play a role in explaining the high rates of cooperation and affiliation, it is not the entire story."

— David Tenenbaum

• The Limited Impact of Kinship on Cooperation in Wild Chimpanzees, Kevin E. Langergraber, John C. Mitani, and Linda Vigilant, PNAS, April 2007 cgi doi 10.1073 pnas.0611449104

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