Female Chimps Wait Their Turn Climbing Social Ladder

While male chimps advance their status in a group by directly challenging their betters, the ladies wait until vacancies arise, a study finds.

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Rank has its privileges in the chimp world. The higher a chimp is on the social ladder, the better its chances for food and mating opportunities.

Female chimps employ a much more patient approach to climbing that ladder than do their male counterparts, according to a new study out of Duke University. Males move up the old-fashioned way: by directly challenging - through chases or attacks, for example - higher ranking group members. But females, the study finds, accept the rules of chimp society and wait until a higher-ranking female dies for their chance to move on up.

"We found that, after entering the adult hierarchy, there was a complete absence of successful challenges for rank increases among females," Duke researcher and study lead Steffen Foerster said in a statement. "It's like a formal queue."

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"If a male has a high rank even for a short time but manages to fertilize a lot of females, he achieves high reproductive success," explained Duke anthropology professor and study co-author Anne Pusey. "A female is only able to raise one offspring at a time, so her reproductive success depends largely on how long she lives."

And there's the rub, the scientists suggest. There's a lot at stake in trying to cut in line, and patience is prudent.

According to Foerster, for female chimpanzees "it is potentially dangerous to challenge each other. You may get injured. Your offspring may be killed, if you have a little baby. Finding that females actually do not fight for rank tells us how costly these challenges must be for them."

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To get a handle on social climbing in chimp hierarchies, the Duke researchers studied more than four decades of daily behavioral records kept for some 100 chimpanzees in Tanzania's Gombe National Park. They found that societal rank for females increases as they get older, the rank order itself remaining stable along the way.

By contrast, the team noted, males tend to peak early, hitting their prime rank years in their early 20s, before declining in status as they aged.

Too, the males nearly always start on the lowest rung of the social ladder, while females enter the hierarchy at varied places.

The scientists say they've yet to determine exactly how the females arrive at that beginning rank - early evidence points toward a preferential boost for chimps whose mothers are also in the group - but one thing is certain: the entry point is critical for the females.

"That seems to be a crucial moment for them," Foerster said, "because after they enter the hierarchy, at about 12 years of age, they can't really change anything about their position unless something happens at the top and individuals die."

The Duke team's findings have been published in the journal Scientific Reports.

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