Chimpanzee culture is driven by its females, suggests a new analysis of six long-term chimp studies.

The number of cultural traits in each colony is linked to the number of females. How many males there are makes no difference.

"Our results suggest that females are the carriers of chimpanzee culture," wrote study co-authors Johan Lind and Patrik Lindenfors, both evolutionary biologists at Stockholm University's Center for the Study of Cultural Evolution.

Lind and Lindenfors' paper, published March 24 in Public Library of Science ONE, was prompted by two sets of observations. First, as becomes more evident with each passing month, chimpanzees possess complex learned behaviors that vary between colony and region. They have culture.

Second, the culture resides in the females. They use tools more frequently than males, and spend more time teaching tricks to their young. And while male chimpanzees tend to stay in the same colony, females will sometimes transfer. Culture would travel with them.

From this, Lind and Lindenfors reasoned that the driving force behind chimpanzee culture ought to be females. They pulled together data from six decades-long studies of chimpanzee colonies in the jungles of Central and West Africa. The data supported their hypothesis.

"The reported number of cultural traits in chimpanzee communities correlates with the number of females in chimpanzee communities, but not with the number of males," they wrote.

That's a different dynamic of cultural transmission than appears to have existed in early humans, where computer models suggest that population density was key. Once there were enough people, cultural evolution accelerated rapidly. After a 2 million-year-long Stone Age, civilization flourished in a comparative handful of millennia.

When trying to understand how chimpanzee culture works, "Some of the general theory behind human cultural evolution cannot strictly be applied to chimpanzees," said Lind. Neither should chimpanzee dynamics be seen as an automatic window into our own past.

"The variation in sociality in now-living apes is phenomenal. We have monogamous gibbons, and then gorillas who live in harems. We have two species of chimpanzees, and their social structures are completely different," said Lind. "According to the best data, we're just as closely related to the bonobo. We could look at them and ask, why don't we have sex rather than kissing on the cheek? There's nothing default about chimpanzees."

An open question is how cumulative chimpanzee culture is, said Lind. Whereas human cultural innovations are "stacked," with innovations building on each other to produce ever-more-complex tools and behaviors, that doesn't seem to be the case with chimpanzees, at least not to a comparable degree way.

Maybe chimpanzees aren't capable of that, or haven't reached their own cultural tipping point, said Lind. Or perhaps we've started to study them too late, with human development having left only isolated pockets of chimpanzee culture.

"When we watch chimpanzees, we look at some scattered remains from previous, much larger populations," said Lind. "I just hope that those remaining spots where chimps can live today will remain."

Images: 1) Mark Fosh/Flickr. 2) Graph of female group size and cultural traits observed in six chimpanzee studies/PLoS ONE

See Also:

Citation: "The Number of Cultural Traits Is Correlated with Female Group Size but Not with Male Group Size in Chimpanzee Communities." By Johan Lind and Patrik Lindenfors. PLoS ONE, Vol. 5 No. 3, March 24, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.