Article under the heading of SCIENCE in Wired
by Brandon Keim
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
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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.
What determines the number of cultural traits present in chimpanzee (Pan troglodytes) communities is poorly understood. In humans, theoretical models suggest that the frequency of cultural traits can be predicted by population size. In chimpanzees, however, females seem to have a particularly important role as cultural carriers. Female chimpanzees use tools more frequently than males. They also spend more time with their young, skewing the infants' potential for social learning towards their mothers. In Gombe, termite fishing has been shown to be transmitted from mother to offspring. Lastly, it is female chimpanzees that transfer between communities and thus have the possibility of bringing in novel cultural traits from other communities. From these observations we predicted that females are more important cultural carriers than males. Here we show that 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. Hence, our results suggest that females are the carriers of chimpanzee culture.
Chimpanzee (Pan troglodytes) cultures exhibit considerable variation between communities . Some of the variation in culture among apes can be explained by local ecological conditions and the diffusion and differentiation of cultural traits between communities –. We here want to propose another important correlate of chimpanzee culture: female group size.
Cultural traits are carried by individuals and inherited through social learning. Thus, the number of cultural traits that can exist in a population depends on the number of individuals that are available to learn from. The diversity of cultural traits present in human populations can be theoretically predicted to increase with community size –. This relationship potentially explains the geographic variation in the timing of the first appearance of modern behaviour, as manifested through advanced human culture, during the Pleistocene without invoking increased cognitive capacity . Here we test if the relationship between cultural diversity and community size holds true also for chimpanzees.
However, there are four key reasons for why adult females can be suspected to be of particular importance for cultural transmission in chimpanzees. First, it can be predicted that young chimpanzees learn more from their mothers than from any other individual in the community since young chimpanzees depend on their mothers up to eight years whereas male involvement is scarce –. Second, tool use is central in chimpanzee culture and females use tools more frequently than males , –. Hence, an important part of chimpanzee culture is mainly exhibited by females. Third, in a detailed study of how chimpanzees in Gombe learn to fish for termites it was found that the time the mother spent termite-fishing was positively correlated to the offspring's acquisition of critical elements of the skill . Since mothers spend more time with their offspring in general, the same pattern can reasonably be expected also for other traits. Fourth, it is the females that transfer between communities in chimpanzees, not the males . Thus, traits learnt by males stay within the community, while traits learnt by females can be transferred to other communities. Even if only a sub-section of females' cultural repertoires are unique to each particular female, the diversity of cultural traits can be predicted to be larger among females than among males.
Because females express and transmit more culture than males, and because females transfer between communities bringing with them their cultural knowledge, the number of cultural traits present in any given chimpanzee community should depend on the number of females in that community. Thus, we hypothesize that the number of cultural traits in chimpanzee communities should correlate with the average number of females in chimpanzee communities, but not with the average number of males.
Since variation in research effort potentially can bias the diversity of cultural traits reported in different communities, we tested if the length of each long-term project affected the reported number of cultural traits (we used the following start dates for the six different projects: Bossou: 1976, Taï: 1979, Gombe: 1960, Mahale: 1965, Kibale: 1987, Budongo: 1990). There was no correlation between the length of the studies and the number of reported cultural traits (rs = 0.450, p = 0.312, n = 6) so we suspect no such bias.
To ascertain the independence of the cultural and community data we estimated λ, a statistic that varies between 0 (phylogenetic independence) and 1 (species' traits covary in direct proportion to their shared evolutionary history) . This parameter did not differ significantly from 0.0 in any test (p>0.4 for all parameters) indicating no presence of a phylogenetic signal in our data (but see 5). Henceforth we therefore only report the results of non-phylogenetic tests.
We found a significant correlation between the number of females in chimpanzee communities and the reported number of cultural traits (rs = 0.873, p = 0.010, n = 7) (Fig. 1). Interestingly, we found no such correlation between reported number of cultural traits and male group size (rs = 0.018, p = 0.969, n = 7), and accordingly only an indication of a correlation with total community size (rs = 0.727, p = 0.064, n = 7).
The correlation between female group size and the reported number of cultural traits indicates that chimpanzee cultural carrying capacity depends on the number of females in chimpanzee communities. This implies that females are critical in chimpanzees for transmitting cultural traits and maintaining cultural diversity. The reported pattern may be explained by the fact that females transfer between communities, bringing with them novel cultural traits and consequently increasing the cultural diversity of the community as a whole.
Vast differences exist between the community sizes of humans living in modern societies and chimpanzees, so it may be tempting to infer that the difference in cultural evolution between humans and chimpanzees depends on differences in community size. However, early hominids lived at much lower population densities than contemporary humans and still, as inferred from the archaeological record , exhibited more culture than chimpanzees . For example, the presence and diversity of early Oldowan stone tools, dating as far back as more than 2 000 000 years, imply a far more complex and diverse culture than what is observed in chimpanzees . The difference between humans and chimpanzees therefore most probably depends on other traits rather than demography.
In humans, culture can grow exponentially as innovation rates depend on the number of cultural traits already present . The lack of a similar exponential growth of chimpanzee culture (inferred from the fact that they do not possess a large amount of culture at present1) might reflect that chimpanzees do not have the mental capacity necessary for making use of established cultural traits when innovating novel traits. This merits further studies of the underlying processes of chimpanzee cultural evolution , –.
As chimpanzee communities continue to dwindle in Africa , more diversity is at stake than biodiversity. If ever lower numbers of chimpanzees results in the transmission of a reduced number of cultural traits over generations and between communities, we risk losing an important possibility of understanding cultural evolution in our closest living relative.