Thirty-five years ago, researchers studying chimpanzees in the wild noticed that neighboring communities had distinct grooming behaviors that could not be explained by differences in their environments. They contended that these behavioral idiosyncrasies were learned, or "cultural," and other scientists soon began noting group-specific tool uses and courting behaviors that also didn't appear to be environmental. But in a new study, researchers say some of these behaviors may be genetic after all.
Before that 1975 revelation, few researchers had observed different communities of wild chimpanzees, and no one had even recognized that these behavioral differences existed. Investigators have been arguing about whether chimps truly have culture ever since. Proponents of culture published a landmark Nature paper in 1999 documenting 39 behaviors that were frequently observed in some communities and never seen in others. In the article's wake, a flood of reports began to appear about culture in other species, and the debates roiled on, with endless discussions about the meaning of the word itself.
The new study, published online tomorrow in the Proceedings of the Royal Society B, examines partial sequences of the mitochondrial DNA (mtDNA) from wild chimpanzees in nine different groups. This DNA is handy because it's inherited only from mothers, and only chimp females typically move to new communities. Team members examined the links between the groups and 38 of the 39 supposed cultural variants documented in the earlier report. The study does not link behaviors to specific genes or even conclude that there is a genetic explanation. Rather, it assesses whether genetic differences can be excluded as an explanation for each behavior; it finds that they cannot more than half the time.
This distinction may seem subtle, but the idea of animal culture turns on the requirement of first excluding ecological forces as an explanation for behaviors. The study now adds yet another hurdle to clear before making bold claims about culture. "I have no horse in this race," says lead author Kevin Langergraber, a molecular ecologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. "I saw some studies that claimed they were settling this question, and I had gathered data that spoke to quite a different explanation."
The findings, as might be expected in this controversial field, are receiving a mixed reaction. The first author of the 1999 Nature study, evolutionary psychologist Andrew Whiten of the University of St. Andrews in the United Kingdom—who did not contribute to the new study—says Langergraber and colleagues have done "a very careful and rigorous job." But Whiten contends that they have given too much weight to the "the relationships between behavioral and genetic differences they found." Specifically, he contends that the sequencing of small regions of mtDNA as well as the relatively few documented behavioral differences are "very crude overall measures" of the true genetic and behavioral differences. He further singles out several experiments that he and others have conducted with unrelated captive chimpanzees that clearly demonstrate sophisticated social learning skills, especially for tool use. "Given all we know about chimpanzee social and individual learning, it seems unlikely that there are any chimpanzees that, because of their genetic constitution, cannot observationally learn all the kinds of tool use seen in Africa."
Ethologist Frans de Waal of Emory University in Atlanta has a more generous take on the new work. In 1999 de Waal wrote an accompanying editorial in Nature that said Whiten and colleagues had provided a "record so impressive that it will be hard to keep these apes out of the cultural domain." The new work, de Waal contends, "is not dismissive of the culture concept, but adds a complication to the picture."
De Waal notes that individuals of a species often have similar behaviors that are not controlled by genes. "No one would assume a gene for ant fishing in the chimpanzee in the same way that no one would assume that some humans have a knife-and-fork gene and others a chopstick gene," says de Waal. Still, he says the new findings likely will make the nature vs. nurture discussion more interesting. "If we simply accept that chimpanzees have cultural habits that spread by means of social learning and then add this genetic picture to it, we get in fact a view closer to what we know about humans, and a broader debate that we have hardly had before," says de Waal.
Langergraber, who studies the evolution of cooperation and social relationships in wild chimpanzees, notes that there's compelling evidence in finches, crows, and gorillas that some behaviors—like learning to use tools or eat nettles that will sting unless they are handled just so—have genetic underpinnings. And the same is true of humans, he notes. "Some things you'd never think are genetically determined are highly inheritable." Genes, for example, appear to play a role in whether a person is an extrovert who wears loud clothing or an introvert who dresses for comfort.
But he stresses that in wild chimpanzees, especially since females often migrate to different communities, it will be particularly difficult to sort the genetic from the cultural. "They're not moving only their genes, but it could be behavior as well," says Langergraber. "So you could get a positive correlation between genetic and behavioral similarity even if it were 100% cultural." Langergraber says he'd make a more conservative point, "You can't rule out that it's genetic."