Over a 12-year span, two groups of chimpanzees maintained distinct, consistent styles of clasping hands while grooming one another, according to a study published May 26 in Biology Letters. The study findings represent a step forward in understanding chimp sociality and chimp culture—the behavioral patterns that are learned from others in a social group.
“The fact that different groups of chimps have different repertoires of gestures is something we’ve known for some time,” says Mary Lee Jensvold, the associate director of the chimp sanctuary Fauna Foundation, who was not involved in the research, but it was not clear how stable these behaviors were. “The longevity of [this study] is new,” she adds, noting that the semi-wild setting of the Chimfunshi Wildlife Orphanage Trust in Zambia where the research was conducted means that the study’s findings likely apply to wild chimps, too.
Chimp researcher William McGrew, then affiliated with the University of St. Andrews in Scotland, first reported observing handclasp grooming in wild chimps in 1978, and this behavior among chimp dyads has attracted increasing research interest in recent years. While the function of handclasping is not known, University of Antwerp researcher Edwin van Leeuwen, the study’s author, says he’s not bothered by the ease of comparing these handclasps to human handshakes, themselves a cultural behavior.
“We now have a lot of evidence of cultural differences across wild chimpanzee study sites, but much of that is about tool use and foraging behavior, so this evidence concerning a social custom is particularly valuable,” Andrew Whiten of the University of St. Andrews, writes in an email to The Scientist. He was not involved in this latest work but collaborates with van Leeuwen.
Handclasping behavior is unique among cultural characteristics of chimp groups in the sense that it does not appear to directly promote chimp survival. “To the best of my knowledge, this handclasp grooming is the only convincing evidence of social conventions in chimpanzees,” Susan Perry, an evolutionary anthropologist and field primatologist at the University of California, Los Angeles, who was also not involved in the work, writes in an email to The Scientist.
The study of social conventions would be advanced by thinking carefully about how these behaviors function in the animals’ lives.—Susan Perry, University of California, Los Angeles
Between 2007 and 2019, van Leeuwen, the chair of the research board at Chimfunshi, tracked 71 chimpanzees in two groups who live in separate, 160- to 190-acre enclosures that closely mimic the animals’ wild habitat. Van Leeuwen and varying teams of researchers and university students observed the animals from outside their enclosures and noted all instances of handclasping behavior during observation periods, typically from 9 to 11:30 a.m., that ranged from 17 days to more than six weeks.
Van Leeuwen’s team observed a significant difference in clasping styles between the two groups, with one group having a more pronounced palm-to-palm style than the other. Within groups, female-female chimp dyads were more likely to clasp their hands palm-to-palm, while male-male pairs were more likely to clasp wrists, potentially to assert dominance, van Leeuwen says. Despite the departure of 20 chimps from the original groups (through death or, in one case, a relocation to preserve a chimp’s welfare), and the maturation of 23 new handclasping members (the behavior usually starts around age five, van Leeuwen says), group-specific handclasp styles did not change.
Perry applauds the study’s rigorous methodology and findings. “Much of the chimpanzee culture literature has been fairly haphazard, relying on memory of researchers rather than using rigorous reporting methodologies to record the relevant events as they occur,” she writes. “So it is nice to see more careful methodology, designed for a specific research question, employed over a long timespan in this study.”
The study raises questions about the meaning of these behaviors, says van Leeuwen. While he suspects handclapsing plays a role in social bonding, that has yet to be demonstrated, he says, noting that the behavior also has no known benefit in terms of survival, fitness, or group identification. Perry agrees: “The study of social conventions would be advanced by thinking carefully about how these behaviors function in the animals’ lives.”
Van Leeuwen sees a potential link between this chimp cultural behavior—and specifically, their ability to learn and then maintain customs over long periods of time—and the evolution of human social behavior. Given chimps’ close genetic relationship with humans, he suspects social learning was a quality found in the common ancestor of the two species. “The motivation/drive to learn socially is most likely a widespread phenomenon that is both spurred by similar selection pressures and phylogenetically preserved,” he writes in an email to The Scientist.
Perry is less convinced, suspecting that chimps and humans could have evolved independently to learn socially and build culture. “In the absence of broader comparative data it is hard to say whether this shared feature is more due to a shared phylogenetic history or to similar need for social conventions (convergent evolution).”
Van Leeuwen and his colleagues are currently researching potential differences in how chimps learn handclasp behaviors—for example, from peers, a parent, or other adults—and whether this acquisition is flexible. The team is also looking for signs of dominance in hand clasps and other potential functions of this behavior.
The difficulty, as in this study, lies in the limitations of observational data, van Leeuwen notes. “I only see what the chimpanzees show me. . . . There might be a lot of hand clasping going on that I will never know.”