Adult male chimps form enduring friendships, maintaining them by grooming each other (Image: John Mitani)
Everyone needs a best friend, even chimpanzees. A decade-long study shows that nearly all adult male chimps form enduring social bonds with other males, exchanging back scratches, sharing meat, and generally chumming around.
On average these bonds lasted seven years, says John Mitani, a primatologist at the University of Michigan, who observed chimpanzees in Uganda several months a year for 10 years.
The colony, in the jungles of Kibale National Park, is about three times the size of other chimpanzee populations in Africa, but is no more social than others, he says.
For the study, Mitani spent a block of time recording the interactions of a specific adult male chimp, including every individual he interacted with, while noting grooming behaviour. Females tend to leave their colony once they reach maturity and therefore forge fewer social bonds, Mitani says.
As with human friendship, the strongest bonds seemed to be based on mutual respect. Chimpanzees that groomed each other for roughly equal amounts of times tended to stay friends longer.
Fraternity also played an important role in chimpanzee friendships, Mitani found. Animals that shared a mother were more likely to form lasting bonds than other pairs. However, chimpanzees with a common father weren't any more likely to become buddies.
Nearly every chimpanzee that Mitani tracked formed at least one long-term social bond, and some had multiple "best friends". Out of 35 males, two never formed close friendships with other adults during the study period. However, both found friendship in a younger, still adolescent brother, Mitani notes.
Exactly why chimpanzees form these stable bonds is unknown, Mitani says. It could be that having a best friend boosts reproductive success or survival somehow. But this will require "staying out there to see who does what with whom, and how often, and counting up the babies," he says.
Joan Silk, a primatologist at the University of California in Los Angeles, notes that chimp friendships aren't so different from the baboon she studies.
Equitable grooming and sisterhood seemed to determine friendships among female baboons in Botswana, she says. "These similarities suggest that there are common principles for building strong bonds which extend across species."