Friendship provides solace and support for humans in stressful situations. The companionship of a long-time friend or family member can help people stay calm in the face of stress.
But that's not unique to human relationships. It turns out chimpanzee friendships provide the same sort of comfort – on a physiological level.
The presence of a chimp's "bond partner," or an individual the ape has a strong social bond with, seems to regulate a chimp's stress-related hormones in a stressful, even dangerous, situation, according to research published Tuesday in the journal Nature Communications.
"We believe humans are very special because they can have these interesting relationships between each other that last over the years," study lead author Roman Wittig, a primatologist at the Max Planck Institute for Evolutionary Anthropology, tells The Christian Science Monitor. But "this is nothing that's typically human. The feeling of good friendship, of strong bonds is something that chimpanzees can feel, too."
It has long been known that chimpanzees are a highly social bunch. The great apes groom each other, share food, and form bands to patrol their group's territory. And chimps are known to form particularly close social bonds with some individuals through consistent cooperation and social behavior, termed "bond partners."
So Dr. Wittig and his colleagues took advantage of these naturally occurring relationships in their research.
The team tested the urine of 17 chimpanzees for stress-related hormone levels in three different scenarios: stressful intergroup encounters, the everyday social interaction of grooming, and a resting period for control. In each type of scenario they tested the individual chimps when a bond partner was present or interacting with them and again when it was a stranger, or an individual without a strong social bond to them.
When one chimpanzee group encounters another it can be quite dangerous, and therefore probably generates a lot of stress for an animal. The groups could be competing for resources along the border of their territories or encroaching in some other way, but these conflicts among rivals can be deadly.
Before a band of chimpanzees goes on a patrol they'll often build up their confidence by grooming one another and moving together, Wittig explains.
In these situations, the stress hormones can help generate energy for a standoff or an escape, but afterward, the animals will need to calm down again. According to Wittig's research, the presence of a bond partner seems to help mitigate that physiological response.
Interestingly, the researchers found that it wasn't just in the stressful situations that the presence of a social bond partner regulated an ape's stress-related hormone levels. This was also the case in the more everyday scenarios as well.
Wittig says this suggests strong social bonds may help mitigate more chronic stress on a day-to-day basis.
But Martin Muller, an anthropologist at the University of New Mexico who was not part of the research, says more research on the effects of social bonds on a chimpanzee's baseline hormone levels could provide those insights. Perhaps, he suggests, researchers could compare the hormone levels in chimpanzees that have stronger social bonds with those in individuals who are less well bonded.
Dr. Muller says it isn't surprising that this new research found evidence that social bonds affect chimpanzees' physiology, as previous research has shown them to be a highly social species. But, he tells the Monitor, "it's always nice to see things that we think are true shown to be true," and measuring the stress hormone levels "is a way to very clearly measure the impact that a social bond can have."
This research serves as a reminder that the close evolutionary relationship between humans and their closest living relatives – chimpanzees – goes beyond genetic material, Wittig says. The two species share things like emotion, some cognitive abilities, and now the support of strong social bonds.