Monkeys have been known to possess many human-like qualities, but a Duke researcher’s findings suggest human friendship may actually be monkey-like.

Lauren Brent, a postdoctoral associate at the Center for Cognitive Neuroscience, studied cooperation and other social bonds in rhesus macaques, for seven years in Cayo Santiago, Puerto Rico. The monkeys were found to exhibit pro-social behavior­s—voluntary acts meant to benefit another—such as friendship. Brent found that the act of friendship is prevalent within the species, and that the basis of these bonds in monkeys and humans may be genetic.

“We’re finding this kind of pro-social behavior in many species, in non-human primates, but in other animals too,” Brent said. “The function that these friendships serve [in other animals] is actually very similar to what these social bonds do for us.”

Although friendships among rhesus macaques do not last an entire lifetime as they can with humans, the monkeys do maintain long-term friendships, Brent said. This suggests that the foundation for these social bonds consists of something more profound than simply the benefit an individual could gain from another.

“It’s just the desire for companionship,” Brent said. “It’s not like we’re still best friends with people we knew when we were five, but most friendships tend to be long-term, and that’s consistent in macaques as well.”

She also found that sociability of individual macaques could be an inherited trait. By meticulously identifying the relationships between individuals to determine family trees, she was able to tell which monkeys were most well-connected, and who their parents or grandparents were. Analyses revealed genetic variation in the “sociability trait” significant enough to determine that friendship is an inherited trait, as the most sociable rhesus macaques were often the offspring of the most sociable adults.

This evidence suggests that friendship could be an ancestral trait that was, perhaps, present in the common ancestor of all mammals.

“Of course the argument can be made that it’s a novel trait that evolved independently many times, but if we look at it parsimoniously, [friendship is] most likely ancestral,” she said.

Long-lasting friendships are known to come with health benefits, Brent said, noting that having a close social bond with other members of a group decreases stress and increases lifespan. Humans who surround themselves with friends are known to have less stress, which is correlated with better health. Brent said this has been seen when studying animals other than humans.

“Those with friends show lower stress, which results in the release of fewer damaging immune cytokines,” she said. “We’ve found that could relate directly to longevity.”

Similar to humans, rhesus macaques also exhibit aggression and competition. Brent noted that pro-social behavior is interrelated to these types of behavior, which are bound to emerge in larger groups with many social interactions.

“Because you have competition with others in the group, you have to have friendships and alliances with certain members too,” Brent said.

Although rhesus macaques are now known to exhibit friendship, humans are still known as the most cooperative species, Brent said.

Michael Platt, director of the Duke Institute for Brain Sciences, said humans are able to cooperate so well together because it is necessary for the human species to solve problems as a group.

“We’ve been trying for millennia to determine what it is that makes us so cooperative,” Platt said. “Whether it’s culture, language or something else, we can be altruistic, and that suggests the capacity for early humans to form strong social bonds.”

For Brent, studying monkeys not only reveals more about mankind’s evolutionary past, but also reveals how humans can be understood as animals. She noted that there is nothing to be gained from thinking that we are completely unrelated to these other species.

“It quickly dispels the idea of something being more evolved than something else,” Brent said. “We’re all still evolving, so that’s the wrong way to think, and it’s something that we lose sight of.”

Brian Hare, an associate professor of evolutionary anthropology, also said there are still lessons we can learn from our primate relatives, even in things we already excel at, such as cooperation.

Hare has studied the act of sharing in one of our most closely related species, the bonobo. He found that when given the option to share food with a friend or a total stranger, bonobos strongly prefer to share food with a stranger. This contrasts with humans, who prefer to share food with a friend, Hare said, adding that chimpanzees—another of the species most closely related to humans—would most likely eat everything without sharing a piece.

“For bonobos, there’s no cost to sharing with a stranger, but there’s the potential benefit of making a new friend,” Hare noted. “That’s how they think: ‘Why would I want to share with this guy, he’s already my friend, so why not make a new friend?’”

Even in terms of cooperation, there’s still room for improvement in the human species.

“We’re like chimps. Xenophobic,” Hare said. “We should all try to be a little more like bonobos.”