Want to live a long life? Have lots of friends. Studies in humans have made clear that people with stronger social networks have greater longevity. Now a new analysis shows the same is true for baboons. The research adds to growing evidence that friendship is an adaptation with deep evolutionary roots.
Little is known about how social bonds influence longevity in nonhuman animals, in part because tracking animal relationships over many years is very difficult. Nevertheless, recent evidence shows that social bonding enhances reproductive success, an important indicator of evolutionary fitness. A study last year of female horses, led by Elissa Cameron, a zoologist at the University of Pretoria, showed that mares with the weakest social ties had about half as many surviving foals as those who were most sociable . And in 2006, a team led by University of California, Los Angeles, anthropologist Joan Silk reported that the infants of female baboons with close social ties to unrelated females survive longer  than those that do not have such ties.
In the new work, a team led by Silk looked at the correlation between social bonding and longevity, another important indicator of fitness. Silk studied wild baboons in Botswana's Moremi Game Reserve, teaming up with a long-term project led by University of Pennsylvania biologist Dorothy Cheney and psychologist Robert Seyfarth. From 2001 to 2007, the researchers closely watched 44 female baboons, recording how often they approached each other, how long they groomed each other, and other measures of social interaction. (The researchers looked at females because, in many species, only females form these kinds of social bonds, whereas males are off doing other things and are competitive with each other rather than cooperative.) From these data, the researchers determined each baboon's top three partners in any given year. Thus the team could estimate the strength of each baboon's relationships with its closest partners over the years and the extent to which each baboon stuck to her best buddies.
Silk and colleagues report  online today in Current Biology that females who had the strongest, most stable, and longest-lasting relationships with other baboons lived significantly longer than those whose social ties were more fragile and unpredictable. To illustrate their findings, the researchers divided the baboons into three groups according to the quality of their relationships with others. Members of the least sociable group lived from about 7 to 18 years; the middle group lived from about 9 to 25 years; and the friendliest group lived from 10 years on up, as some were still alive when the study ended.
Such findings in a nonhuman primate, the authors write, "suggest that the human motivation to form close and enduring bonds has a long evolutionary history." The researchers speculate that friendship helps buffer the effects of stress and boost physiological repair mechanisms. They note that previous research in humans has shown that socially isolated people suffer more from high blood pressure and sleep disorders and have longer wound-healing times.
This and the other recent studies, Cameron says, "provide us with huge insights into the evolutionary significance of human friendship and of social bonds" in other animals.