Courtesy of Rachel Morrison

For Some Primates, Survival of the Nicest

Baboons, like people, really do get by with a little help from their friends. Humans with strong social ties live longer, healthier lives, whereas hostility and "loner" tendencies can set the stage for disease and early death. In animals, too, strong social networks contribute to longer lives and healthier offspring—and now it seems that personality may be just as big a factor in other primates' longevity status. A new study found that female baboons that had the most stable relationships with other females weren't always the highest up in the dominance hierarchy or the ones with close kin around—but they were the nicest.

Scientists are increasingly seeing personality as a key factor in an animal's ability to survive, adapt, and thrive in its environment. But this topic isn't an easy one to study scientifically, says primatologist Dorothy Cheney of the University of Pennsylvania. "Research in mammals, birds, fish, and insects shows individual patterns of behavior that can't be easily explained. But the many studies of personality are based on human traits like conscientiousness, agreeableness, or neuroticism. It isn't clear how to apply those traits to animals," Cheney says.

Along with a group of scientists—including co-authors Robert Seyfarth, also at the University of Pennsylvania, and primatologist Joan Silk of Arizona State University, Tempe—Cheney has studied wild baboons at the Moremi Game Reserve in Botswana for almost 20 years. Besides providing detailed, long-term observations of behavior in several generations of baboons, the research has yielded a wealth of biological and genetic information.

In previously published research, Cheney and co-workers showed that females lived longer, had lower stress hormone levels, and had more surviving offspring when they had close, long-lasting relationships with other females (characterized chiefly by spending time together and grooming). Although dominance rank was significant for male baboons—alpha male baboons may live longer than lower-ranking males—this wasn't true for the females. Nor was an abundance of kin the key to longevity. Not all of the longer-lived, less-stressed females had large families.

To find out more about how female baboons forge bonds, Cheney and co-authors focused on detailed records of observations of 45 female baboons from 2001 to 2007. As a personality gauge, the researchers used specific behaviors, including how often the females were alone, how often they touched other females, how often they behaved aggressively, how often they were approached by others, and how often they grunted when approaching other females of various ranks. Among female baboons, grunting is a sign of good will, Cheney says. Using these criteria, the researchers characterized the baboons as "nice," "aloof," or "loner." The team also tested the baboons for levels of stress hormones known as glucocorticoids.

The researchers took particular note of how often a female grunted at a lower-ranking female that didn't have an infant. "Female baboons are besotted with babies -- they love to look at them and touch them," Cheney says. The researchers assumed that a female grunting at a lower-ranking animal with no baby had nothing to gain, and, therefore, must just be nice.

Females who scored high on the "nice" meter were friendly to all females, the researchers report online today in the Proceedings of the National Academy of Sciences. They were approached most often by other females, were most sociable in general, and had stable relationships. "Aloof" females, though less sociable, also had stable relationships. Not surprisingly, "loner" animals were most often left alone and their partner relationships were less stable. They also had strikingly higher glucocorticoid levels than did the other two groups, suggesting that stress takes a greater toll on the less social. "This is a highly innovative study," says anthropologist Sarah Hrdy of the University of California, Davis. "It uses behavioral measures that are meaningful to the baboons themselves to probe the relationship between fitness and personality style." Hrdy says the paper clarifies previous work by these and other authors showing that close social bonds—"friendships, if you want to call them that," she says—help ensure the survival of a female's offspring as well as her own longevity.