Monkey's Best Friend

Prudy is one of the most popular baboons in her group. When her fellow monkeys pass by, they raise their tails in deference. When her fur grows dusty there's always a volunteer to give it a good grooming. For eight years, she's even had that rare thing in the baboon world: a steady male companion. Rocky would carry her children on his back and accompany her on foraging expeditions through the savannah at the foot of Kenya's Mount Kilimanjaro. Even though Rocky left the group in August, Prudy still has female friends and relatives she can count on. In 23 years, she's given birth to 11 kids, of which eight have survived.

Viva, a female from a nearby group, is a social pariah. When she approaches her fellow baboons, they often lunge at her threateningly or raise their eyebrows, flashing the pale pigment of their eyelids, to scare her off. She's also been less fortunate in childbirth. In 18 years, she's delivered eight children, but only four have survived.

One of the biggest cliches in psychology is that in times of crisis, women turn to friends, while men either withdraw or curse and gesticulate. The male response--a manifestation of the "fight or flight" survival instinct--has been rigorously studied. A paper published last week in the journal Science now sheds some light on the female response. Scientists studying baboon populations have found that the mothers who develop the most elaborate social networks tend to raise children with lower mortality rates. The findings suggest that a woman's social network is more than just a diversion: it's basic to the survival of the species.

The study focused on 108 female baboons in Kenya's Amboseli basin, who had been observed continually from 1984 to 1999. The three scientists who authored the report--Joan Silk of the University of California, Los Angeles; Susan Alberts of Duke University and Jeanne Altmann of Princeton University--were part of a group that studied the baboons in their natural habitat, living in tents and following them on foot and by car from sunrise to sundown. They were struck in general by how much time female baboons spend socializing--about 10 percent of their day. "It's a very expensive activity," says Silk. Baboons need all the time and energy they can muster to forage for food, care for their offspring and guard against predators. "The phenomenon begs the question of why evolution constructed an organism that's so sensitive to social contact," says Alberts. "Ultimately, it has to be because it affects Darwinian fitness. It affects how well you propagate your lineage."

Knowing that socializing plays a role in the evolution of a species is one thing, but exactly how it works remains a mystery. Silk and her colleagues have several guesses: proximity to fellow baboons is known to discourage predators, especially if a female teams up with a male. And because most of baboon socializing involves grooming--picking dirt or insects out of each other's fur--it may promote better health. (A lonely baboon will self-groom, but there are always those hard-to-reach areas, like the back and ears.)

The leading theory, though, is that socialization relieves stress. Data gathered on baboons in captivity have shown that grooming reduces the animals' heart rate and promotes the release of endorphins. Human studies have also linked social ties and good health. Chris Dunkel Schetter, a psychologist at UCLA, has found a correlation between the social network of an expectant mother and the birth weight of her baby. "The more support a woman receives, the more healthy her pregnancy is," she says. Friends and family will pitch in with everything from helpful advice to transportation and money for prenatal visits. But Robert Sapolsky, a Stanford anthropologist and author of "Why Zebras Don't Get Ulcers," says the benefits of friendship go far beyond material support. "The sociologists who think about relationships use the term 'social capital.' And they use it in the sense of: OK, how many people in an emergency can you borrow money from or have watch your kids? How many people would you trust with the keys to your house? That's a collective measure of stability, and there's an enormous literature that shows every aspect of health gets better as a result of more social capital." Scientists may need more time to nail down exactly how socializing confers an evolutionary advantage. In the meantime, let's just assume that meeting friends for a drink is a biological necessity.