The more time wild female baboons spend in the company of other adult baboons, particularly while occupied with grooming activities, the more likely their offspring are to live until their first birthday, the team reports in the Nov. 14 issue of Science.
"Until now, social scientists assumed that because females invest a lot in social relationships, they must gain a lot from those relationships, but we've never been able to make a direct link to reproductive success," said Joan B. Silk, the study's lead author. "These findings provide the first evidence that there's a link between the amount of social involvement and having offspring who survive the critical first year of life."
The connection is noteworthy because "reproductive success is the gold standard in evolutionary biology," said Silk, an anthropology professor in UCLA College. "A trait can't really be determined to have an evolutionary advantage unless it has a positive impact on reproductive performance. Socializing and grooming are traits that help baboons pass along their genes."
Along with evolutionary biologists from Duke and Princeton Universities, Silk pored over 16 years of data collected in Kenya's Amboseli Basin, which is located at the foot of Mt. Kilimanjaro. Since 1984, researchers have measured the social behavior of more than 100 wild savannah baboon females, recording daytime activities six days a week in 10-minute intervals.
Among the resulting 34,000 records of discrete activities, Silk's team looked for examples of social behavior among female baboons, including the propensity to spend time within at least 15 feet of another adult and to groom other females or to allow other females to pick dirt, ticks and other parasites from their own hair.
The researchers then studied each baboon's reproductive history, including all pregnancies, births and deaths.
The most social females enjoyed a reproductive success rate that was about one-third higher than the least social females, the team found.
"It's increasingly apparent that social skills are of great importance in the evolution of primates," said Mark L. Weiss, a program director at the National Science Foundation, which helped fund the research. "These researchers have not only demonstrated just how important it is for mothers to be social, they also demonstrate the great importance of supporting long investigations of natural populations so that we can appreciate the long-term consequences of the animals' activities."
The gregarious primates live in large, mixed-sex groups with clearly defined dominance hierarchies, or pecking orders. In many primate groups, high rank enhances reproductive success. Nevertheless, the researchers found that strong social bonds improved infant survival, regardless of the mother's position in the pecking order.
Because droughts and other ecological conditions also affect reproductive performance, the team took care to compare females living in the same habitat and at the same time.
"We don't exactly know how sociality helps females, but our data make it very clear that females who are more social have higher infant survival," said Susan C. Alberts, an assistant professor of biology at Duke University.
Additional research is needed to determine just why social interaction improves infant survival rates, but the team speculates that the proximity of company may deter predators. More elusive benefits may also be at work said the primatologists, citing research that has shown the value of social support for humans.
"Social isolation increases the risk of disease, accidents and a range of mental disorders, and the disruption of social ties due to death, divorce or separation is a major source of stress," the study notes.
Although infant survival rates in humans have yet to be linked directly with social factors, research has shown that low-income women with extensive social networks give birth to heavier infants -- a key marker of viability -- than their more isolated peers.
It's not clear whether social interaction provides baboons the same benefits as humans, the researchers said. But research has shown that social integration among male baboons in Amboseli reduces basal levels of the hormone cortisol, a key marker of stress. Grooming has also been shown to reduce heart rate in some monkey species.
Nevertheless, the amount of time that baboons spend grooming -- some devote as much as 10 percent of the day to the activity -- is striking, particularly given the demands of life in the wild. Socializing with males may offer direct advantages, because some close male associates tend to shield females from harassment, support their offspring in conflicts with others and even protect infants from predators.
"We take it for granted that women socialize, but when you stop and think about what it means to be social from a biological point of view, the benefit isn't clear," Silk said. "But in nature, animals compete with one another, and baboons have a really tough life. They spend most of their time of walking from one food site to another, eating as much as they can as quickly as they can, which leaves them with little free time. So if they place a premium on social interactions, there has to be some kind of benefit."
Since baboons share a long evolutionary history with humans, their behavior is thought to provide a window into human nature, particularly among human ancestors.
"In baboon social interaction, we see the roots of the human inclination to come together in families and stable communities," Silk said. "Being social seems to go back very far back in our evolutionary history."
The third author was Princeton evolutionary biologist Jeanne Altmann, who co-directs the Amboseli Baboon Research Project with Alberts.
In addition to the National Science Foundation, the project received support from the Leakey Foundation, National Geographic Society and Brookfield Zoo.