For female baboons, too, it's good to have friends

"Our results suggest that close, stable social relationships have significant reproductive benefits," said Joan Silk of the University of California, Los Angeles. "The data add to a growing body of evidence from humans and other animals that females with a strong, supportive social network are healthier and have greater reproductive success."

Silk said that she and her colleagues were surprised to discover that the effects are at least partially independent of a given female's status in the group. In other words, the benefits of social ties don't just derive from greater competitive ability or greater access to resources for those of higher dominance rank.

"Given that high-ranking females have priority of access to food resources, one might have predicted that rank would be the primary factor influencing longevity," said Dorothy Cheney of the University of Pennsylvania. "Our results indicate instead that the quality of a female's social bonds with other females is more important, suggesting that subordinate females may be able to offset the competitive disadvantage of low rank through their social relationships."

The quality and stability of baboon relationships trump quantity, the researchers say. Females who were able to maintain the same grooming partners from one year to the next lived longer and had more surviving offspring.

Baboons most often form close bonds with relatives, the researchers said, and the stability of those connections is partly a matter of luck since lions and leopards often prey upon baboons. But it appears that isn't the whole story. The researchers found that 80 percent of females that switched close grooming partners from one year to the next did so despite that fact that their earlier companion was still in the group.

Silk and Cheney said they aren't exactly sure how the relationships lead baboons to live longer lives, but it might have something to do with lower stress levels in those with dedicated grooming partners. It's also possible that females with such networks are more likely to be resting and foraging near other animals, making them less vulnerable to predators.

The findings in baboons may lend humans some added insight into our own social lives, the researchers said.

"Having friends is important for us and for female baboons -- and maybe for some of the same reasons," Silk said. "Our findings are strikingly similar to evidence from humans showing that social ties have important effects on our mental and physical health and our longevity. We suspect that the human motivation to form close and lasting friendships has a very long evolutionary history."

The researchers include Joan B. Silk, University of California, Los Angeles, Los Angeles, CA; Jacinta C. Beehner, University of Michigan, Ann Arbor, MI; Thore J. Bergman, University of Michigan, Ann Arbor, MI; Catherine Crockford, University of St. Andrews, St. Andrews, UK; Anne L. Engh, University of Pennsylvania, Philadelphia, PA; Liza R. Moscovice, Binghamton University, Binghamton, NY; Roman M. Wittig, University of St. Andrews, St. Andrews, UK; Robert M. Seyfarth, University of Pennsylvania, Philadelphia, PA; and Dorothy L. Cheney, University of Pennsylvania, Philadelphia, PA.