In the baboon world, female bonding can lead to a greater chance of survival.

According to a joint Penn and University of California, Los Angeles study, female baboons who have strong social relationships with other females give birth to offspring who are one-and-a-half times more likely to survive to adulthood than baboons reared by less social mothers. The results support a growing body of research on humans — especially women — indicating that strong social networks are crucially important to health and reduced stress.

“Females who raise offspring to a reproductive age are more likely see their genes pass along, so these findings demonstrate an evolutionary advantage to strong relationships with other females,” according to researcher Dorothy Cheney, biology professor in Penn’s School of Arts and Sciences. “In evolutionary terms, social moms are the fittest moms, at least when it comes to baboons."

The improved survival rate may be linked to daily activities such as grooming, which lowers the cortisol, or stress hormones, in baboons.

Cheney worked with other researchers, including Penn professor of psychology Robert M. Seyfarth, to look at 17 years of records on more than 66 adult female baboons in the Moremi Game Reserve, a 2,000-square-mile national park in Botswana. They found the strongest social bonds exist between mothers and adult daughters—which are three times greater than those between sisters, and 10 times stronger than relationships with other females.

"The benefit comes not from being wildly social. It's about having close social bonds," said Cheney, who runs the Moremi baboon-tracking project with Seyfarth.

This study provides the first direct evidence that social relationships among female baboons convey fitness benefits.