PHILADELPHIA –- Female baboons who have strong social relationships with other females give birth to offspring who are much more likely to survive to adulthood than baboons reared by less social mothers, according to a new study by researchers at the University of Pennsylvania, the University of California, Los Angeles, and others. 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.
Offspring from the most social mothers turned out to be about one-and-a-half times more likely to survive to adulthood than offspring from the least social mothers.
The findings are significant because "survivorship to reproduction is the gold standard in evolutionary biology," said co-author Dorothy Cheney, professor in the Department of Biology in Penn’s School of Arts and Sciences. “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. In evolutionary terms, social moms are the fittest moms, at least when it comes to baboons."
Additional research is needed to determine how the female bonds improve infant survival, but it may have to do with such stress hormones as cortisol, said Joan Silk, the study's lead author and a UCLA professor of anthropology. Research has shown that prolonged elevations of stress hormones in primates can lead to cardiovascular disease and other serious health problems. Research has also shown that grooming tends to lower these stress hormones in baboons.
"Our research suggests that somehow there is a link between the kind of social relationships you form and the natural, normal stresses that occur in everyday life, and that seems to have -- at least in baboons -- a long-term effect on reproductive success," Silk said.
The strongest social bonds were measured between mothers and adult daughters, followed by sisters and all other potential relationships, including aunts, nieces, cousins and baboons with no familial ties. Bonds between mothers and adult daughters proved to be three times stronger 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 Robert M. Seyfarth, Penn psychology professor.
To hear a Robert Seyfarth lecture on the study, given at the University of Delaware in May 2009, visit http://www.ums.udel.edu/podcast/watch?c=244.
The study, appearing online in the Proceedings of the Royal Society B, was conducted by researchers from Penn, UCLA, the University of Michigan and the University of St. Andrews and analyzed 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.
Conducted in the field by primatologists who tracked the baboons six days a week, 12 months a year from 1 year of age through sexual maturity at age 5, the study analyzed the sex and survival rates of baboon offspring, as well as telling details of the mothers' social lives, including their ranking within the group as measured by the direction of approach/retreat interactions and the amount of social interactions they had with each of the group's other females.
It provides the first direct evidence that social relationships among female baboons convey fitness benefits. In a group of free-ranging baboons, the offspring of females who formed strong social bonds with other females survived at higher rates and lived significantly longer than the offspring of females who formed weaker social bonds. Importantly, these survival benefits were independent of maternal dominance rank and number of kin and extended into offspring adulthood.
These effects held even when researchers controlled for the class of social relationship. In particular, females who formed stronger bonds with their mothers and adult daughters experienced higher offspring-survival rates than females who formed weaker bonds with these relatives. For females lacking mothers or adult daughters, offspring survival was closely linked to bonds between maternal sisters. Although the causes of this higher offspring survival are not yet known, it seems that natural selection has favored females who are able to form strong, long-term bonds with other females.
"If you're a baboon, the strength of your mother's relationship with other females is the best predictor of whether you'll live to have children yourself," Silk said. "The study adds to mounting evidence of the biological benefits of close relationships among females."
Primates are unusual among animals because they establish highly differentiated bonds with other group members. Such bonds are particularly pronounced among females in species like baboons, where females remain in their natal groups throughout their lives and maintain close bonds with their close female relatives. Although females’
relationships with other females seem to confer a number of short-term benefits, the long-term consequences of social bonds among adult females have not been well established.
In humans, greater social integration is generally associated with reduced mortality and better physical and mental health, particularly for women. As with baboons, the strength and quality of these bonds are more important than their number. Similarly, the help that women receive from their own mothers and adult daughters (the “grandmother” effect)
appears to have a significant influence on offspring fitness.
“It therefore seems possible that the capacity and motivation to establish and nurture close social relationships with other females have been under strong selective pressure in the primate lineage for many millions of years,” Cheney said.
The research was funded by the National Geographic Foundation, the Research Foundation of the University of Pennsylvania, the Institute for Research in Cognitive Science at the University of Pennsylvania, the National Institutes of Health and the National Science Foundation.