Mares who form stronger social bonds produce more and healthier offspring, according to a new study. The finding adds to the growing evidence that friendship is an adaptation with deep evolutionary roots.
Numerous human studies, especially of women, have found that friendships lead to better health--and healthier babies. The effect seems to also hold for other animals: In 2003, a research team led by anthropologist Joan Silk of the University of California, Los Angeles, reported that female baboons with close social ties to unrelated females produce infants that survive longer. Similar effects have even been claimed for house mice, but it's been unclear how widespread the benefits of friendship are to nonprimates.
The new study, led by zoologist Elissa Cameron of the University of Pretoria in South Africa, focused on 55 adult mares in the hills of New Zealand's North Island. The mares are part of a group that has run wild since the mid-1800s, and they all belong to the same "band"--a stable breeding group consisting of one to four stallions and many more unrelated mares and their offspring.
Cameron and her co-workers observed the mares, which they could individually identify by facial markings and other signs. For nearly 3 years, they gathered evidence of social bonding such as mares grooming each other or coming into close contact (less than two body lengths away). The team also noted "harassment" of the mares by stallions apparently asserting their sexual dominance, which earlier work had suggested diminishes reproductive success.
The team racked up more than 2000 hours of observations, averaging 38 hours per horse. Then the researchers calculated a "composite social-integration score" for each mare. Social bonding and reproductive success were highly correlated: Mares with the weakest social ties had about half as many surviving foals as those that were the most sociable, the researchers report online this week in the Proceedings of the National Academy of Sciences. Mares with higher social-bonding scores also suffered less harassment from stallions. The authors speculate that solidarity might increase a mare's reproductive success by deterring harassment.
Silk says the findings support the idea that friendship can be important to survival. "Natural selection is an efficient process, so it would be surprising if animals devoted substantial time to activities like social grooming if they were not related to reproductive success," she says. As for why friendship might have evolved in the first place, Silk and other researchers have suggested that it might have helped animals band together to avoid predators. It may also reduce stress in pregnant mothers, which has been shown to adversely affect the growth rates and survival of their offspring.