(4 of 7)
Silk looked for a similar reproductive benefit among the Amboseli baboons. She ran a new analysis, comparing the number of offspring a female had with her number of friendships. Here too there was a statistical baby bump. While female baboons with strong friendships were not necessarily likely to produce more young, the offspring they had were likelier to stay alive than the babies of females with shallower friendships.
The mechanism behind this wasn't clear, so Silk decided to team up with Robert Seyfarth, a primatologist at the University of Pennsylvania, and his wife Dorothy Cheney, who have studied friendships among chacma baboons in Botswana. For this study, the scientists looked at the longevity of the friendly adults. On average, they found, the survival rate to age 15 for female baboons with strong friendships is four times as high as that of those with weak ones. Long-lived mothers should increase the odds, at least in theory, for long-lived babies.
Silk's research has spurred other scientists to see what effects friendships have in other species. In New Zealand, Elissa Cameron of the University of Tasmania studies a population of 400 feral horses in the Kaimanawa Mountains. The horses live in bands that are typically not made up of close relatives. Sometimes the horses are aggressive. One might bite another or chase it away. But they can be sociable too. They run around together playfully. They use their teeth not to bite but to groom each other's manes. "Sometimes they stand with their heads resting on each other," says Cameron.
After collecting four years of data, she went through her records. She found that pairs of mares would establish strong bonds, and those bonds endured throughout her study. Cameron then did what Silk had done: she compared the strength of a mare's friendships to her reproductive success. And similar to Silk, she discovered that the more close friends a mare had, the more foals she could rear.
Never Mind the Genes
The principal explanation biologists always had for social behavior between unrelated animals is the favor-for-favor arrangement of reciprocal altruism. This would be particularly true among males, which don't have such a heavy investment in raising long-lived babies and thus would expect more immediate payback. There's little question that this plays a powerful role. But Seyfarth doesn't think animal friendship can be reduced to just a marketplace of immediate favors.
"In chimps, if you study them over a short period, you'd see a bit of meat sharing, a bit of cooperation on forming alliances," he says. But if you look at chimpanzee pairs that have established friendships, these favors are separated by long periods of time. "There are often many days or weeks that pass in between successive acts, so they can't be done for immediate benefit. Over six months, it's much more balanced, and over two years, it's more balanced still. Animals are happy to tolerate a temporary imbalance because what matters is the long-term relationship."