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Appealing as such scenes of amiable play are, however, most scientists think they fall well short of true friendship. Echoing King, they note the lack of evidence in dogs of the constancy, reciprocity and mutual defense observed in species such as chimpanzees and dolphins. They also point out that dogs evolved from wolves or wolflike mammals, and scientists don't see friendships in wolf packs. Thanks to domestication, dogs have become capable of being sweet and loyal to humans, but it's likely that they treat us more as guardians than friends. Dogs are neither our best friends nor one another's--which is not to say they're not warm and wonderful company all the same.
Studies of animal friendships may deepen our understanding of how complex the nonhuman world is, but there are more tangible benefits as well. The better we understand how friendships change an animal's physiology--improving its health in the process--the more we can learn about the power of those processes in ourselves.
Lauren Brent, a postdoctoral researcher at Duke University, is one of the leaders in this field. Brent conducts her work on a small island off the coast of Puerto Rico called Cayo Santiago that is home to about 1,000 rhesus monkeys. Brent spent four years on Cayo Santiago, carefully observing one 90-monkey group. Once she identified probable friend pairs, she wanted to determine if their relationships influenced their hormone levels--specifically glucocorticoids, which are produced in response to stress. Drawing the monkeys' blood would have been a stressful experience in itself, skewing the results. Fortunately, it's now possible to measure levels of hormones and other molecules from urine and feces.
"You just follow your monkeys around and wait for a sample to be deposited," Brent says. The only trouble came when the monkeys figured out what Brent was up to. They'd sometimes fight her for their feces. "Some of them just get possessive," says Brent. "I have no idea why."
It was worth the battle. Brent found that the amount of glucocorticoids in the rhesus monkeys varied with the strength of their social networks. When monkeys had strong friendships with a few other monkeys, their glucocorticoid levels were low. Less sociable types had higher readings. Seyfarth and his colleagues found similar results in baboons. When members of that species lose close family members, their glucocorticoids soar. They respond by making new friendships with other baboons, offering to groom them and perform other favors. Soon their hormone levels fall to normal. Research on nonprimates also lines up with these findings. In studies of domesticated horses outfitted with sensors, researchers found that when friends groom each other, their heart rate slows. Wells plans to study hormones in dolphins by taking tiny skin samples from them.