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By Jeffrey Kluger
April 16, 2015

Humans are irrational in a whole lot of ways, but nothing quite compares to our love for our dogs. They provide us neither food nor conversation nor, in most cases, protection. What’s more, they cost us a fortune—a big share of the $60 billion Americans spend on all pets per year goes to the 70 million dogs living in 43 million U.S. households.

But never mind. Dogs and humans have created an improbable bond that is nearly as close as the one we share with our own kind. Now, a study in Science reveals one of the reasons the two species love each other so: the secret, it turns out, is in the eyes.

The average dog spends a lot of its time gazing at it owner adoringly, and owners—whether they know it or not—spend a lot of time gazing back. That’s very different from the way things work with other species—particularly the dog’s close cousin, the wolf—which typically use eye contact as a threat display or a means of domination.

To test the effect of the human-dog gaze, a team of researchers headed by Miho Nagasawa of Japan’s Azabu University conducted a pair of experiments, both of which involved the hormone oxytocin, nicknamed the cuddle chemical because it facilitates bonding in humans and many other species. Oxytocin levels skyrocket in people who are in love and in new parents, and breastfeeding blows the doors off the concentrations of the stuff in the mother’s blood and milk, which means it goes straight to the babies, making them feel the love too.

In the first part of Nagasawa’s study, urine samples were collected from 21 pairs of dogs and owners, both before and after experimental sessions in which the owners petted the dogs, talked to the dogs, and often simply gazed at the dogs. As a control group, 11 pairs of owners and hand-raised wolves also provided samples and also performed the interactions.

Consistently, the oxytocin levels of both the dogs and the humans were higher at the end of the sessions—and usually by about the same percentage for each owner-dog pair. But it was among the pairs in which there was a lot more gazing and a lot less touching and talking that the levels were highest—high enough to cross the threshold of statistical significance. None of this was true in the wolf-human pairs.

“The duration of the dog-to-owner gaze…significantly explained the oxytocin-change ratio,” the investigators wrote.

In the second experiment, the investigators similarly collected before-and-after urine samples from dog-human pairs. But this time, either oxytocin or an inert solution was administered to the dogs nasally before the interactions began. Each dog was then released into a room with its owner and two strangers, and though the dogs typically approached their owners and nuzzled them, the humans were instructed neither to talk to the dogs nor touch them back, but simply to meet their gaze.

Of all the dogs, the females that had received the oxytocin gazed at their owners most—and it was those females’ owners whose oxytocin levels were the highest afterwards. Female dogs, the researchers believe, are simply more susceptible to the effects of oxytocin than males—no surprise since they’re the ones who bear and nurse puppies. To the extent that the males were affected by the intranasal dosing at all, the impact might have been blunted by the mere fact that there were strangers in the room.

“The results of experiment 2 may indicate that male dogs were attending to both their owners and to unfamiliar people as a form of vigilance,” the researchers wrote.

Whatever the explanation for the dogs’ behavior, it’s clear that it works. It’s been many thousands of years since dogs climbed aboard the human caravan—guarding our campfires and protecting our livestock in exchange for food and a warm place to sleep. But as with all good friends, the relationship deepened, and as with all good friends too, the right chemistry—literally—is one of the reasons.

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