If you think of your dog as your “fur baby,” science has your back. New research shows that when our canine pals stare into our eyes, they activate the same hormonal response that bonds us to human infants. The study—the first to show this hormonal bonding effect between humans and another species—may help explain how dogs became our companions thousands of years ago.

“It’s an incredible finding that suggests that dogs have hijacked the human bonding system,” says Brian Hare, an expert on canine cognition at Duke University in Durham, North Carolina, who was not involved in the work. Hare says the discovery might lead to a better understanding of why service dogs are so helpful for people with autism and post-traumatic stress disorder. “A finding of this magnitude will need to be replicated because it potentially has such far-reaching implications.”

Dogs are already renowned for their ability to interact with humans. It’s not just the walks and the Frisbee catching; canines seem to understand us in a way that no other animal does. Point at an object, for example, and a dog will look at where you’re pointing—an intuitive reading of our intentions (“I’m trying to show you something”) that confounds our closest relatives: chimpanzees. People and dogs also look into each other’s eyes while interacting—a sign of understanding and affection that dogs’ closest relatives, wolves, interpret as hostility.

It was this mutual gazing that piqued the interest of Takefumi Kikusui, an animal behaviorist at Azabu University in Sagamihara, Japan. Kikusui’s lab studies oxytocin, a hormone that plays a role in maternal bonding, trust, and altruism. Other groups have shown that when a mother stares into her baby’s eyes, the baby’s oxytocin levels rise, which causes the infant to stare back into its mother’s eyes, which causes the mother to release more oxytocin, and so on. This positive feedback loop seems to create a strong emotional bond between mother and child during a time when the baby can’t express itself in other ways.

Kikusui—a dog owner for more than 15 years—wondered if the same held true for canines. “I love my dogs, and I always feel that they’re more of a partner than a pet,” he says. “So I started wondering, ‘Why are they so close to humans? Why are they connected so tightly to us?’ ”

Kikusui and his colleagues convinced 30 of their friends and neighbors to bring their pets into his lab. They also found and reached out to a few people who were raising wolves as pets. When each owner brought his or her animal into the lab, the researchers collected urine from both and then asked the owners to interact with their animal in a room together for 30 minutes. During this time, the owners typically petted their animals and talked to them. Dogs and their owners also gazed into each other’s eyes, some for a total of a couple of minutes, some for just a few seconds. (The wolves, not surprisingly, didn’t make much eye contact with their owners.) After the time was up, the team took urine samples again.

Mutual gazing had a profound effect on both the dogs and their owners. Of the duos that had spent the greatest amount of time looking into each other’s eyes, both male and female dogs experienced a 130% rise in oxytocin levels, and both male and female owners a 300% increase. (Kikusui was one of them, participating in the experiment himself with his two standard poodles, Anita and Jasmine.) The scientists saw no oxytocin increase in the dogs and owners who had spent little time gazing at each other, or in any of the wolf-owner duos.

In a second experiment, the team repeated the same essential procedure, except this time they gave the dogs a nasal spray of oxytocin before they interacted with their owners. There were also no wolves this time around. “It would be very, very dangerous to give a nasal spray to a wolf,” Kikusui laughs. Female dogs given the nasal spray spent 150% more time gazing into the eyes of their owners, who in turn saw a 300% spike in their oxytocin levels. No effect was seen in male dogs or in dogs given a nasal spray that contained only saline.

The results suggest that human-dog interactions elicit the same type of oxytocin positive feedback loop as seen between mothers and their infants, the team reports online today in Science. And that, in turn, may explain why we feel so close to our dogs, and vice versa. Kikusui says the nasal spray may have affected only female dogs because oxytocin plays a greater role in female reproduction, being important during labor and lactation.

This positive feedback loop, he says, may have played a critical role in dog domestication. As wolves were morphing into dogs, only those that could bond with humans would have received care and protection. And humans themselves may have evolved the ability to reciprocate, adapting the maternal bonding feedback loop to a new species. “That’s our biggest speculation,” says Kikusui, who suggests that because oxytocin decreases anxiety, the adaptation may have been important for human survival as well. “If human beings are less stressed out, it’s better for their health.”

“I definitely think oxytocin was involved in domestication,” says Jessica Oliva, a Ph.D. student at Monash University in Melbourne, Australia, whose work recently showed that the hormone enhances the ability of dogs to understand human pointing. Still, she says, mutual gazing doesn’t happen in a vacuum; most of these dogs probably associate the behavior with food and playing, both of which can also boost oxytocin levels. So although we may view our dogs as our babies, they don’t necessarily view us as their mothers. We may just be cool friends who give them an occasional massage.

For more on the origin of the human-dog bond, check out a story in this week’s Science about solving the mystery of canine domestication.

For more on man's best friend, see Science's latest coverage of doggy science.