No wonder our canine companions often seem like part of the family—dogs have evolved to hijack the same mechanisms in our brains that create the strongest social bonds, including those between mother and child. This powerful example of interspecies affection is fueled when dogs and humans gaze into each other's eyes, a new study shows.

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Those loving looks cause both dog and human brains to secrete the hormone oxytocin, which has previously been linked to strengthening emotional bonds between mothers and babies and between other mammal pairs. This study is the first to show oxytocin at work in both members of different species, and it suggests that the effect co-evolved over the long history of dog domestication.

"Dogs successfully coexist with humans because they have adapted the bonding mechanism [used in] relations with humans,” author Miho Nagasawa, a researcher at Japan's Azabu University, says in a video statement released with the study. “On the other hand, humans also likely went through some sort of evolution that allowed them to bond with another species.” The human-dog bond may even be a unique relationship, the team says. Wolves, the closest relatives to dogs, do not share the same behaviors or brain responses with people—even when those wolves were raised by humans.

When human mothers and children gaze at each other, each of their brains secrets the hormone oxytocin, which has been linked to maternal bonding and other trust relationships. Similarly, rodent studies show that a pup's attachment behavior leads to oxytocin release in its mother's brain, which then leads to nurturing behaviors from mom. That in turn causes the pup to secrete more oxytocin, which drives more attachment behavior, fueling a positive feedback loop.

To find out whether the hormone was behind the sometimes parent-like relationships we have with pets, Nagasawa and her colleagues ran a series of experiments, described this week in the journal Science.

In one experiment, 30 dogs of various breeds were left in a room with their owners to interact freely for 30 minutes. The dogs that gazed at their owners longer showed increased oxytocin levels in their urine at the end of that period. Intriguingly, so did their owners. This suggests the existence of a similar oxytocin feedback loop, in which dogs' gazing behavior caused their owners' brains to secrete more oxytocin, which led those owners to interact more with their dogs, which in turn facilitated more oxytocin secretion in the dogs' brains.

In a second experiment, 27 dogs were administered oxytocin via nasal spray, while another group was given saline spray. The dogs were then allowed to move freely about a room with their owners and two unfamiliar persons. Hormone-dosed female dogs gazed longer at their owners than those with salty noses, the study shows. Intriguingly, the oxytocin levels of the dosed dogs' owners also rose even though the humans hadn't been administered any of the hormone.

“It's thought that [administering the oxytocin] enhanced female dogs' gazing behavior, and as a result their owners' oxytocin secretions also increased,” Nagasawa explains to Smithsonian. “This points to the existence of a positive feedback loop.”

When a female dog was sprayed with oxytocin (left), she gazed longer toward her owner, and the owner's oxytocin levels increased as a result. Credit: Miho Nagasawa

However, the application of nasal oxytocin didn't increase gazing in male dogs in the second experiment with strangers present. The hormone has also been tied to canine awareness and aggression towards unfamiliar people—while the male dogs didn't get aggressive, it's possible their increased awareness of strangers balanced out their urge to gaze.

“The effects of the oxytocin seem to depend on the person that the dog is with,” Nagasawa says. “So if a stranger is present perhaps the increased awareness behavior comes first in males, and the friendly, gazing behavior effects of oxytocin will be seen mostly with their owners.”

The compelling results are consistent with what scientists know about oxytocin's role in other social relations, like mother-infant or pair bonding, says Emory University's Larry Young, who wasn't affiliated with the study. “The neural mechanisms present in all mammal species to promote the mother-infant bond have also been adapted to regulate bonding between mates,” he notes. “Our work in prairie voles is an example. Oxytocin promotes the pair bond between monogamous mates. So it makes sense that this same mechanism could also be shaped during co-evolution of species where interspecies bonding occurs."

Nagasawa's team also tried the same tests with wolves, and the results showed that this phenomenon was shared only between humans and dogs. Even wolves that had been raised by humans, just as the dogs had, did not communicate by eye gazing and did not experience an oxytocin feedback loop. This strongly suggests that these behaviors were also absent in dog ancestors and only appeared at some point in their later evolutionary history.

“I think that it is a very exciting evolutionary story as well,” Young says.

Over the course of domestication, "there was likely strong selection for dogs that could elicit a bond with the owner and become bonded to a human owner. Evolution took the easy route and used the neural mechanisms already in place to create mother-infant bonds, tweaked them slightly, perhaps through neoteny, or preservation of infant-like traits into adulthood.”

The research could prove a boon to scientists studying the origins and evolution of dogs and perhaps those looking at human civilizations. But Nagasawa hopes that it can have at least one immediate benefit for people who live with family dogs every day: “Many people think that they have to teach a dog everything and take total control of a dog,” she notes. “But our research shows that dogs are able to be friends with humans very naturally.”