The long, loving gazes. The ritualized, often high-pitched, expressions of affection. The heroic self-sacrifice one would readily endure for the other.
What is it about the bond between human and dog that is not like the relationship between parent and child?
Now science offers a new explanation for the similarity. When our dogs gaze into our eyes with that "you are everything to me" look, our bodies — and theirs as well — are flooded with oxytocin, the hormone of love and nurture that cements the bond between people.
In fact, the more that dog owners and their canine companions gazed into each others' eyes, the more concentrated the burst of oxytocin both human and canine produced, Japanese researchers discovered. And the more we humans return a pet's gaze and the greater the resulting surge of oxytocin, the more emphatically we believe ours is the best dog in the whole wide world. (Yes, she is...)
These findings, published in Friday's edition of the journal Science, come courtesy of 30 beloved pets, including golden and Labrador retrievers, miniature schnauzers and dachshunds, standard and toy poodles, and a boxer, border collie, German shepherd and Shetland sheepdog.
For the sake of comparison, the researchers also analyzed the spontaneous interactions between 11 wolves and the animal management professionals who had raised, fed and played with them. In those encounters, they saw neither the mutual gaze (not surprising, since wolves generally lock eyes with other wolves to threaten them) nor the oxytocin surge.
In a separate experiment, the researchers gave dogs a dose of oxytocin before they spent 30 minutes with their owners. The hormone boost increased the number of times and the duration for which female dogs locked eyes with their owners, in turn sending more oxytocin into their owners' blood. The same response was not noted in male dogs.
The findings — "that humans may feel affection for their companion dogs similar to that felt toward human family members," as the researchers put it — are likely to be greeted by dog lovers with a "duh."
But for scientists, the experiments help solve a perplexing evolutionary mystery: How did two species from very different branches of the evolutionary tree come to live together in such close harmony?
The answer is that over eons of co-evolution, dogs probably insinuated themselves into human society by co-opting the behavior and the neural machinery that draw humans together in tight pair-bonds.
The study highlights the power and versatility of oxytocin, said Stanford University neurobiologist Robert M. Sapolsky, who was not involved in the Japanese research. Across many species, the neuropeptide plays a key role in mother-infant bonding. But it's influence doesn't stop there, he said.
"Evolution is a tinkerer, and when a small subset of species came up with the novel business of forming monogamous pair-bonds, the oxytocin system got co-opted to fuel the bonding," Sapolsky said. "When humans and dogs came up with this even stranger, more unique relationship, it looks like oxytocin got co-opted for that as well."
Indeed, oxytocin's impact may help explain why "looking at cute puppies and cute babies activates the same reward system in the brain," Sapolsky said.
That striking similarity may go a long way toward explaining why and how oxytocin could be useful in the treatment of a wide range of neuropsychological problems, according to a pair of animal cognition researchers from Duke University who wrote an essay that accompanied the study.
Service dogs, which are bred and trained to develop particularly powerful bonds with their owners, are proving their worth with patients, wrote Evan L. MacLean and Brian Hare. It's probably no coincidence, the pair suggested, that supplemental oxytocin is also showing promise as a treatment to reduce anxiety in people with post-traumatic stress disorder and helps those on the autism spectrum build social skills.
Bigger studies with more diverse populations of dogs and people will be needed to confirm that pets and service animals can function as furry, four-legged oxytocin dispensers, Hare and MacLean wrote.
In the meantime, they added, the Japanese researchers "have provided more evidence that when your dog is staring at you, she may not just be after your sandwich."