Do you love your dogs like they're your children? There's a reason for that—actually, many reasons, from their cute faces to their affectionate natures.

 In a new study, researchers in Japan find hints of yet one more: When dogs and their owners look into each other's eyes, it raises, in both human and canine, levels of a hormone associated with bonding and social behaviors. In fact, a rise in the hormone in a pet dog is able to trigger a parallel rise in the owner. It's like a positive feedback loop of love.

The study provides evidence that humanity and its best friend might have a truly special bond, built by evolution. That relationship could be based on the same materials—the same hormones, the same processes—that work in certain one-on-one human relationships, like one between a parent and child. "Evolution is notoriously thrifty, often recycling old mechanisms for new purposes," two Duke University researchers write in an essay that the journal Science is publishing today as a companion to its report about the Japanese study.

Humanity and its best friend might have a truly special bond, built by evolution.

To test the idea that dogs and their owners have a love feedback loop, biologists from various universities in Japan performed two experiments. In one, study volunteers and their dogs gave the researchers samples of their urine before and after interacting. Those human-canine pairs who spent the most time gazing into each other's eyes had the steepest rises in their urine levels of the hormone oxytocin. However, the same thing didn't happen among the other test group, who had raised wolves. This suggests the feedback loop evolved sometime after dogs diverged from their wolfish ancestors.

In the second experiment, researchers spritzed oxytocin into the nostrils of dogs. Compared to dogs who got a dose of salt water in the nose instead, oxytocin-ed female dogs looked at their owners significantly longer (although male dogs did not do the same). Afterwards, the owners of female dogs had elevated levels of oxytocin in urine samples, evidence that a dog's oxytocin levels can affect her owner's levels.

Recent research has found that oxytocin is involved in many human relationships. One study, published last year, found that mothers' oxytocin levels rise when they look at their babies. Could the same be happening when dog owners look at their pets? There's other evidence that domestic dogs evolved to work with people. A few studies have found puppies quickly understand what it means when people point with their fingers, whereas wolf pups require extensive training to get the same message. So it's plausible they've evolved around us in other ways as well. Don't think of it as a hijacking of our human parenting urges, however. Remember, their oxytocin levels rise when they look at us too. We're in this together, as we have been for more than 10,000 years.

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