The bond between dogs and their humans is weirdly strong. Despite belonging to entirely different species, we treat them like family, and they really do seem to love us. When they stare at us — and we’re not holding food — it feels real. It feels right.

Now, we’re one step closer to figuring out why that is. As it turns out, when humans gaze into their dogs’ eyes, they experience a rise in oxytocin — a hormone linked to human bonding and some other, less cuddly behavior. Moreover, when dogs stare back at their humans, they experience a similar rise in oxytocin. Taken together, the findings hint that the reaction may have evolved simultaneously in both humans and dogs, as these animals were domesticated. It may also explain why humans and their dogs bond so tightly.

Affection "similar to that felt toward human family members."

"These results suggest that humans may feel affection for their companion dogs similar to that felt toward human family members," the researchers write in the study, published today in Science. That's because when dogs make eye contact with their owners, they trigger biological signals similar to the ones that are activated when a mother looks at her child.

To figure out what role oxytocin plays in dog-human relationships, researchers conducted two experiments. In the first experiment, researchers compared oxytocin levels in urine belonging to 30 dogs and their owners before and after a 30-minute interaction. They also measured how long dogs and owners stared at each other, and how often they touched. Then, they did the same things with pairs of wolves and their owners; the wolves has been raised by humans their entire lives. The idea here was to see if undomesticated animals that are closely related to dogs would lead to the same result.

Credit: AAAS/Carla Schaffer

The researchers found that the dogs and their owners had increased levels of oxytocin in their urine after gazing into each other’s eyes. This didn’t occur in the wolves, or the humans who own them, however. Moreover, the owners whose dogs looked at them the most experienced the largest increase in oxytocin — an increase that their dogs mirrored.

Female dogs were more affected by the drug

To see if there was a causal relationship between the increase in oxytocin seen in the dogs and the increase seen in humans, the researchers conducted a second experiment. This time, they administered oxytocin — or a saline solution devoid of oxytocin — to the noses of a new group of 27 dogs. This didn't affect the male dogs. But in female dogs, researchers saw an increase in the amount of time they spent staring at their owners. Also, the dog owners experienced a larger-than-normal increase in their own oxytocin levels when they interacted with their doped-up female dogs.

The researchers aren’t sure why female dogs were more affected by the oxytocin than the male dogs. This difference wasn’t observed in the first experiment, so it’s possible that female dogs are simply more sensitive to the artificial administration of the drug. There may also be some dog behaviors at work here that we don’t quite understand yet; scientists will have to conduct more experiments like these to figure that out.

One thing to note, however, is that oxytocin hasn’t just been linked to caring, love, and trust. Some studies have linked it to increased envy in humans, and decreased cooperation. So, you probably shouldn’t refer to it as the "love hormone" — as many have in the past. That said, it does appear to play a role in human social interactions in general, regardless of whether the interactions are positive. And if this study stands the test of time, we’ll probably be able to add dog-human bonding to the "love" side of the oxytocin list — which is a pretty ingenious evolutionary phenomenon, when you think about it.

When dogs look at us — and we're not holding food — it feels real

After all, the rise in oxytocin seen in humans when they look at their dog is pretty similar to the rise in oxytocin seen in mothers when they look at their kids. And humans who report feeling very satisfied with their dog-human relationship tend to maintain a mutual gaze with their pet for longer periods. This means that dogs, through domestication, may have figured out a way to insert themselves in a hormonal pathway that was once exclusive to humans. It’s a neat biological and evolutionary trick — one that likely secured the dog’s spot as humanity’s "best friend."