Have you ever gazed into your dog's eyes and sensed a remarkably strong connection – almost as if that little fur ball were your child? The results of a newly published study could help explain why this is — and how dogs evolved from wild wolves into the domesticated companions we know (and love) today.

Photo Credit: Tony Hammond via flickr | CC BY-NC-SA 2.0

In this week's issue of Science, researchers led by Miho Nagasawa of Azabu University report that when dogs and their human owners gaze into each other's eyes, members of both species experience a rise in the hormone oxytocin. The mechanism closely resembles one previously shown to facilitate bonding between human mothers and their infants. It's a fascinating observation for multiple reasons — shedding light not only on the unique social relationships that exist between dogs and humans, but the evolutionary history that we share with our canine companions.

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Like most hormones, oxytocin's biochemical duties are various and complex. About the most general thing that can be said about this neuropeptide, without cramming it too far down the widely hyped "love drug" pigeonhole, is that it often plays a role in social behavior and bonding between members of the same species (a human mother and her child, for instance). But could such a mechanism facilitate bonding between species, as well?

Nagasawa and her team believe it can. "[Our] findings support the existence of an interspecies oxytocin-mediated positive loop facilitated and modulated by gazing," the researchers write, and observations collected from three studies of canine-human interaction lend their hypothesis compelling empirical support.

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In the first experiment, the researchers had 30 dogs and their owners interact one-on-one in a room for thirty minutes. These sessions were recorded with a video camera. The researchers also monitored urinary oxytocin levels in both species, by comparing the concentration of the neuropeptide concentration before and after the interaction. They found the dogs that gazed at their owners the most experienced the greatest spike in urinary oxytocin levels, and that the spike was mirrored in the dogs' owners, as well. (These findings also corroborate observations Nagasawa made during a previous investigation.)

To assess whether the link between mutual gaze and oxytocin release is causal, the researchers conducted a followup experiment. Nagasawa and her colleagues sprayed either oxytocin or saline solution into the noses of a second group of 27 dogs, and placed each pooch in a room with its owner and two strangers. Compared to those that received saline, the female dogs that were administered oxytocin directly spent significantly more time gazing at their owners (for reasons that remain unclear, oxytocin administration to male dogs had no such effect, though the researchers speculate that female dogs may be more sensitive to the hormone):

Tellingly, urinary oxytocin concentration was shown to increase significantly in the owners of the female dogs that had received oxytocin directly, even though the owners, themselves, had not. By controlling for other behavioral variables – e.g. talking to or touching the dogs – the researchers were able to attribute changes in oxytocin levels to the gazing behavior, specifically. In other words, an artificial boost in oxytocin levels in dogs was shown to lead to an increase in human oxytocin levels, as well.

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Taken together, these observations lend strong support to an interspecies chemical loop that unites the brains of humans and their canine companions. "[It's] a powerful mechanism, through which dogs win our hearts—and we win theirs in return," write Evan MacLean and Brian Hare, both of Duke University's Canine Cognition Center, in an editorial that accompanies the study in this week's issue of Science.

Intriguingly, evidence for this mechanism was shown to be missing in wolves. In a third experiment, Nagasawa and her team repeated the first trial in their investigation, only this time they monitored wolves as they interacted with the owners who had reared them from their first days of life. In this study, the researchers write, "the duration of wolf-to-owner gaze did not correlate with the oxytocin change ratio in either owners or wolves, and wolf-to-owner gaze did not explain the oxytocin change ratio in owners and wolves." This finding is not entirely surprising, as eye-contact among wolves is commonly seen as a threat-display.

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But it also hints at a deep evolutionary connection bond between dogs and humans – one that extends tens of thousands of years into the past, back to the early days of canine domestication. As MacLean and Hare put it, dogs, in the course of their co-evolution with our species, appear to have "hijacked" a bonding pathway that is central to one of the deepest connections known to humankind, namely that between a parent and its child.

To say that we love our dogs as much as we love our offspring may sound, to some people, like a bit of a stretch – but a growing body of evidence suggests that we feel for them in remarkably similar ways. And our dogs, it seems, feel the same way.


Contact the author at rtgonzalez@io9.com.