The Washington PostDemocracy Dies in Darkness

What making puppy eyes with your dog says about evolution

You and your favorite dog may be locked in an oxytocin feedback loop -- a chemical boost for the both of you that leaves both parties better at bonding and reading social cues -- every time you lock eyes. And it's no mistake, either: Scientists think this is one of the evolutionary turns that took modern, domesticated dogs away from their wild wolf ancestors. Their findings were reported in a study published Thursday in Science.

Oxytocin isn't the "moral molecule" that some have sold it as, but this hormone is definitely important. Researchers are pretty sure that it has something to do with pair bonding, particularly between mothers and infants. A study published earlier this week found that oxytocin causes virgin female mice to respond to the cries of unrelated young the way a mother would, suggesting that it might make us recognize stimuli that might otherwise be ignored.

"Rather than virtue incarnate, oxytocin is more of an all-purpose social molecule," Ed Yong wrote for his blog at National Geographic. "It probably acts as a spotlight that draws our attention to social cues. We then react differently, depending on our temperament, or whether those social cues are positive or threatening."

Now it seems that dogs and humans may benefit from the chemical in a similar way. In the new study, researchers found that dog owners and their dogs had higher levels of oxytocin in their urine after sustaining eye contact with each other. Wolves that were raised as pets didn't show the same boost when interacting with their owners. The study also found that exposure to oxytocin spray before interactions with humans led to more eye contact from dogs (and higher oxytocin levels in the humans gazing back at them), but this only held true for female dogs, and the researchers aren't sure why.

In the same way that dogs are known to respond to pointing in ways that wolves do not -- understanding the gesture as a cue from humans and following it accordingly -- it could be that they've developed the ability to gain meaning, both practical and emotional, from eye contact. For humans, making eye contact is a valuable social tool, and one that promotes bonding between parent and child. It may be that puppies are using this mechanism as a shortcut to our hearts.

[You really do love your dog like it’s a baby]

"The origin of the function of oxytocin is breast feeding and nurturing," study author Takefumi Kikusui of Azabu University told The Post. "However, in our study we demonstrated that a non-reproductive relationship -- a human and dog -- show the same positive loop. This is beyond the expected function of oxytocin."

Kikusui and his colleagues believe that this feedback loop explains the warm feelings humans have towards their dogs, and the animals' ability to relieve stress.

They suspect that some dog ancestors were able to interact with humans by way of eye gaze -- something not many cross-species pairs can manage -- and that over time we selected for ones that were able to bond with us this way.

"There's a possibility that dogs cleverly and unknowingly utilize a natural system meant for bonding a parent with his or her child," Kikusui said. "On the other hand, humans are also unique in using eye gaze with other species, namely dogs, which suggests that both humans and dogs co-evolved during this process and both of them can obtain the advantages of this bonding."

Want more animals? Give these a click:

Study finds that dogs can be pessimists

Meet Derby, the dog who runs on 3-D printed legs

What the history of dogs tells us about civilization in the Americas

Scientists investigate the cat genome to see how wildcats turned into lazy furballs