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Dogs Get Teary Eyes When Reunited With Owners, Study Says

I'm not crying, you're crying.

A smiling human bends to greet a tall gray poodle. They both look very happy.
A happy, teary scene.
Madoka Nakamura

Is someone chopping onions in here? No, it might just be that you got back together with your dog after being away. 

A team of researchers investigated an interesting phenomenon one of them noticed: a dog that got teary eyes when nursing her puppies. That observation led to a study published in the journal Current Biology on Monday that looks at dog tears when the canines reunite with their beloved humans.

"We found that dogs shed tears associated with positive emotions," co-author Takefumi Kikusui of Azabu University in Japan said in a statement on Monday. "We also made the discovery of oxytocin as a possible mechanism underlying it." Oxytocin is known as the "love hormone." In humans, it's connected to bonding activities, like hugging.

Scientists already knew dogs and their people experience a release of oxytocin when they interact, so they worked out an experiment involving 18 dogs.

The team measured tear volume in dogs before and after reunions with their owners and compared that data to when the dogs met an unfamiliar nonowner instead. The tear volume went up during the owner reunifications, but not when the dogs got together with the nonowners. Additional work found that adding oxytocin to a dog's eyes increased tear volume.

The researchers also had humans rate responses to pictures of dogs and found that humans had more positive responses to dogs with teary eyes. "These results suggest that emotion-elicited tears can facilitate human-dog emotional connections," the study said.

The sample size was small, and it doesn't necessarily mean dogs are experiencing the same sort of emotions humans do. 

The research leads to some questions. Do dogs also get teary in response to negative emotions? Do they get teary eyes when they meet up with canine friends? Those are subjects for future studies. Said Kikusui, "We had never heard of the discovery that animals shed tears in joyful situations, such as reuniting with their owners, and we were all excited that this would be a world first."