Maybe it’s a tight hug from a loved one you haven’t seen in years, or kindness from a stranger after a ridiculously difficult day. Whatever the happy occasion, we’ve all shed a couple of joyful tears.
But Japanese researchers have found that this emotive response isn’t entirely exclusive to humans. In fact, canines cry tears of joy too – when reunited with their owners.
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Dogs, like humans, have tear ducts that well up to keep their eyes clean. But those tears haven’t been linked to emotion until now. The study published on Monday in the science journal Current Biology would be the first evidence that emotions cause tears in nonhuman animals.
“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!” Takefumi Kikusui, one of the coauthors of the study and a professor at the Laboratory of Human-Animal Interaction and Reciprocity at Azabu University, said in a press release.
Hirofumi Ito, who owns a Shiba Inu named Mame, said it made his pet even cuter, if that was possible. He noticed Mame would have wet eyes when he’d come back home from weeks-long vacations. “So it’s nice to know that he’s just really happy to see me,” he told VICE World News.
Kikusui first made the discovery about six years ago, when his standard poodle had puppies. He noticed that there were tears in his poodle’s eyes and though they didn’t bawl, there was no mistaking the wet eyes.
“That gave me the idea that oxytocin might increase tears,” Kikusui said, referring to a hormone that’s responsible for some of the positive emotions you feel during attraction and is also known as the “love drug.”
To research their findings, Kikusui and his team first studied the volume of tears produced by 18 dogs when at home with their owners using the Schirmer Tear Test. This entailed placing a piece of paper for one minute inside the dogs’ eyelids. More moisture on the paper meant more tears.
Then, the scientists compared this volume of tears with the amount produced within the first five minutes when a dog was reunited with its owner, after five to seven hours of separation. Researchers found that the pets produced about 10 percent more tears when they were reunited.
What’s more, when the scientists contrasted this data with the volume of tears produced by 20 dogs when they were reunited with a human that wasn’t their owner, there was no increase.
To test whether the oxytocin had any significance, the team put a solution containing the hormone in the dogs’ eyes. They found that the amount of tears significantly increased.
In past studies, it’s been proven that dogs recognize human emotions. Using visual and auditory cues from humans, dogs can determine how to respond to a situation.
Though the scientists have yet to determine the social function of dog tears and whether dogs cry when reunited with other dogs, the study nonetheless proves that dogs are indeed people’s partners, Kikusui said.
