Dogs' eyes well up with tears of joy when reunited with their owners, study suggests – USA TODAY

You knew your dog gets happy whenever you come home from work, school or a vacation – but how happy?
New research from Japanese scientists published this week in the journal Current Biology suggests a dog’s eyes may well up with tears of happiness when reunited with their owner after a period of absence.
Takefumi Kikusui, a professor at the Laboratory of Human-Animal Interaction and Reciprocity at Azabu University in Japan, investigated dog tears after watching one of his two standard poodles when she had puppies six years ago. He noticed that her eyes got teary as she nursed her puppies, he said in a news release.
“We found that dogs shed tears associated with positive emotions,” Kikusui, who coauthored the study, was quoted in the news release.
To test whether tear production in dogs was associated with happiness, researchers added a solution with oxytocin – a hormone associated with love – to dogs’ eyes and found it increased tear volume, “suggesting that oxytocin might mediate tear secretion during owner-dog reunions,” according to the study. 
The study found dogs shed more tears when reunited with their owner following hours of separation than when reuniting with a familiar non-owner.  
Through generations of domestication, dogs have established “high-level communication abilities with humans using eye contact,” which has resulted in deeper mutual relationships and interspecies bonding, the study says.
Do dogs have nightmares? Your canine’s dream state, explained.
Why does my dog tilt its head? Trying to understand your pet’s nonverbal communication.
And have you ever fallen for that teary-eyed look you get from a dog? Your response could be backed by science.
The study “hypothesized that canine tear production during reunions with owners can facilitate human caregiving, a phenomenon which has similarly been reported in human children.”
Researchers had humans rate photos of dogs, some with and some without artificial tears, in terms of how much they wanted to care for them. The dogs with tears provoked a much higher caregiving response from humans than those without, according to the study. 
“A dog’s gaze initiates interactions with its owner, and stimulates secretion of oxytocin, a key hormone involved in bond formation, in owners,” the study reads. 
Natalie Neysa Alund covers trending news for USA TODAY. Reach her at [email protected] and follow her on Twitter @nataliealund.

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