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By Jess Thomson
Science Reporter
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Dogs get so emotional when their owners come home to them that they cry tears of joy, scientists have found.
According to a paper published in the journal Current Biology, dogs are capable of emotional tears, especially when seeing someone they are very fond of.
"We found that dogs shed tears associated with positive emotions," Takefumi Kikusui of Azabu University in Japan, a co-author of the paper in a statement. "We also made the discovery of oxytocin as a possible mechanism underlying it."
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Humans cry emotional tears, both when they are sad and happy. This is thought to have developed as a non-verbal communication cue, as well as a therapeutic expression of grief or overwhelming joy.
Formal investigations into happy tears in dogs and other animals haven't been done much before, however, animals have been anecdotally observed to show signs of joy or excitement when their owners come home, such as by wagging their tails and running around erratically.
The researchers performed something called a Schirmer Tear Test on a group of 22 dogs, which involves measuring the animal's tear volume. They found that the amount of tears produced by the dog's eyes increased significantly when they were reunited with their owner compared to someone who they were familiar with but wasn't their owner.
"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," said Kikusui.
An additional finding of the study was that the dogs' tear volume increased further when an oxytocin solution was applied to their eyes. This suggests that oxytocin—a bonding hormone produced in the human body after childbirth and falling in love—might mediate tear secretion during an owner-dog reunion.
The authors posit that this response may have evolved to make dogs more likely to be cared for by humans, similarly to "puppy dog eyes'"
"Unlike any other animals, dogs have evolved or have been domesticated through communication with humans and have gained high-level communication abilities with humans using eye contact," wrote the authors in the paper.
"Dogs have become a partner of humans, and we can form bonds," Kikusui said in a statement. "In this process, it is possible that the dogs that show teary eyes during interaction with the owner would be cared for by the owner more."
However, the sample size used in this study is pretty small at a meager 22 dogs, so more testing at larger scales would serve to consolidate the findings.
Additionally, the authors hope to test whether dogs produce tears in response to negative emotions, and if the dogs cry happy tears when they reunite with other dogs, which would have implications on dog socialization as well as domestication by humans.
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