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A wagging tail, a loving lick, a playful jump, a heartbreaking look. You know when your dog is happy to see you.
Now, new research from Japanese scientists suggests that a dog’s eyes can well up with tears of happiness when they are reunited with their owner after a period of absence. The tears can help cement the bond between humans and dogs — a relationship that dates back tens of thousands of years.
Just like humans, dogs have tear ducts that well up with tears to keep their eyes clean and healthy. But tears in dogs, which do not tend to fall as when humans cry, had not previously been associated with emotion.
Takefumi Kikusui, a professor in the Laboratory of Human-Animal Interaction and Reciprocity at Azabu University in Japan, decided to investigate dog tears after seeing one of his two standard poodles when she had puppies six years ago. He noticed her eyes watered as she nursed her puppies.
“We found that dogs shed tears associated with positive emotions,” Kikusui, who co-authored the study published Monday in the journal Current Biology, said in a press release.
“We also made the discovery of oxytocin as a possible mechanism underlying it,” Kikusui said, referring to the hormone that in humans is sometimes referred to as the love or mother hormone.
To investigate the link, Kikusui and his team measured the amount of tears in 18 dogs using a standard test known as the Schirmer Tear Test. It It involved a paper strip that was placed in the dogs’ eyelids for a minute before and after they were reunited with their owners after five to seven hours of separation.
“The tear volume was assessed by the length of the wet part on the STT. The baseline was approximately 22mm and owner reunification increased by 10%,” explains Kikusui via email.
With the help of 20 dogs, researchers then compared the amount of tears before and after reunions with their owners and people with whom the animals were familiar. Only seeing the owner again increased the amount of tears.
To understand whether oxytocin played a role in producing the tears, a solution containing the hormone was applied to the surface of 22 dogs’ eyes. The amount of tears increased significantly after the oxytocin was applied, compared to a control solution.
There’s still a lot that researchers don’t know about dog tears. People often cry in response to negative emotions, but researchers haven’t tested whether dogs did the same. They also don’t know if a dog’s ability to tear up plays a social function in the dog world.
Kikusui said it was possible that people would take better care of dogs that got tears in their eyes. His team showed 74 people pictures of dog faces with and without artificial tears in them and asked them to rank the animals. People reacted more positively when they saw dogs with teary eyes.
“Dogs have become partners with people,” Kikusui said in a statement, “and we can bond.”