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(CNN) --
A tail wag, an affectionate lick, a playful jump, a look that melts the heart.
You certainly know how to identify when your dog is happy to see you.
Now, new research from Japanese scientists suggests that a dog's eyes may well fill with happy tears when reunited with its owner after a period of absence.
Tears can help cement the bond between humans and dogs, a relationship that dates back tens of thousands of years.
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Just like humans, dogs have tear ducts that fill with tears to keep their eyes clean and healthy.
But dog tears, which don't usually fall like when humans cry, hadn't been linked to emotion before.
Takefumi Kikusui, a professor at the Human-Animal Interaction and Reciprocity Laboratory at Azabu University, Japan, decided to investigate dog tears after observing one of his two poodles when she had puppies six years ago.
He noticed that his eyes filled with tears when he nursed his pups.
"We found that dogs shed tears associated with positive emotions," Kikusui, a co-author of the research published Monday in the academic journal Current Biology, said in a press release.
"We also found that oxytocin is a possible underlying mechanism," Kikusui said, referring to the hormone that in humans is sometimes called the love or maternal hormone.
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To investigate the relationship, Kikusui and his team measured the amount of tears in 18 dogs with a standard test known as the Schirmer tear test, or STT.
It involved placing a strip of paper inside the dogs' eyelids for a minute before and after they were reunited with their owners after five to seven hours of separation.
"Tear volume was assessed by wet spot length on STT. Baseline was about 22mm, and owner reunion increased it by 10%," Kikusui explained via email.
With the help of 20 dogs, the researchers compared the number of tears before and after reunions with their owners and people with whom the animals were familiar.
Only the reunion with the owner increased the number of tears.
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To find out if oxytocin played a role in tear production, a solution containing the hormone was applied to the surface of the eyes of 22 dogs.
The amount of tears increased significantly after the application of oxytocin, compared to a control solution.
There's still a lot that researchers don't know about dog tears.
Humans often cry in response to negative emotions, but the researchers didn't test to see if dogs did the same thing, too.
They also don't know if a dog's ability to cry serves a social function in the canine world.
Kikusui said it was possible that humans cared more about crying dogs.
His team showed 74 people photos of dog faces with and without artificial tears and asked them to rank the animals.
People gave more positive responses when they saw teary-eyed dogs.
"Dogs have become companions to humans," Kikusui said in a statement, "and we can create bonds."
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