Now, new research from Japanese scientists suggests that a dog's eyes may well up with tears of happiness when reunited with their owner after a period of absence.
The tears may help cement the bond between human and dog -- a relationship that goes back tens of thousands of years.
Like humans, dogs have tear ducts that well up with tears to keep their eyes clean and healthy.
But tears in dogs, which tend not to fall as they do when humans cry, hadn't been linked with emotion before.
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.
To understand whether oxytocin played a role in producing the tears, a solution containing the hormone was applied to the surface of 22 dog' eyes.
"Dogs have become a partner of humans," Kikusui said in a statement, "and we can form bonds. »