They found that absence did in fact make the heart grow fonder, and the volume of tears was greater when the owners returned home after a few hours.
'High-level communication abilities'
"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 Professor Takefumi Kikusui, the study author from Azabu University.
"In this study, we demonstrated that dogs secrete tears when reuniting with their owner, and our data suggests that this tear secretion is mediated by oxytocin," the scientists write in their paper, published in Current Biology.
"This is the first report demonstrating that positive emotion stimulates tear secretion in a non-human animal, and that oxytocin functions in tear secretion.
"Unlike any other animal, dogs have evolved or have been domesticated through communication with humans and have gained high-level communication abilities with humans using eye contact.
"Through this process, their tears might play a role in eliciting protective behaviour or nurturing behaviour from their owners, resulting in the deepening of mutual relationships and leading to interspecies bonding."
Researchers looked at how dogs responded to their owners coming home, and also another familiar person who was not their owner, and found the tears of joy were only triggered by their owner.
The team now hope to find out whether or not dogs have the same joyous tears when reunited with other dogs, and if they also cry due to sadness.
The inspiration for the study came when the pet poodle of Professor Kikusui had a litter of puppies six years ago, and the scientist spotted tears in his dog's eyes as she nursed the newborns.
Professor Kikysui wondered if oxytocin was responsible for the lacrimation, as previous research has found that oxytocin is released in vast quantities by both humans and canines when they are together.
Dr Anna Machin, an anthropologist from Oxford University, said earlier this year that there is real, genuine love between humans and dogs, with the dynamic similar to that of a mother and her child.
"I think the evidence is showing more and more, particularly with dogs, that the relationship [dogs] have with their human is, by any definition, a loving relationship," she said earlier this year.
And the new study from Japan tested to see if the hormone was responsible for the happy tears, and found it likely was. When dogs were given extra oxytocin, the amount of tears produced increased, they found.
"Tear volume was significantly increased after oxytocin administration," the researchers say.
In the experiment, the team also took a photo of a dog and digitally doctored it to include tears in the eyes, and found that when people looked at these photos, the photos made them more inclined to look after the animal.
"A pair of photos was prepared for each dog: a picture of the dog with unaltered eyes and one of the dog where sterile saline was applied to the eyes," the researchers write in the study.
"Ten photos from five dogs were randomly presented on the computer screen, and the participants scored on a five-point scale from positive (want to touch, and give some care) to negative (fearful and avoid) for each photo,"
The scientists found that people rated the teary photo "significantly higher than the normal tearless dog photographs" when it came to how much a person wanted to care for them.
