A new study has found dogs can tear up when reunited with their human owners.
Published Monday in the journal , the study out of Azabu University in Japan measured the tear volume of 22 dogs under different circumstances.
Researchers say the dogs shed "significantly" more tears of joy after getting back together with their owners compared to meeting someone who was not their owner.
This response is believed to be tied to the release of oxytocin, the so-called maternal or "love hormone," during these interactions, the researchers .
"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," Azabu University researcher Takefumi Kikusui said.
Kikusui said one of his two poodles, who had puppies six years ago and would get teary eyed while nursing them, inspired him.
For the study, the researchers performed a to measure the dogs' tear volume. The test works by allowing water in tears to travel along a paper test strip, with the rate of travel being proportional to the rate of tear production.
The researchers measured the dogs' tears in their normal home environment while the owner was present to set a baseline and later within the first five minutes of a reunion once the owner had been away for five to seven hours.
The researchers say the dogs secreted larger tear volumes when reunited with their owners compared to meeting others whom they would also be familiar with.
Tear volume also increased after adding an oxytocin solution to the dogs' eyes, which the researchers say supports the notion that the hormone has a role to play in tear production when dogs and their owners are reunited.
Lastly, the researchers asked people to rate pictures of dogs' faces, with and without artificial tears, in terms of how much they wanted to care for them.
The researchers say people responded more positively to seeing a dog with teary eyes, suggesting dogs produce tears in situations humans would consider "happy."
"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 further leading to interspecies bonding," the study says.
However, the researchers say they have not tested whether dogs produce tears in response to negative emotions or when reunited with other dogs.
"Dogs have become a partner of humans, and we can form bonds," Kikusui said. "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."
