Dogs shed tears of joy when they reunite with their beloved owners after spending several hours apart, a recent study by a Japanese research team has found.
Although animals have been known to shed tears to protect their eyes, this is the first time it has been confirmed that dogs can also cry as an emotional response, said Miho Nagasawa, assistant professor of animal behavior at Japan's Azabu University who contributed to the study.
The volume of tears in a dog likely increases due to the production of oxytocin, commonly called the "love hormone," when it becomes excited after seeing its owner, according to results published in science journal Current Biology.
"Eyes are important to humans, and over time we have come to show affection to dogs that cry," said Nagasawa from the university in Kanagawa Prefecture, near Tokyo.
She added that "as a result of dogs acquiring the ability to cry over a long history of domestication, it seems we have developed a close relationship with them that transcends species."
In an experiment where 18 dogs were apart from their owners for periods of five to seven hours, the volume of dogs' tears was found to be around 10 percent higher after they reunited than before they were separated.
The same did not occur when the dogs were paired with 22 people who were not their owners.
Tears were also found to increase when dogs were given eye drops containing oxytocin.
In another experiment in which photos of dogs before and after being given the eye drops were shown to 75 people, participants were found to experience a stronger urge to care for the teary-eyed dogs.
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