Dogs shed "happy tears" when their owners come home and greet them, according to a new study. The research was carried out at Japan's Azabu University after a scientist spotted his pet pooch would seem to tear up when she nursed her puppies.
Professor Takefumi Kikusui explained that dogs produce tears but they don't fall like humans' do - so he was fascinated to learn more about their emotions. He said: "That gave me the idea that oxytocin might increase tears."
Prof Kikusui followed up his idea and found that dogs had more tears in their eyes than normal when reunited with their owners, reports the Mirror. Their base level of tears didn't change when they met someone they didn't know.
When the researchers added oxytocin to the dogs' eyes, they filled up with tears too. The supported the theory that the release of oxytocin causes dogs to cry when they're back together with their human families.
It was already known that the hormone is released in dogs and their owners when they play, but studying the reunification of the two is new. The Japanese team also asked people to rate pictures of dogs' faces with and without artificial tears in them, and people loved the teary-eyed pets more.
This suggests dogs' tears help forge stronger connections between people and their dogs. Prof Kikusui, who was surprised by the results, said: "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."
He added: "Dogs have become a partner of humans, and we can form bonds. 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."
The study, published in the journal Current Biology, showed dogs shed tears in happy situations, but they haven't tested it on negative emotions yet. The research team is yet to discover if dogs cry when they reconnect with other dog pals, but they hope to figure out if this tearful reaction also has a social function in the canine world.
