A dog will shed tears when reunited with its owner, strengthening the bond between pet and owner, reported a new study in Current Biology.

The discovery was made when Takefumi Kikusui of Azabu University in Japan saw his dog nursing its puppies and noticed a change in its face – there were tears.

That gave Kikusui the idea that maybe oxytocin, which is also known as the ‘love hormone’ increases tears. From earlier observations, Kikusui and his researchers knew that oxytocin is released in both dogs and owners during interactions, which led them to run a reunification experiment and see if it brought dogs to tears.

“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!”

For the study, researchers used a standard test to measure dogs’ tear volume before and after reuniting with their owners. They found that tear volume indeed went up when they got back together with the familiar human and not with a person they didn’t know.

Interestingly, when people were asked to rate pictures of dogs with and without artificial tears, people gave more positive reactions when they saw dogs with teary eyes. Suggesting that dogs’ tear production helps to forge stronger connections between people and their dogs.

“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,” said Kikusui.

Not yet tested is whether dogs produce tears in response to negative emotions, nor if dogs produce tears when reunited with other dogs.

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