Dogs Shed Tears of Joy When Reunited With Their Owners, Scientists Say

Humans are thought to have unique tears of joy or sadness. There is no other animal that can make you cry.

Researchers in Japan have taken on that assumption with a series of controversial experiments. They argue that our dogs have eyes that are filled with feelings as well.

Dogs tear up with excitement when they are with their owners after a day away, according to a study.

There was a 10 percent increase in tear volume when the dogs greeted their owners.

The dogs didn't fare well when they met a new owner.

A slip of paper was put against a dog's eye for 60 seconds and recorded how far the wet was.

In our species, a higher volume of tears is linked to higher arousal of emotion, and the current findings suggest that it is positive emotions that cause the extra tears in dogs.

When a solution containing the 'love hormone' oxytocin was added to the eyes of 22 dogs, more tears were produced.

Extra tear production in dogs is thought to be caused by an elevation in the level of the brain chemical oxytocin.

"We were all excited that this would be a world first, because we had never heard of the discovery that animals shed tears in joyful situations, such as reunification with their owners, and we were all excited that this would be a world first!" says animal behaviorist Takefumi Kikusui of

It would be revolutionary to discover happy tears in our dogs. Other scientists are unconvinced by the experiment.

Humans are the only animals that cry as an emotional reaction, but other animals use their eyes for other things. Adding oxytocin to a dog's eyes may cause irritation, not happiness.

The control substance used in his experiments did not cause more tears among dogs.

The difference between an eye cleaning tear and an emotional tear is not easy to measure. One study isn't enough to decide the issue.

More research is needed to determine if dogs cry tears to express emotions or if their watery eyes are just an adaptive way to appeal to our own species.

When Kikusui and colleagues showed 10 photographs of dogs to 74 people and asked them to rate how it made them feel, they found that the dogs with damp eyes elicited greater feelings of care.

Humans and dogs can form bonds.

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Dogs are clearly attracted to our species. Is it possible that we only see what we want to see?

There's still more to know about our furry friends after thousands of years of co-evolution.

The study was published in a scientific journal.