Everyone who’s had a dog knows that they can get really excited and happy. Sometimes, even just someone returning home is enough to send them into emotional overdrive. But there’s a lot we still don’t know about how dogs manifest their emotions and sometimes, even experts were surprised.

This includes Takefumi Kikusui of Azabu University in Japan. Kikusui, an experienced veterinarian researcher, noticed something surprising with one of his dogs.

“I have two standard poodles and I had one female pregnant 6 years ago. When she was nursing her puppies, her face became so cute (she is cute as always but she became even cuter). We realized that she had teary eyes,” Kikusui tells ZME Science.

Kikusui started to suspect that maybe the dog was crying with happiness. So he decided to test it out with a reunion experiment.

They had 18 dogs reunite with their humans or with other people, measuring their tear volume before and after. As expected, the tear volume did indeed increase when they got back together with the familiar human and not with a person they didn’t know. The researchers were thrilled when they saw the results.

“I was surprised. We had never heard of the animals shedding tears in joyful situations, such as reuniting with their owners, and we were all excited that this would be a world first!”

But by the time the experiment was carried out, Kikusui also had an idea why this was happening: oxytocin.

Oxytocin, he explains, is the maternal or “love” hormone. The research team also observed that oxytocin is released both in dogs and owners when interacting, and it would also be secreted when a mother would be caring for her cubs. So they implemented another twist to the experiment, adding oxytocin to the dogs’ eyes — when this happened, their tear volume also went up, supporting the idea that oxytocin does play a role in joyful crying.

Basically, dogs can cry tears of happiness.

“This is the first report, as far as we know, the animals, not including humans, show emotional tears. We think that this is tear secretion related to positive (happy) emotion. We have not tested yet in sad situations in dogs. But in mice, fear-related memory can stimulate tear secretion,” Kikusui tells ZME Science.

In the last experiment, human participants were asked to rate how cute they find dogs with or without artificial tears, and they assigned more positive scores to the photos with tears — which suggests that emotion-elicited tears can facilitate human–dog emotional bonding. It could even be an example of convergent evolution, though the role of tears is not fully understood, neither in humans nor in dogs.

It’s not yet clear if different breeds react in a similar way, but the researcher says that it’s the link between dogs and humans that made this experiment possible. It’s possible that other species also exhibit similar behavior, but we just haven’t discovered it yet.

“I speculated, as mentioned above, animals can show emotional tears, such as wolves and other mammals. But measuring tear is so hard in animals. Dogs can be trained and accustomed to measuring tears. One important thing is that dog can form bonds with the owner, so owner-related emotional changes would be specific to dogs.”

“Dogs have become a partner of humans, and we can form bonds,” Kikusui says. “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.”

Journal Reference: Murata et al. “Increase of tear volume in dogs after reunion with owners is mediated by oxytocin” https://www.cell.com/current-biology/fulltext/S0960-9822(22)01132-0