Dogs cry tears of joy when their favourite humans return home.
Our canine companions often bark and enthusiastically lick their owners’ faces when they get home, but they also become tearful, a Japanese study found.
The study’s results, published in the Journal of Current Biology, stated dogs’ tears appear to be linked to oxytocin, the bonding hormone.
It was the first study to find ‘happy tears’ in an animal other than humans, the research team told The Guardian.
“This is the first report demonstrating that positive emotion stimulates tear secretion in a non-human animal, and that oxytocin functions in tear secretion,” they said.
How did they find out?
The researchers recruited 20 dogs for their study and set up a range of scenarios.
In the first scenario, owners would stay home and interact with their dogs as per usual; this was used as a baseline reading.
In the second scenario, owners would leave the house for five to seven hours.
And in the third scenario, researchers observed the dogs as they interacted with people they were familiar with, but weren’t their owners.
They placed strips of paper inside the dogs’ lower eyelids in each scenario and measured how far along each strip the moisture travelled.
They found there was a significant increase in tear production when the dogs were reunited with their owners after a long period of time, compared to the other scenarios.
One of the report’s authors, Takefumi Kikusui, a professor at Azabu University, said he was inspired to conduct the study after he observed his two poodles at home years ago.
He noticed one dog, which had just given birth to a litter of puppies, had a more tender face, and her eyes would well up with tears when nursing her puppies.
“That gave me the idea that oxytocin might increase tears,” he told The Guardian.
“We previously observed that oxytocin is released both in dogs and owners when interacting. So we conducted a reunion experiment.”
Interspecies bonding
The researchers then investigated whether a dog’s tears evoked an emotional response in humans.
The researchers hypothesised that a dog’s tears would cause humans to care for them more.
They presented 74 participants with 10 photos of five dogs that depicted with or without tears in their eyes.
They asked participants to rate on a five-point scale just how much they wanted to care for them.
“We found that teary eyes of dogs can facilitate human caregiving. Dogs have become a partner of humans, and we can form bonds,” Professor Kikusui told Gizmodo.
“In this process, it is possible the dogs that show teary eyes during interaction with the owner would be cared for by the owner more.”
But other questions remain: Professor Kikusui said it is not known whether tears are produced when dogs reunite with other dogs, or how dogs use tears to communicate with each other.
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