Dogs' eyes well up with tears when they reunite with their owners, researchers say

• Dogs produce more tears when filled with emotion from reuniting with their owners, per a new study.
• The researchers said the same increase wasn't seen when dogs meet humans they aren't familiar with.

If your dog looks like it's about to cry when it greets you, it's possible those are tears of joy, according to new research by Japanese scientists.

The researchers said that they've found a connection between a dog's emotions and the volume of tears it produces, in their study published in the peer-reviewed journal Current Biology.

Dogs have tear ducts that can well up with tears just like humans do, but this is the first time that an animal's tear production has been tied to its happiness.

The team said they conducted the Schirmer test — which involves placing a paper filter on a dog's lower eyelid to collect moisture — on 18 dogs who were reunited with their owners after being separated for five to seven hours.

They said the dogs secreted significantly larger tear volumes in the first five minutes of meeting their owners again, compared to when they were alone.

The researchers also ran another experiment in which 20 dogs met a human they weren't familiar with. This time, the dogs didn't produce more tears than normal, they said.

The team also hypothesized that oxytocin — the hormone that helps with bonding and feelings of love — is related to a dog's tear production.

In a third experiment, the researchers administered oxytocin into the eyes of 22 dogs. The researchers said the dogs began to produce more tears when this happened, but the increase was not observed when they dropped a non-oxytocin solution into the dogs' eyes.

Lead author Takefumi Kikusui, a professor at the Laboratory of Human-Animal Interaction and Reciprocity at Azabu University in Japan, told The Guardian that the idea for the study came from when he noticed that one of his poodles became teary-eyed when she was nursing her puppies.

"That gave me the idea that oxytocin might increase tears," he told the outlet. "We previously observed that oxytocin is released both in dogs and owners when interacting."

The researchers wrote in their study that dogs exhibit excitement and emotion when they reunite with their owners after extended periods, such as by jumping up, licking their owner's face, or wagging their tails.

The research team suggested that teary eyes may also help a dog forge better bonds with its owner.

Kikusui and his colleagues conducted another test with 74 human participants, who were each shown 10 photos of five dogs, some of which had moist eyes.

The participants were asked how much they wanted to care for each dog on a scale of one to five. The test results showed that dogs with tears welled up in their eyes had 10% to 15% more people saying they wanted to care for them.

"In humans, infants use tears to transmit negative feelings to their parents, and in response to that the receivers show caregiving behavior," they wrote. "In this study, we found that a dog's face with artificial tears can also stimulate caring emotions in humans."