By wagging their tails, panting and jumping, dogs make it clear when they’re happy. But new research suggests there might be another way that dogs show joy: producing tears.
In a small study, researchers found that dogs produce significantly more tears after being reunited with their owners compared to when their owners stayed at home with them.
The findings suggest “dogs shed tears associated with positive emotions, such as a reunion with the owner,” Takefumi Kikusui, one of the study’s authors and a veterinary researcher at Azabu University in Japan, told Gizmodo’s Ed Cara in an email.
If this is true, the study marks the first time that researchers have tied dogs’ tears to emotion. However, multiple scientists told the New York Times’ Elizabeth Landau that the paper’s conclusions are hard to accept, saying the methodology used in the experiments might not provide a reliable conclusion.
In the new study, published Monday in the journal Current Biology, the researchers placed a strip of a paper inside each dog’s lower eyelid and measured how far the tears traveled along the strip, writes the Guardian’s Nicola Davis. In one experiment, they established a baseline by measuring tear production when the dog and owner were at home, then they compared the results to when the dog and human were reunited after five to seven hours of separation.
Tear production increased by 10 percent in the reunited dogs, Kikusui told CNN’s Katie Hunt in an email. The researchers also found that reunions with owners led to more tear production than reunions with familiar non-owners.
In a second experiment, researchers tested if oxytocin, a hormone thought to be connected to emotional bonding in humans and dogs, could be causing the tear production. After all, Kikusui and his team concluded in a 2015 paper that both humans and dogs produce more oxytocin when they spend time together, reports Gizmodo. In the new research, the scientists found that dropping oxytocin in dogs’ eyes increased tear production, while a control solution did not, per the Guardian.
Previous studies have shown that dogs are in tune with human emotions, per the Times, and can have human-like social skills, per Gizmodo. Daniel Mills, a veterinary behavioral medicine specialist at the University of Lincoln in England who did not contribute to the paper, tells the Times that other research has shown dogs have emotional categories, such as “You are somebody I care about, therefore, I’m pleased to see you,” and “You are somebody I don’t care about, so I can ignore you most of the time.”
Still, some scientists think it’s a stretch to say that dogs cry tears of joy. Clive Wynne, a canine behavior specialist at Arizona State University, tells the Times that “it would take a lot to convince me” to accept the evidence of this research.
He tells the Times that using paper in the dogs’ eyelids might have affected this experiment’s outcome: More excited dogs could have moved around more, causing the paper to rub against their eyes and produce more tears. Likewise, the oxytocin eyedrops might have caused tears simply because they irritated the dogs’ eyes, Lauren M. Bylsma, a clinical psychologist at the University of Pittsburgh not involved in the research, says to the Times.
Bylsma and Wynne still support the idea that only humans are known to shed emotional tears, according to the Times.
Whether or not dogs’ tears are tied to emotion, scientists do seem to agree that the tears influence their relationships with their owners. In a third experiment, the researchers had 74 human participants rate photos of dogs based on how much they wanted to care for the animals, per the Guardian. People expressed greater interest in caring for the teary-eyed canines.
“It might be that things like a more glossy eye or the presence of tears do encourage nurturing tendencies in us,” Mills tells the Times.