Researchers in Japan say they have discovered that dogs produce tears when they are reunited with their owners. In addition, boldness appears to be linked to levels of the ‘bonding hormone’ oxytocin. “This is the first report showing that positive emotion stimulates tear secretion in a non-human animal and that oxytocin functions in tear secretion,” the team said. Writing in the journal Current Biology, they describe how eye contact between humans and dogs encourages the former to care for the latter, while a dog’s gaze can trigger the release of oxytocin in its owner. Dogs have also evolved the ability to raise their inner eyebrows, a trait scientists say motivates humans to breed them. Now researchers in Japan have discovered that tears can have a similar effect. “I have two standard poodles and I had a female pregnant six years ago,” Professor Takefumi Kikusui, co-author of the study at Azabu University, told the Guardian. Noticing that her face was more tender than usual when nursing her pups, Kikusui realized that her eyes were teary. “That gave me the idea that oxytocin might increase tears,” he said. “We previously observed that oxytocin is released in both dogs and owners when they interact. So we performed a reunion experiment.” In the first step, the team measured the volume of tears produced by 18 dogs when they were in their normal home environment with their owner, using Schirmer’s test. This involves placing a special strip of paper inside the lower eyelid and measuring the distance along the strip that moisture travels. The team compared this volume to that produced within the first five minutes of dogs being reunited with their owners after being separated for more than five hours. The researchers say the dogs produced a significantly higher volume of tears when they were reunited with their owners than when they wandered the house alone. This increase was not observed, however, when 20 dogs were reunited in the same way with a familiar human who was not their owner. A further experiment involving 22 dogs showed that dropping oxytocin into their eyes increased the volume of tears they produced – an effect that was not seen when another solution without oxytocin was used. The team then showed 74 participants 10 photos of five dogs, depicting each animal either with or without watery eyes, and asked them to rate on a five-point scale how much they wanted to avoid or care for the animal. Kikusui said the results revealed that teary-eyed dogs won 10 to 15 percent more people who wanted to care for them. This, the team says, suggests that a dog’s watery eyes evoke such feelings in humans. The researchers add that unlike other animals, dogs have acquired high-level abilities to communicate with humans using eye contact. “Through this process, their tears may play a role in eliciting protective or nurturing behavior from their owners,” they write, noting that it can deepen mutual relationships and bonds between humans and their canine companions. . But Kikusui said that while the team had found that dogs show an increase in tears during reunification with their owners, questions remained. “We don’t yet know if dogs show an increase in tears during a dog-dog reunion. We also don’t know how dogs use tears to communicate with each other,” he said. “We need to clarify the social function of dog tears.”
