According to researchers in Japan, dogs produce tears when reunited with their owners.
“This is the first report demonstrating that positive emotion stimulates tear secretion in a non-human animal, but also that oxytocin plays a role in tear secretion,” said the team behind the study.
Writing in the journal Current Biologythey describe how eye contact between humans and dogs encourages the former to care for the latter, while a dog’s gaze can stimulate the production of oxytocin in the body of the owners.
Dogs have also developed the ability to raise their eyebrows, a trait that prompts humans to feed them, according to scientists.
Eye contact, important for both dogs and humans
Researchers in Japan have found that tears may have a similar effect.
“I have two poodles and had a female pregnant six years ago,” he told The Guardian Professor Takefumi Kikusui of Azabu University, co-author of the research. Noticing that her expression was softer than usual when nursing her cubs, Kikusui realized that her eyes were full of tears.
“This gave me the idea that oxytocin might play a role in the formation of these tears,” he said. “We have previously observed that oxytocin is released in both dogs and owners when they interact. So we conducted a reunion experiment.”
How was this study carried out?
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 the Schirmer test. This involves placing a special strip of paper inside the lower eyelid and measuring the moisture on it.
The team compared the amount of tears produced in the first five minutes after the dogs were reunited with their owners after a separation of more than five hours.
According to the researchers, the dogs produced a significantly higher volume of tears when they were reunited with their owners than when they were alone at home. This increase was not observed, however, when 20 dogs were similarly reunited with a human familiar but not their owner.
A significantly higher volume of tears
A further experiment involving 22 dogs showed that adding oxytocin to their eyes increased the amount of tears they produced – a result that was not seen when another solution, without oxytocin, was used.
On the other hand, the team presented 74 human participants with 10 photos of five dogs, depicting each animal with or without wet eyes. They were asked to rate on a five-point scale how much they wanted to avoid or care for the animal. According to the results, watery-eyed dogs won 10 to 15 percent more people willing to take care of them. This suggests that watery eyes in dogs trigger such emotions in humans.
Unlike other animals, dogs have acquired high-level communication skills
The researchers add that, unlike other animals, dogs have acquired high-level communication skills with humans using eye contact.
“Through this process, dogs’ tears could play a role in triggering protective or nurturing behavior on the part of their owners,” the researchers write.
However according to Kikisui, there are still questions that need to be answered. “We don’t know yet if they can 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.”