Hardly anywhere do so many tears of joy flow as in the reception halls of train stations or airports. Excessive emotions drive the water into people's eyes when they are allowed to clasp their loved ones in their arms again. In fact, even dogs shed tears of joy when they meet their masters or mistresses again. This is what a team led by the Japanese researcher Takefumi Kikusui from Azabu University in Japan writes in the journal "Current Biology".
The veterinarians wanted to know whether dog eyes also fill tears in emotional situations. And indeed: "The tear volume increased significantly during the reunion with the owner, but not with unknown people," they write. Oxytozin, also known as "cuddly hormone", plays a crucial role.
Kikusui and his colleagues developed an interest in the research question after one of his two poodles had puppies. He noticed that his bitch's face changed when she suckled the puppies: she was crying. "This gave me the idea that oxytocin could promote the formation of tears," says Kikusui, according to a press release from the journal. The hormone oxytocin produced in the brain plays a major role at birth as well as for living together among people. It initiates labor, stimulates milk production, strengthens the bond between mother and child, and can also strengthen couple bonding and build trust in other people among adults.
According to older studies, dogs have human-like social-cognitive skills that have probably developed in parallel to their domestication. The eye contact between dog and humans, for example, increases the bond. Dogs have also developed muscles that are responsible for lifting the inner eyebrows, which triggers caring behavior in humans. Why shouldn't tears also have a similar function?
So, the researchers measured the volume of tears in dogs before and after they saw their owners again. They compared this with the dogs' reaction to a reunion with a person they did not know. They found that the amount of tears actually increased significantly more when they got back together with the familiar person. When they dripped oxytocin into the dogs' eyes, their tear volume also increased measurably. This result supports the idea that oxytocin plays a role in tear production when dogs and their humans meet, the scientists write.
In a further step, the researchers asked people to evaluate pictures of dogs with and without tears in their eyes. It was shown that people reacted more positively when they saw dogs with damp eyes. These results indicated that the tear production of dogs contributes to strengthening the bond between humans and their dogs, says Kikusui. However, the results would have surprised him. "We had never heard before that animals shed tears in joyful situations, such as reunion with their owners," he notes.
However, it is still unclear whether dogs produce tears even with negative emotions. So far, the researchers have not known whether dogs cry when they meet other dogs again - whether tears also have a social function in the dog world.
