Even dogs cry with happiness

Spektrum der Wissenschaft

3.9.2022

Translation: machine translated

When their owner comes home, dogs wag their tails happily - and shed a tear of joy. Oxytocin may play a role in this.

Nearly nowhere do so many tears of joy flow as in the reception halls of railway stations or airports. Overflowing emotions bring tears to people's eyes when they can hold their loved ones in their arms again. In fact, dogs also shed tears of joy when they see their masters or mistresses again. This is what a team led by Japanese researcher Takefumi Kikusui from Azabu University in Japan wrote in the scientific journal "Current Biology". . The veterinarians wanted to know whether dogs' eyes also fill with tears in emotional situations. And indeed: "The volume of tears increased significantly during the reunion with the owner, but not with strangers", they write. Oxytocin, also known as the "cuddle hormone", possibly plays a decisive role here.

Kikusui and his colleagues became interested in the research question after one of his two poodles had puppies. He noticed that his dog's face changed when she nursed the puppies: she cried. "That gave me the idea that oxytocin might promote tear formation," Kikusui said, according to a press release from the journal. The hormone oxytocin, which is produced in the brain, plays a major role in birth as well as in cohabitation among humans. It induces labour, stimulates milk production, strengthens the bond between mother and child and can also strengthen the pair bond among adults and build trust in other people.

According to older studies, dogs have human-like social-cognitive abilities, which are thought to have evolved alongside their domestication. The gaze contact between dog and human, for example, reinforces bonding. Dogs have also developed muscles responsible for raising the inner eyebrows, which triggers caring behaviour in humans. Why shouldn't tears 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 seeing a person they did not know again. They found that the amount of tears actually increased significantly more when they were reunited 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 rate pictures of dogs' faces with and without tears in their eyes. It turned out that people reacted more positively when they saw dogs with moist eyes. These results suggested that dogs' tear production helps strengthen the bond between people and their dogs, Kikusui says. However, he says the results surprised him. "We had never heard of animals shedding tears in joyful situations, such as reuniting with their owners, before," he notes.

It is still unclear, however, whether dogs also produce tears during negative emotions. Nor do researchers yet know whether dogs cry when they reunite with other dogs - in other words, whether tears also have a social function in the canine world.

Spectrum of Science

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