Dog owners know the sheer joy of returning home from a long journey and being greeted by their tail-wagging, uncontrollably jumping, face-licking companion.
But these ecstatic canines might do more than just shed fur on your clothes — they might also tear it, according to a new study published Monday in the journal Current Biology.
“We had never heard of the discovery that animals shed tears in joyful situations, such as reuniting with their owners,” said Takefumi Kikusui, one of the authors of the study, which he called a possible “world first.”
The scientists measured the amount of tears in the dogs’ eyes using the widely used Schirmer test, in which a special strip is placed under the eyelids. For a baseline, they performed the test on dogs during normal interaction with their owner.
When dogs were reunited with their owners after five to seven hours of separation, they “significantly” increased tear production over the following five minutes, the researchers found.
They also discovered that the dogs’ tear volume was higher when reunited with their owner than with other people the dog is familiar with.
According to the researchers, this tearing reaction is likely related to the release of oxytocin, also known as the “love hormone” because of its connection to building bonds.
The scientists then tried to test whether the tears could have an emotional effect on the owners. They asked owners to rank different photos of their dogs with and without artificial tears based on how much they wanted to take care of them.
“The dog photos with artificial tears ranked significantly higher than the normal dog photos without tears,” the Japanese research team wrote.
“It’s possible that the dogs that show tears in their eyes during the interaction with the owner would care more about the owner,” Kikusui suggested.
In humans, the authors note, infants communicate their negative feelings through crying, leading to more nurturing from parents.
Unlike any other animal domesticated by humans, dogs have evolved specific communication skills over time. Eye contact has been shown to play a role in forming the relationship between a dog and its owner.
In future studies, the researchers want to test whether dogs produce tears when they meet other canine buddies.
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