Like humans, dogs have tear ducts that fill with tears to keep their eyes clean and healthy. But tears in dogs, which tend not to fall the way they do when humans cry, hadn’t been tied to emotion before.
“We also discovered oxytocin as a possible underlying mechanism,” Kikusui said, referring to the hormone that in humans is sometimes called the love or maternal hormone. .
To study the link, Kikusui and his team measured the amount of tears in 18 dogs with a standard test known as the Schirmer Tear Test. This involved a strip of paper placed inside the dogs’ eyelids for one minute before and after they were reunited with their owners after five to seven hours apart.
“The tear volume was assessed by the length of the wet end on the STT. The baseline was around 22mm and the meeting with the owner increased by 10%,” Kikusui explained via email.
With the help of 20 dogs, the researchers then compared the amount of tears before and after the reunion with their owners and people with whom the animals were familiar. Only the reunion with the owner increased the amount of tears.
To understand if oxytocin plays a role in tear production, a solution containing the hormone was applied to the surface of the eyes of 22 dogs. The amount of tears increased significantly after application of oxytocin, compared to a control solution.
There’s still a lot researchers don’t know about dog tears. Humans often cry in response to negative emotions, but researchers haven’t tested to see if dogs do the same. They also don’t know if a dog’s ability to tear plays a social function in the dog. world.
Kikusui said it was possible that humans took better care of dogs that had tears in their eyes. His team showed 74 people photos of dog faces with and without artificial tears and asked them to classify the animals. People gave more positive responses when they saw dogs with tears in their eyes.
“Dogs have become a partner to humans,” Kikusui said in a statement, “and we can form bonds.”