Published Monday in the journal Current Biology, the study from Azabu University in Japan measured the tear volume of 22 dogs under different conditions. The researchers say the dogs shed “significantly” more tears of joy after being reunited with their owners compared to meeting someone who wasn’t their owner. This response is believed to be linked to the release of oxytocin, the so-called maternal or “love hormone,” during these interactions, the researchers said in a press release. “We had never heard of the discovery that animals cry in happy situations, such as being reunited with their owners, and we were all excited that this would be a world first,” said Azabu University researcher Takefumi Kikusui. Kikusui said one of his two poodles, who had puppies six years ago and got teary-eyed while nursing them, inspired him. For the study, the researchers performed a Schirmer test to measure the dogs’ tear volume. The test works by allowing the water in the tears to travel along a paper test strip, with the rate of travel proportional to the rate of tear production. The researchers measured the dogs’ tears in their normal home environment while the owner was present to set a baseline, and later within the first five minutes of a reunion after the owner was gone for five to seven hours. The researchers say that dogs shed larger volumes of tears when they were reunited with their owners compared to meeting others with whom they would also be familiar. Tear volume also increased after an oxytocin solution was added to the dogs’ eyes, which the researchers say supports the idea that the hormone plays a role in tear production when dogs and their owners are reunited. Finally, the researchers asked people to rate photos of dogs’ faces, with and without artificial tears, on how much they wanted to care for them. The researchers say that people responded more positively to seeing a teary-eyed dog, indicating that dogs produce tears in situations that people would consider “happy.” “Through this process, their tears may play a role in eliciting protective or nurturing behavior from their owners, resulting in deepening mutual relationships and further bonding between species,” the study says. However, the researchers say they have not tested whether dogs produce tears in response to negative emotions or when reunited with other dogs. “Dogs have become human companions and we can form bonds,” Kikusui said. “In this process, it is possible that dogs that show teary eyes during owner interaction are more cared for by the owner.”
