In the battle to be mankind's most compatible companion, dogs may have just taken the lead against cats with a new scientific finding. A new study published by Current Biology in August of 2022, states that dogs cry tears of joy when reunited with their owners after prolonged periods of time. This unprecedented finding provides a new perspective on the behavior of our four-legged friends and their range of emotions. The study was led by Takefumi Kikusui, a professor at the Laboratory of Human-Animal Interaction and Reciprocity at Azabu University in Japan. Kikusui undertook the research after observing his own dog “tear up” when nursing her newborn puppies.
Kikusui believed that oxytocin, the hormone in humans dubbed “the love hormone” or “maternal hormone,” was aroused in dogs, resulting in an increase in tears during a reunion. This link was investigated in 18 dogs. Each underwent a standard test known as the Schirmer tear test, where tears were tracked using a paper slip inserted in the eye a minute prior and a minute following the reunion. A 10% increase in tear production was observed, alluding to a correlation between happiness and crying in dogs. Furthermore, these tears were seen to only be triggered by owners. More specifically, the 10% increase was neither observed when meeting strangers nor when encountering acquaintances. To support the notion that oxytocin was at the root of dog tears, another test was performed. The hormone was applied to the dog eyes to see if it would elicit an identical response in inducing tear production. It proved successful in igniting tears.
Previously, dog tears were known to solely function as a cleansing mechanism in the eyes with no correlation to emotion. These data allow scientists to infer that the purpose of tears extends beyond just hygiene. Now, scientists and dog-owners can wonder about whether tears may also be linked to negative emotions in dogs as well and what evolutionary purpose tears serve within the species.