You know how we don’t only cry when we’re sad but also when we’re relieved, as a kind of catharsis when the sadness has ended? It appears that dogs form tears when they are relieved, too — specifically, when they are reunited with us after many hours of separation.
In an experiment by Japanese researchers reporting in the journal Current Biology, the volume of tear formation was assessed in 18 dogs both in their normal home environment with their human family present and also after 5 to 7 hours of having been at doggie day care. What was measured was the volume of tears that formed in the dogs’ eyes; dogs’ tears do not drop from their eyes onto their cheeks the way ours do.
The investigators found that the dogs had a significantly higher volume of tears within the first 5 minutes of reuniting with their people than when they just happened to be hanging out with them. Moreover, the dogs produced more tears when finally back together with their human family members than when they were united with people they knew who were not members of their household. That is, it took their closest loved ones to elicit the effect most profoundly.
A hormonal connection?
It may be the feel-good hormone oxytocin (released when mothers nurse their young) that is at least partly responsible for the formation of tears. To test that theory, the researchers put either solutions of oxytocin on the surfaces of the dogs’ eyes or “control” solutions that had some of the same chemical properties of oxytocin but were in fact different substances. The upshot: oxytocin produced a significant increase in tear development, but the control solution
did not.
It may all be part of an evolutionary adaptation. The scientists found that the wet, shining eyes of the dogs after oxytocin administration stimulated caring emotions in people who looked at photos of them. Human infants also demonstrate their need for care and comfort with tears. In other words, a dog whose eyes become bright with tears may have a better chance of receiving the emotional soothing and attention he needs than if his eyes remained relatively dry upon reintroduction.
Whatever the case, it appears dogs are not just showing us with wagging tails, jumping, and whining how happy they are to see us. They may just be crying tears of joy, too.
