Dogs shed tears of joy when reunited with their human companions

The Secret Life of Pets gave us a brief, if comedic, window into what it might be like to be a companion animal in the modern world. Its success at the box office shouldn’t be all that surprising considering how important dogs and other pets are to us.

When you stop to think about it, it’s truly weird that we have these little furry animals that just live inside our houses. Our connection with dogs is the result of tens of thousands of years of bonding and symbiotic relationships. Many breeds are so far removed from their natural ancestors as to be nearly unrecognizable, and they depend on our love and care to survive.

As a result, they live out lives of luxury, taking naps, smelling things we can’t perceive, and barking at the neighbors. They have entire inner lives we’re not aware of and you can’t help but wonder what’s going on inside their heads. Perhaps more importantly, do they love us as much as we love them?

A new study from scientists at the School of Veterinary Medicine at Azabu University suggests that they do. In fact, they love us so much that they cry tears of joy when we come home. This is the first documented evidence of a non-human animal shedding happy tears. The study was published in the journal Current Biology.

It all started several years ago when one of the study authors, Takefumi Kikusui, was at home with one of his dogs who had recently had puppies. While watching the dog nurse, he noticed it had tears in its eyes. One could shrug that off as the exhaustion of new motherhood, but Kikusui wondered if it might be related to a release of oxytocin, commonly called the love hormone, something that was likely happening during nursing.

Previous research has demonstrated that oxytocin is released in both dogs and humans when they interact with one another, particularly if they have an existing relationship. So, it stood to reason that if oxytocin was responsible for the welling up seen in Kikusui’s dog, then it should manifest in other scenarios. Maybe we could observe it occurring when dogs reunite with their human companions.

To find out, scientists measured the volume of tear production in dogs before and after a reunion with someone they knew. They found that tear production increased when they were reunited with their own human family members but not when encountering an unknown person. Again, that could be interpreted in a number of ways, so scientists took their experiments a step further.

Oxytocin was introduced manually, directly to the dog’s eyes and tear production was measured. Once again, researchers observed an increase in the production of tears. That seems to have clinched it. Oxytocin results in increased tear production, and oxytocin is released in the brain when dogs reunite with humans they know and love.

You might wonder why we’re just figuring this out, considering that we’ve been living with dogs since before recorded history and many of us spend considerable amounts of time with them every day. One contributing factor is the difference in what crying looks like in dogs as opposed to humans. Dogs well up, their eyes glisten over with moisture, but it’s uncommon for tears to fall over their face in the same way it does when we are overcome with emotion. When a dog cries, it just isn’t that obvious.

Scientists also wondered if the production of tears might have any effect on the people involved in the interaction. To that end, researchers presented study participants with photos of dogs exhibiting varying levels of teary eyes and asked them to rate them. They found that dogs with higher levels of tear production were rated more positively than their dry-eyed counterparts. This suggests there might be a symbiotic emotional relationship happening. Dogs get happy when they see us, their oxytocin levels spike, and they tear up. Then, we see those tears, we think they’re cuter, and we love them back even harder. The relationship reinforces itself.

Future research will look into whether or not dogs cry when they’re sad and if they undergo a similar reaction when reuniting with other dogs, or if this is something special just for us. Either way, take an extra minute today to give your dog an extra squeeze, we truly don’t deserve them.