Add this to the list of reasons why we don't deserve dogs: they miss us so much when we're gone for the day, they cry literal tears of joy when we return.
A recent study from a group of Japanese scientists found that dogs actually tear up when reunited with their owners. It's the first research of its kind to link tears in dogs to an emotional response, the scientists said.
The study, published in the journal Current Biology, suggests that the reaction is simple biology.
"We found that dogs shed tears associated with positive emotions," Takefumi Kikusui, co-author of the research at Azabu University in Japan, said in a statement. "We also made the discovery of oxytocin as a possible mechanism underlying it."
Oxytocin is often referred to as the "love hormone" and is associated with feelings of trust, connection and bonding.
"We previously observed that oxytocin is released both in dogs and owners when interacting, so we conducted a reunion experiment," Kikusui told The Guardian.
For the study, researchers measured tear volume in dogs before and after reunions with owners and familiar non-owners in multiple scenarios. The study noted that dogs' eyes get watery and well with tears, but that the tears don't run down as the way humans cry.
In the first experiment, a dog's tear volume was measured following a five to seven hour separation with the owner. The researchers found that tear volume was "significantly greater" during the reunion than before the owner left.
The second experiment compared tear volume before and after reunions with owners, as well as other people familiar to the dog. Researchers found that dogs teared up more with their owners than compared with their friends.
The researchers also examined how a dog with tears of joy might affect its owner. Owners were asked to report their feelings while looking at different photos of dogs with or without artificial tears. The results indicated that individuals assigned more positive scores to the photos of dogs with artificial tears.
The study indicates that canine tear production can stimulate caring emotions in humans and strengthen the bond between owners and pets, according to the research team.
"Unlike any other animals, dogs have evolved or have been domesticated through communication with humans and have gained high-level communication abilities with humans using eye contact," the study states. "Through this process, their tears might play a role in eliciting protective behavior or nurturing behavior from their owners, resulting in the deepening of mutual relationships and further leading to interspecies bonding."
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