Tearful Dog Head: Will they cry with joy when they are reunited with their owners?

In May this year, a Bichon Frise named "Coffee" was transferred to a quarantine center because its owner tested positive for nucleic acid. It was forced to be separated from its owner and lived alone at home for 19 days.

During the 19 days, it went from refusing to eat or drink on the first day to slowly interacting with the voice of its owner through the camera, eating and sleeping obediently, occasionally receiving food from the neighbors, and then continuing its long wait.

On the day of the reunion, Coffee's happiness could be felt through the screen, and it ran around its owner. The owner finally saw the dog he had missed so much that he cried with excitement.

It is said that dogs are animals that understand human nature. When they are reunited with their owners, will they also cry with joy?

Recently, a research team from Azabu University in Japan and its collaborators pointed out in a study that dogs' eyes also filled with tears when they were reunited with their owners. The researchers believe that dogs shed tears due to positive emotions, and the release of oxytocin promotes tear production.

The research, titled “Increase of tear volume in dogs after reunion with owners is mediated by oxytocin,” has been published in the scientific journal Current Biology.

The "fierce dog" shed tears, perhaps because of the "hormone of love"

The study was inspired by researcher Takefumi Kikusui's dog-raising experience. Six years ago, he had two poodles, one of which gave birth to puppies. He noticed that the mother dog shed tears while feeding the puppies.

This made Kikusui think that oxytocin might cause an increase in tears. Oxytocin is often vividly called the "hormone of love". Many mammals' intimate behaviors are affected by oxytocin. It not only promotes female childbirth, but also affects the relationship between couples. It can even cross the boundaries of species and affect the relationship between owners and pet dogs.

As early as 2015, the research team of Azabu University found that the gaze of a pet dog on its owner would increase the oxytocin level in the owner's urine, and the longer the gaze, the higher the oxytocin content in the owner's urine. Moreover, the increase in the owner's oxytocin makes the relationship between the owner and the dog closer, which in turn promotes the dog's oxytocin secretion, forming a positive feedback mechanism.

When dogs are reunited with their owners, they display high-affinity behaviors, including gazing at their owners, wagging their tails, jumping up, and licking their owners’ faces. Physiologically, dogs also experience an increase in oxytocin levels during reunion with their humans.

In this work, the research team decided to explore further and study whether dogs shed tears when reunited with their owners, whether there is a regulatory role of oxytocin behind it, and what impact it will have on the relationship between dogs and their owners.

The research team measured the tear volume of dogs 5 to 7 hours after separation from their owners and within 5 minutes after reuniting with their owners. Using linear mixed model analysis, they found that the tear volume of dogs increased significantly when they were reunited.

In addition, the team also conducted a comparative experiment to measure the amount of tears when the dogs were reunited with their owners and when they were reunited with other familiar people. The results showed that the dogs secreted more tears when they were reunited with their owners.

To evaluate the role of oxytocin in the production of tears in dogs, the researchers dripped an oxytocin solution and a control peptide solution with the same amino acid composition as oxytocin but a different arrangement into the dog's eyes and measured the tear volume. The results showed that the dog's tear volume increased significantly after the oxytocin solution was dripped, but there was no increase when the control peptide solution was dripped. This finding verified the team's hypothesis that the release of oxytocin would promote tear production when the dog and its owner are reunited.

Dogs that cry have the best fate?

How does a dog's tears affect the relationship between it and its owner?

In parent-child interactions, babies convey their emotions to their parents through crying, and in response, parents display caring behaviors. The research team speculated that perhaps dogs may also trigger caring behaviors from their owners when they shed tears.

To test this hypothesis, the researchers conducted a test in which participants were shown two photos of the same dog, one of which had artificial tears in its eyes. Participants could choose which dog they would like to take care of more. The results showed that the photo of the dog with tears in its eyes was more likely to arouse human caring emotions, trigger protective behavior in the owner, and establish a stronger emotional connection.

"Dogs have become companions to humans and have developed deep bonds," the researchers said. "In this process, dogs that shed tears when interacting with their owners may receive more affection from their owners."

The experiment successfully verified the research hypothesis that dogs secrete tears when reunited with their owners, and that this tear secretion is mediated by oxytocin, which helps dogs and their owners to establish a closer bond. This is also the first time that scientists have revealed that positive emotions can stimulate tear secretion in animals and that the release of oxytocin promotes tear production.

However, this research can be further expanded. The research team hopes to continue to explore whether dog tears have social functions. For example, when dogs greet each other, sniffing and licking near the eyelids are common behaviors, which may indicate that dog tears can serve as a social signal. In addition, this study confirms that dogs seem to shed tears when humans consider them "happy", and the researchers will further experiment whether dogs shed tears due to negative emotions.

Author Jeffrey Mason said, "If a dog loves you, it will always love you, no matter what you do, what happens, or how much time passes."

So, when you meet your dog again after a long separation, remember to touch its head and say “don’t cry, don’t cry.”

Reference Links:

https://www.cell.com/current-biology/fulltext/S0960-9822(22)01132-0

https://www.eurekalert.org/news-releases/962142

https://www.guokr.com/article/440185/

https://pubmed.ncbi.nlm.nih.gov/25883356/

https://www.youtube.com/watch?v=So7VXu-BUmo