Dogs tear up with joy when they are reunited with owners, according to a new Japanese research

Dogs are usually extremely excited to see their human beings after a long time, wagging their tails, barking cheerfully and jumping on you with excitement. However, typically, dogs don't cry as people do and their tears have never previously been associated with emotion. Now, new findings from a Japanese study indicate that when reunited with their owner after a separation, a dog's eyes may well up with tears of joy, reports CNN. Just like human beings, dogs also have tear ducts that produce tears to keep their eyes hygienic. The tears may also help strengthen the age-old connection between dogs and humans.  

After observing one of his two standard poodles tear up when his dog was feeding her puppies six years ago, Takefumi Kikusui, a professor at the Laboratory of Human-Animal Interaction and Reciprocity at Azabu University in Japan, decided to look into dog tears. According to a press release from Kikusui, who co-authored the study that was published Monday in the journal Current Biology, "We found that dogs shed tears associated with positive emotions." Referring to the hormone that is sometimes referred to as the maternal hormone or the hormone of love in humans, he added, "We also found oxytocin as a potential mechanism underlying it." 

Kikusui and his team used the Schirmer tear test, a common test, to measure the amount of tears in 18 dogs to observe the connection. It involved placing a paper strip inside the dogs' eyelids for a minute before and after their five- to seven-hour separation from their owners. "Tear volume was evaluated by the length of the wet part on the STT. The baseline was about 22 mm, and the reunion with the owner increased by 10%," Kikusui told CNN via email.

The amount of crying before and after reunions with owners and people the dogs were familiar with was then compared by researchers using 20 dogs. The only thing that made them cry more was seeing the owner again. For studying the link between oxytocin and dog tears, a solution containing oxytocin was applied to the surface of 22 dogs' eyes. The amount of tears significantly increased after the oxytocin was administered.

Dogs have previously also shown that they are aware of human emotions. Researchers have been able to establish that dogs have categorized emotions such as “You are somebody I care about, therefore, I’m pleased to see you,” and “You are somebody I don’t care about, so I can ignore you most of the time,” said Dr. Daniel Mills, a veterinary behavioral medicine specialist at the University of Lincoln in England who was not involved in this study, according to The New York Times.

Researchers didn't test to see if dogs cried when they felt bad like humans frequently do. They also don't know whether a dog's capacity for crying serves a social purpose in the canine community. But, according to Kikusui, it's possible that people would treat the dogs that cry better. His team asked 74 participants to rank the dogs based on images of their faces with and without artificial tears. When people saw dogs with teary eyes, their reactions were more favorable.

"Dogs have become a partner of humans," Kikusui said in a statement, "and we can form bonds."

“If we accept the evidence of this paper, this is one of the most stunning discoveries in animal expression of emotions of all time,” Clive Wynne, a canine behavior specialist at Arizona State University, told The New York Times adding that it would take "a lot" to convince him.

Dr. Mills, however, does support the researchers' assertion that dog tears may play a role in human-dog relationships. “It might be that things like a more glossy eye or the presence of tears do encourage nurturing tendencies in us,” Dr. Mills said, “in the same way that short-nosed breeds of dogs with high foreheads make us want to care for them more.”