Dogs are known for being a man’s best friend because of the close bond many dog owners share with their pups. This bond is solidified when dogs gaze at their owners, according to a study published in the April 17 edition of the journal Science.

Researchers at Azabu University in Japan have found that when dogs and humans exchange gazes, both experience rushes of oxytocin. Oxytocin, often called the “the love hormone,” is often associated with feelings of trust, nurturing and attachment. The brain also experiences an oxytocin rush when a mother looks into her child’s eyes, or when a couple is falling in love.

“Oxytocin is a hormone that is important for many functions, such as giving birth, lactation and sexual behavior,” said Steve Chang, an assistant professor of psychology at Yale University, in an email. “Oxytocin is also critical in mediating the bonding between mother and infant. More recently, accumulating evidence suggests that oxytocin is involved in regulating various types of social cognition even among strangers.”

A human’s oxytocin levels are greatly influenced by social interaction, Chang said.

“[The] oxytocin system is engaged in many everyday social interactions,” Chang said. “Oxytocin in the brain seems to enhance social attention and social motivation. When oxytocin controls these processes, it can amplify either pro-social or antisocial behaviors depending on the context and individuals.”

In the study, owners and dogs played for half an hour. Urine tests conducted before and after the play session determined that oxytocin levels were higher among the owners and dogs that had exchanged more gazes. Researchers then repeated the experiment with domesticated wolves and did not observe an increase in oxytocin levels.

The lack of an oxytocin rush occurring between wolves and their owners may show that dog and human interaction through eye contact is an unique trait, according to Takefumi Kikusui, corresponding author and a professor of companion animal research at Azabu University.

“We [were] surprised that we have [these results] with dogs, especially using eye gaze,” Kikusui said. “Because eye gaze from human to animals are usually threatening, not affiliated.”

To confirm the correlation between oxytocin and gazing, researchers also sprayed the hormone into an experimental group of dogs’ noses and then observed the dogs’ interactions with their human owners. Female dogs gazed at their owners longer than male dogs who had received the same oxytocin dosage, according to the study.

The study alludes to the possibility of co-evolution between humans and dogs, Kikusui said.

“Dogs have acquired the skills to obtain the care from the humans using eye gaze,” Kikusui said. “On the human side, we have no data yet, but there is a possibility to obtain the [affiliated] connections with other species, which can be accelerated by cohabitation with dogs.”

The physiological effect of co-habitation manifests in this study, according to Chang.

“The most interesting part of the study is that the oxytocin system is also involved in social cognitive processes occurring across two very different species—humans and dogs,” Chang said. “Through domestication, it is likely that the brains of both humans and dogs have been ‘repurposed’ to regard each other as important social partners in such ways that the interactions engage the oxytocin system.”