Paul J. Zak, a professor at Claremont Graduate University, whose lab showed the brain produces the chemical oxytocin when someone treats a person with kindness, wrote in The Atlantic that oxytocin was first found to be released when a new mother interacted with her baby and later between lovers during sexual intercourse.
However, it was later found many agreeable human interactions such as trusting also cause the release of oxytocin -- even among complete strangers.
In a small-scale experiment for BBC television, Zak tested the oxytocin levels of different animals at an animal refuge in Arkansas, where numerous animals interacted with each another.
A domestic mixed-breed terrier and a goat regularly played with each other. Zak said after 15 minutes of play, the the dog had a 48 percent increase in oxytocin indicating the dog was quite attached to the goat and viewed the goat as a friend. However, the goat had a 210 percent increase in oxytocin, indicating the the goat might have been in love with the dog.
Zak told the Independent the findings suggested pets might feel love for their owners.
“That animals of different species induce oxytocin release in each other suggests that they, like us, might be capable of love," Zak told the Independent. "It is quite possible that Fido and Boots may feel the same way about you as you do about them. You can even call it love.”
In another experiment, Zak had 100 study participants' blood tested for oxytocin and other baseline physiologic states. After the participants interacted with a dog or a cat -- and then other humans -- oxytocin levels were tested again.
Neither the dog or the cat consistently increased oxytocin in humans -- 30 percent of the humans had an increase in oxytocin after playing with an animal. However, those who had owned dogs in the past were more likely to have increased oxytocin levels than cat owners, or those who had never owned pets.