The Hormone That Could Explain Your Dog's Aggression

By Jane Farrell

Dogs who exhibit “leash aggression” – barking, growling or lunging at other dogs while out for a walk – may be influenced by hormones that have not been recognized until now, new research shows.

The study, led by the University of Arizona’s Evan MacLean, was published in a special issue of the journal Frontiers in Psychology.

Existing interventions for dogs’ behavioral problems often target testosterone and serotonin — the other two most commonly studied hormones in canine aggression. Pet owners commonly neuter male dogs to help manage levels of testosterone, which has been linked to aggression. And to manage levels of serotonin, which is believed to reduce aggression, some dogs are prescribed SSRIs, or selective serotonin reuptake inhibitors, the most common type of antidepressants.

But MacLean and his collaborators looked specifically at oxytocin and vasopressin — hormones that are also found in humans — and found that they too may play an important role in shaping dogs’ social behavior.

Better understanding the biology behind canine aggression could help with the development of interventions, said MacLean, an assistant professor of anthropology and director of the Arizona Canine Cognition Center in the UA School of Anthropology.

“Dog aggression is a huge problem. Thousands of people are hospitalized every year for dog bites, especially kids, and aggression is one of the main reasons that dogs get relinquished to shelters,” MacLean said in a news release from UA. “If there are ways to intervene and affect biological processes that produce aggression, that could have a benefit both for people and dogs.”

MacLean was interested in oxytocin and vasopressin — sometimes thought of as “yin and yang” hormones — because of the growing research on their role in the biology of social behavior.

Oxytocin, which is significant in childbirth and nursing, is sometimes called the “love hormone,” as its levels in humans have been shown to increase when we hug or kiss a loved one. In contrast to oxytocin, vasopressin has been linked to aggression in humans, with previous research suggesting that people with chronic aggression problems have high levels of vasopressin.

For the current study, MacLean and his collaborators recruited pet dogs of varying ages, breeds and sexes, whose owners reported struggles with leash aggression. For each aggressive dog recruited, the researchers found a non-aggressive dog of the same sex, age and breed to serve as a comparison.

During the experiment, each dog was held on a leash by its owner. Across the room, an experimenter played audio of a dog barking behind a curtain, before pulling back the curtain to reveal a lifelike dog model with a human handler.

The dogs in the study were presented in the same way with everyday noises and three common objects — a cardboard box, trash bag and an inflated yoga ball.

The dogs’ responses and hormone levels were measured before and after the interaction.

While none of the dogs in the study reacted aggressively toward the box, bag or ball, many of the dogs in the leash aggression group had aggressive responses to the model dog, including barking, growling and lunging.

The dogs that reacted aggressively showed higher levels of total vasopressin in their systems, suggesting a link between vasopressin and aggression.

The researchers did not observe differences in oxytocin levels between the two groups of dogs. However, when they compared the oxytocin levels of the pet dogs in the study to a group of assistance dogs, which are specifically bred to have non-aggressive temperaments, they found that the assistance dogs had higher levels of oxytocin and higher oxytocin-to-vasopressin ratios. This supports the idea that oxytocin may help inhibit aggression in dogs.

Future research might consider new canine interventions focused on vasopressin and oxytocin, MacLean said.

A piece of good news for pet owners and their pups: One way to boost dogs’ oxytocin levels and decrease vasopressin is through friendly dog-human interactions. And the effect extends to people as well.

“Previous work shows dog-human friendly interactions can create a release in oxytocin in dogs, and when dogs interact with people, we see that their vasopressin levels go down over time,” MacLean said. “These are bidirectional effects. It’s not just that when we’re petting a dog, the dog is having this hormonal response — we’re having it, too.”