For some dog owners, a simple visit to the vet can be an adrenaline-inducing nightmare.

They’ve got their dog by leash, trying to find a seat, praying that the sight of another dog won’t set their dog off on a frenzy of snapping, growling, and lunging.

All that unpredictable aggression could be hormones, according to a new study by researchers at the University of Arizona.

“Dog aggression is a huge problem,” said lead researcher Evan MacLean. “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.”

While a number of studies have looked at the role of hormones like testosterone and serotonin in aggression in dogs, those hormones may be only part of the story, according to MacLean's findings, which were published in the September issue of the journal Frontiers in Psychology.

The study focused on two hormones that, like testosterone and serotonin, are also found in humans—oxytocin and vasopressin. Their findings suggest that both may play an important role in shaping dogs social behavior.

Oxytocin is sometimes called the “love hormone,” because its levels in humans have been shown to increase when we hug or kiss a loved one. It plays a significant role in childbirth and nursing.

Vasopressin is a hormone closely related to oxytocin. But in contrast to the love hormone, vasopressin has been linked to aggression in humans. And previous research suggests that people who have problems with chronic aggression have high levels of vasopressin.

MacLean’s team studied dogs of varying ages, breeds, and sexes, whose owners reported struggles with leash hostility. For each aggressive dog studied, researchers also studied a nonaggressive dog of the same sex, age, and breed to serve as a comparison. They measured the hormone levels of all the dogs throughout the study.

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

The dogs were also exposed to common, everyday sounds and objects, including a cardboard box, a trash bag, and an inflated yoga ball.

Researchers measured the dogs’ hormone levels before and after each exposure.

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

The researchers chose to study leash aggression specifically, because they wanted to investigate aggressive behavior in a highly controlled context, one in which all dogs could be kept a fixed distance from a controlled stimulus used to elicit aggressive responses. 

Conditions similar to what you might find in a hospital waiting room.

And while the researchers found no differences in oxytocin levels between the two groups of dogs the tested, they did find differences when they compared the aggressive dogs to the levels in a group of assistance dogs, which are bred to have nonaggressive temperaments. The assistance dogs had higher levels of oxytocin and higher ratios of oxytocin to vasopressin.

Which suggests that the human love hormone may help inhibit aggression in dogs.

Lead researcher Maclean sees great promise here. “Previous work shows dog-human friendly interactions can create a release in oxytocin in dogs,” he said, “and when dogs interact with people, we see that their vasopressin levels go down over time.”

In other words, hug more, bark less.

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