Scientists May Have Found the Hormone That Makes Dogs Aggressive

Many dog owners believe neutering will curb aggressive tendencies in their pets. Balakate/Shutterstock

Most dogs on leashes pass each other with the usual pleasantries, a greeting that involves a deep whiff of each other’s most intimate parts.

But some dogs, for some reason, get whipped up into a frenzy.

Called leash aggression, the behavior often puts owners in some very awkward situations, heaving at the leash while apologizing for their dog’s sudden turn for the homicidal. Leash aggression has been the bane of dog behaviorists, too, who often try without success to curb it.

Maybe it’s better addressed by science. Researchers at the University of Arizona claim to have zeroed in on the specific hormones — oxytocin and vasopressin — that determine how dogs get along with each other. Understanding how they work, the researchers claim, may be the real key to preventing tragedy for both dogs and humans.

“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," Evan MacLean, the psychologist and anthropologist who led the study, says in a press release. "If there are ways to intervene and affect biological processes that produce aggression, that could have a huge benefit both for people and dogs."

Finding the right balance

Oxytocin and vasopressin, which are also found in humans, are intimately connected, with one hormone balancing the other. At one end of the spectrum, oxytocin levels rise when humans connect lovingly with each other. In fact, it’s often called the “love hormone,” influencing the body when we hug or kiss.

Vasopressin, on the other hand, is like a dark but necessary twin. It’s the hormone often linked with anger and aggression.

Together, these hormones would ideally regulate each other. And in dogs, at least, they can strike a balance, allowing the animals to go ahead and sniff each other intimately — but not too intimately, lest they elicit a snarling warning.

Dog barking on grass at park
The hormones oxytocin and vasopressin have been described as the yin and the yang in balancing a dog's behavior. elbud/Shutterstock

For the study, University of Arizona researchers looked at dogs with a history of leash aggression. Then they paired them with dogs of the same sex, age and breed who weren’t known for being ornery on a leash.

Several walking scenarios were presented to the leash-aggressive dogs, all of them centering on a curtain with barking sounds coming from behind it. When the curtain was lifted, there was either a box, a bag — or a cutout of a dog.

Guess which one got the test dog howling mad? The sight of a dog, even if it was made of cardboard, got the vasopressin flowing.

The peace-loving and leash-raging dogs didn’t show much difference in their “love” hormone levels. That seems to suggest there’s plenty of love to go around, even from the growling, howling dogs. But for some reason, some of these dogs lose their "yin" entirely when their "yang" is fired up.

A better way than going under the knife?

Oxytocin and vasopressin aren’t the first hormones to be linked with dog behavior. Researchers have long looked at the role hormones like testosterone and serotonin play. Many dog owners have tried to regulate aggression through surgery, hoping tht neutering an animal will reduce testosterone levels and, in turn, aggression.

But in studies, the "surgical solution" has yielded decidedly mixed results, with some dogs becoming even more aggressive after being neutered.

The new research may open the door for owners to explore a pharmaceutical solution for their leash-aggressive pals.

"It would be reasonable to think," MacLean says, "that if vasopressin facilitates aggression, you could develop pharmaceuticals that could target the vasopressin system to help in cases where dogs are really aggressive.”