By Dr. Karen Shaw Becker
Aggression in dogs can be a touchy subject, because there are many competing schools of thought on what causes it, as well as a lot of emotion around the issue. However, no matter the cause, we can all agree aggressive behavior in canines is a significant problem, with serious ramifications.
Millions of dog bites occur in the U.S. each year, and thousands of people seek medical treatment for their injuries. In addition, aggressiveness is the leading reason for relinquishment of dogs to shelters. Given the extent and seriousness of the issue, it's important to investigate all potential causes and remedies for canine aggression.
Toward that end, a group of university biologists has been studying the role of two specific hormones in domestic dog aggression, and published their findings recently in the journal Frontiers in Psychology.1
"If there are ways to intervene and affect biological processes that produce aggression, that could have a huge benefit both for people and dogs," says Evan MacLean, Ph.D., assistant professor of anthropology and director of the Arizona Canine Cognition Center in the University of Arizona School of Anthropology.2
Two Hormones That May Play a Role in Dog Aggression
Past studies of aggression in dogs and other animals have explored the role of testosterone and serotonin. MacLean and his collaborators decided instead to evaluate oxytocin and vasopressin because they're often considered the "yin and yang" hormones and may play an important role in shaping social behavior.
Oxytocin, nicknamed the "love hormone," increases in humans when they are physically affectionate with a loved one. Vasopressin, while closely related to oxytocin, is associated with aggression in humans. In fact, studies show high levels of vasopressin in people with chronic aggression problems.
Study of Dogs Shows Link Between Increased Vasopressin Levels and Aggression
For their study, the researchers recruited leash-aggressive family dogs of both sexes and varying ages and breeds. They paired each of those dogs with a nonaggressive dog of the same sex, age and breed (the control group).
Each dog in the study was held on a leash by its owner while a person across the room and behind a curtain played a recording of a dog barking. Then the curtain was pulled back, revealing a life-size dog model with a human. The same scenario was repeated with everyday noises coupled with three common objects: a cardboard box, a trash bag and a yoga ball.
The dogs' responses and hormone levels were assessed both before and after each presentation. None of the dogs showed aggression toward the box, bag or ball. However, many of the leash-aggressive dogs had aggressive responses (growling, barking and lunging) to the life-size dog model, and those same dogs had higher levels of vasopressin in their systems.
The oxytocin levels remained about the same in the two groups of dogs, so the researchers compared their levels to the oxytocin levels in a group of assistance dogs who are selectively bred to have nonaggressive temperaments. The assistance dogs had higher levels of oxytocin than the study dogs, and higher oxytocin-to-vasopressin ratios as well.
"Seeing high oxytocin levels in assistance dogs is completely consistent with their behavioral phenotype — that they're very, very friendly dogs that are not aggressive toward people or other dogs," MacLean said.
These study results suggest a link between vasopressin and aggression, and they also suggest that oxytocin plays a role in inhibiting aggression in dogs.
Findings Could Open the Door for New Ways to Manage Canine Aggression
Past research has focused on the role of testosterone and serotonin in canine aggression, with the result that dog parents are often advised to neuter male dogs to lower their testosterone levels either to prevent or treat aggression. Serotonin is thought to reduce aggression, so it's not uncommon for dogs with aggressive tendencies to be treated with SSRIs (selective serotonin reuptake inhibitors), which are antidepressants. Neither of these solutions is ideal for every aggressive dog.
Research into vasopressin and oxytocin may lead to new approaches to treating aggressive dogs. Unfortunately, with conventional veterinary medicine leading the charge, those new approaches will most likely be in the form of synthetic drugs that manipulate natural vasopressin and/or oxytocin levels in the body. The good news is the holistic veterinary community is often able to use this type of research to discover nontoxic substances with similar actions and few or no side effects.
Life Experiences May Play a Role in Increasing Vasopressin Levels
MacLean believes life experiences may play a role in increasing a dog's level of vasopressin.
"There's a lot of work showing that experiences in your lifetime can change the way hormones function," MacLean told ScienceDaily. "For a lot of dogs that have aggression problems, the owners report that the onset of the aggressive symptoms happened after some sort of traumatic experience.
Often it was that the dog was attacked by some other dog and is in a hypervigilant state after that event — almost like a post-traumatic reaction."
The good news is that it's possible to influence your dog's behavior by interacting with him in a friendly, nonthreatening way. Studies show these types of loving dog-human interactions actually increase oxytocin release in dogs, and lower vasopressin levels. An added bonus is that your hormone levels also respond positively:
"These are bidirectional effects," says MacLean. "It's not just that when we're petting a dog, the dog is having this hormonal response — we're having it, too."
The Ultimate Goal: To Preserve and Protect the Unique and Long-Standing Relationship Between People and Their Dogs
In the final paragraphs of the study, the co-authors make a compelling case for why their work is so important:
"Ultimately, dog aggression is a normal and adaptive social behavior, but expressed in the wrong contexts, or to an extreme extent, its consequences jeopardize the welfare of both humans and dogs in our society. It is likely that dog aggression can be motivated by diverse psychological states, including fear and anger.
These emotional processes may be facilitated by, or produce effects on, OT [oxytocin] and AVP [vasopressin] signaling in the brain. Thus, it is important to consider dog aggression at multiple levels of analysis, addressing both the cognitive processes (e.g., appraisal, learning, inhibition), and underlying physiological mechanisms, which mediate these behaviors.
The studies presented here suggest that OT and AVP may play important roles in these socioemotional processes, and set the stage for future work evaluating whether treatments and interventions for aggression can be improved by considering the roles of these neuropeptides.
Ultimately, we hope that these investigations will lead to increased knowledge of the biology of social behavior, promote human and animal welfare, and help to preserve the unique and long-standing relationship between humans and dogs."3
Dogs with aggressive or fear-based behaviors aren't happy campers, and studies such as this one shed light on how we can make life better for those pets and their humans. Dogs are individuals, and a dog's seemingly inappropriate response to certain stimuli may be entirely appropriate based on his internal wiring and life experiences.
How we manage dogs with aggression is critical in helping them have happier futures, and new treatments coming out of studies like this may provide promising adjunctive therapies to behavior modification programs that help aggressive dogs live more balanced lives.