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The balance between two hormones may account for why some dogs are aggressive. – Psychology Today

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You are peacefully walking your dog on a leash when you encounter someone else coming toward you also walking a dog on a leash. Suddenly, that other dog begins to bark, growl, and lunge at your dog, who has not acted in a threatening manner or shown any aggressive signals. The dog you have encountered is showing something that behaviorists often referred to as “leash aggression“. Recently, researchers have started to zoom in on the reasons why some dogs lash out when on a leash and why others do not.
An investigative team headed by Evan MacLean, who is a professor of anthropology and director of the Arizona Canine Cognition Center at the University of Arizona, started looking at some possible factors associated with the physiology of dogs. Early researchers had focused on the possible effects of testosterone. High levels of this male hormone have been shown to be associated with aggression in humans and some other species. That is why neutering male dogs has often been recommended by veterinarians as a means of lowering aggressive tendencies. Unfortunately, data has been piling up which suggests that not only does neutering not reduce aggressive tendencies, but it actually may be associated with increased levels of aggression. Furthermore, since this is a male hormone, testosterone is unlikely to explain aggression in female dogs.
More success has been had in studies of another hormone, namely serotonin. Specifically, researchers have found that some dogs with a history of aggression also have low levels of serotonin in their blood or in their cerebrospinal fluid. Because higher levels of serotonin seem to lower the aggressive tendencies in dogs, aggressive dogs are often treated with drugs which raise the serotonin level. Particularly, drugs like Prozac, which are technically called “selective serotonin reuptake inhibitors”, have been used.
What caught the attention of this research team was the growing body of research which looked at the hormone oxytocin in dogs. Oxytocin is sometimes referred to as the “love hormone” since its levels in humans have been shown to increase when we hug or kiss a loved one, or have sex, or when mothers are nursing their infants. The levels of oxytocin have been shown to go up in dogs when they are engaged in friendly social interactions with people as well.
The interesting fact is that oxytocin has an evil twin, vasopressin, and researchers sometimes refer to them as “yin and yang” hormones. Research has shown that humans who chronically have aggression problems also tend to have high levels of vasopressin. So the Arizona researchers wondered whether this same thing might be going on in dogs.
In their first experiment, the researchers looked at the behavior and the hormonal levels of two groups of pet dogs. One group consisted of dogs who have a known history of aggression, especially toward unfamiliar things and toward other dogs, while the second group had no such aggressive history. The dogs were carefully chosen so that for each aggressive dog there was a nonaggressive dog of the same sex, age, and breed which could serve as a comparison.
The test for dog-directed aggression involved having each dog held on a leash by its owner. Across the room, there was a curtained area. From behind that curtain, researchers played a sound of a dog barking and then they pulled back the curtain to reveal a lifelike dog model with a human handler. The models resembled a Jack Russell Terrier, a Shetland Sheepdog, or an Old English Sheepdog. To test the reaction of these dogs in situations which did not involve another canine, the dogs were also walked in a room where they could make contact with common control items like a trash bag, a box, or a yoga ball, while random sound effects were being played. The dogs were also exposed to videos of other dogs engaged in miscellaneous activities.
The researchers measured the dogs’ behavioral responses and also their hormone levels before and after each interaction. None of the dogs reacted aggressively toward the inanimate objects or the videos. However many of the dogs in the leash aggressive group had hostile responses to the model dog. These responses included barking, straining at the leash as though they were trying to attack it, making threatening growling noises and so forth.
When they looked at the hormonal levels they found that the dogs that reacted aggressively showed higher levels of total vasopressin in their systems. This seemed to demonstrate the expected link between vasopressin and aggression in dogs. However, when they looked at the oxytocin levels they didn’t find any significant differences.
While the association between vasopressin and aggression appeared to be confirmed, the absence of any differences in the oxytocin levels seemed a bit puzzling, so the research team conducted a second set of measurements comparing the oxytocin levels of the pet dogs in the study to a group of assistance dogs. They used assistance dogs because they are specifically bred and behaviorally screened to be friendly, social, dogs with non-aggressive temperaments. Now the investigators found the expected differences in oxytocin levels, with the assistance dogs showing a larger total oxytocin than the pet dogs.
Lead researcher MacLean believes that these results may be very helpful in dealing with aggressive dogs since vasopressin and oxytocin are antagonistic hormones which tend to neutralize each other in their effects. Current therapies for aggressive behavioral problems in dogs have usually tended to target testosterone or serotonin, but MacLean suggests “It would be reasonable to think that if vasopressin facilitates aggression, you could develop pharmaceuticals that could target the vasopressin system to help in cases where dogs are really aggressive. Oxytocin and vasopressin are being used extensively as therapeutics in humans right now. Regulation of the oxytocin system has been implicated in things ranging from autism to schizophrenia to posttraumatic stress disorder, and there are clinical trials looking at administering oxytocin as a drug to create some kind of behavioral response. It’s interesting to think that maybe some of these same therapies that we are trying with people could be useful in dogs.”
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MacLean EL, Gesquiere LR, Gruen ME, Sherman BL, Martin WL and Carter CS (2017). Endogenous Oxytocin, Vasopressin, and Aggression in Domestic Dogs. Frontiers in Psychology, 8, Article 1613. doi: 10.3389/fpsyg.2017.01613
Stanley Coren, Ph.D., FRSC., is a professor of psychology at the University of British Columbia.
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