Many hormones influence canine aggression, according to a new study by researchers at the University of Arizona titled, “Endogenous Oxytocin, Vasopressin, and Aggression in Domestic Dogs”. This is no surprise given that the hormones testosterone and serotonin have a huge influence on aggressive behavior, but this study provides evidence that high vasopressin levels are associated with aggression, and that high levels of oxytocin are associated with the absence of aggression in dogs. Previous work has shown that oxytocin levels in dogs are elevated by positive interactions with people. (In humans, oxytocin is important in both childbirth and in breastfeeding, and is also known to facilitate social bonding. Vasopressin is also influential in people, with previous research indicating that people with long standing aggression problems have high levels of this hormone.)
Dogs with a history of behaving aggressively to other dogs were recruited for this study, and for every dog recruited, a non-aggressive dog of the same age, sex and breed was also recruited. In one experiment, dogs were on leash and exposed to a recorded sound of a barking dog behind a curtain. then the curtain was pulled back, revealing a realistic dog model with a person. Dogs were also tested with videos showing dogs exhibiting various non-aggressive behaviors. (In control trials, they were also exposed to random sound effects everyday objects such as a box or a yoga ball. No dogs reacted with aggression to these objects.)
In all trials, the dogs’ hormone levels were recorded before and after the exposure to what was behind the curtain. Many of the aggressive dogs did react to the model dog with barking, lunging and growling, but there were almost no reactions to the controls or the videos. The dogs who reacted aggressively had higher levels of vasopressin than dogs who did not react, but no differences in their oxytocin levels were found.
Another experiment in this study compared hormone levels of dogs in an assistance guide dog training program to those of the pet dogs in the study. Researchers found that these assistance dogs had higher levels of oxytocin than pet dogs, but did not find differences in vasopressin levels between these two groups of dogs.
The assistance dogs are from a population of dogs who have been bred for over 40 years for traits such as friendliness, calm temperaments and the lack of aggressive behavior. At the physiological level, they showed a difference in oxytocin levels when compared to pet dogs, suggesting that the selective breeding of these dogs may have been acting on oxytocin levels, and that changes in the levels of that hormone may also influence the likelihood of aggressive behavior.
There is a never ending quest for ways to help dogs overcome aggressive behavior. This study indicates that there may be value in pursuing treatments based on targeting both vasopressin and oxytocin.