First off, biological mechanisms regulating dog aggression are not well understood. It’s complicated and we get that! We hope increasing the knowledge present on the mechanisms that affect aggression, will help promote healthy relationships between people and dogs.
To start, there are three types of signaling molecules that regulate physiology and behavior factors in underlying canine aggression. Hormones, neurohormones, and neurotransmitters influence canine behavior on a biochemical level.
Terminology
Hormones are synthesized by several glands, organs and tissues, and release hormones that make up the endocrine system. They act as chemical messengers responsible for regulating certain activities.
The Hypothalamus is located in the brain and is responsible for making hormones. It plays a role in managing body temperature, hunger, thirst, mood, sex drive, blood pressure and sleep. The hypothalamus is an important link between the endocrine system and nervous system.
Neurotransmitters are the body’s chemical messengers, important modulators of behavior, synthesized in neurons; distributed by neurons at nerve endings.
Neurohormones are hormones produced by nerve cells and distributed by neurons into the blood circulation.
How Does It Work?
At the University of Arizona, Dr. Evan MacLean, director of the Arizona Canine Cognition Center conducted a study on the effects of vasopressin and oxytocin on dogs with leash aggression.
In the study, dogs that reacted aggressively showed higher levels of vasopressin in their systems. In the group of study dogs, no differences in oxytocin were observed. However, when study dogs were compared to assistance dogs, dogs bred to have non-aggressive temperaments, they found that assistance dogs had higher levels of oxytocin – thus supporting the idea that oxytocin may help inhibit aggression.
This correlation is one of the most important aspects in the study. It is also where training begins to become a proficient aspect of assistance dogs developing a deep bond with their partner. Overall, dogs who behaved aggressively towards strangers in the study had higher levels of vasopressin. The balance between oxytocin and vasopressin is hypothesized to mediate emotional states and social behaviors. Or, in simpler terms, they cancel eachother out.
A way to boost oxytocin and reduce vasopressin is through friendly dog-human interaction.
Figure: Comparison of free oxytocin and vasopressin in pet and assistance dogs
Assistance dogs had higher levels of oxytocin and lower levels of vasopressin.
Studies have shown that aggressive dogs have low levels of serotonin in their brain. The Norwegian School of Veterinary Science published a study by Jorn Vage and other researchers on the association of dopamine and serotonin related to canine aggression. Human directed aggression was studied using 50 aggressive dogs and 81 non-aggressive dogs. They report that reduced levels of serotonin and increased levels of dopamine increase levels of aggressive behavior. Serotonin is believed to apply behavioral control with the influence of dopamine.
| Biological Mechanisms Involved in Canine Aggression | ||
| Hormone | Production | Role |
| Vasopressin– Potentially facilitates aggression. | A neurohormone, synthesized in the hypothalamus. | Plays a role in water retention and in regulating aggression toward unfamiliar individuals. Increased levels of vasopressin are associated with increased levels of aggressive behavior. |
| Oxytocin-Love hormone *Deficits in oxytocin might contribute to hostility, fear and mistrust that provide the preconditions of aggressive behavior. | a neurohormone and a neurotransmitter, produced in the hypothalamus. | Involved in the birthing process, nursing and intimate relationships. Thought to inhibit aggression. Released during sensory stimulation such as touch. When oxytocin is increased cortisol is decreased and when stressful events increase cortisol, levels of oxytocin decrease. |
| Testosterone- a sex hormone produced by males and females. | A steroid hormone produced in reproductive organs and the adrenal gland. | Plays a role in growth and behavior. It affects bone growth and strength. Testosterone levels contribute to aggressive behavior in male dogs. |
| Serotonin- “Happy hormone”* Low levels are found in aggressive dogs. Administration of drugs that increase serotonin reduces aggression. | A neurotransmitter that also acts as a hormone.The majority of the body’s serotonin is produced in the intestines while the remainder is produced in the brain stem. | Main function is to stabilize mood and feelings of happiness. When serotonin levels decrease, cortisol levels increase. When cortisol levels decrease, serotonin levels increase. |
| Cortisol- “Stress hormone” | A steroid hormone, produced in the adrenal gland. Released by the hypothalamus. | plays an important role in regulating the body’s stress response. The body releases cortisol when it experiences stress. |
| Dopamine- “Happy hormone” Increased levels of dopamine are associated with aggression | Both a neurotransmitter and a neurohormone. It is synthesized by neurons at the base of the brain and the adrenal gland. Released by the hypothalamus. | Plays a role as a reward center. Controls mental and emotional responses. |
Aggressive behavior is influenced by cognitive, genetic and environmental factors as well as hormonal and physiological processes. A better understanding of the biology behind aggression could help with the development of interventions used to rehabilitate aggressive dogs.
The roles of hormones, neurotransmitters and neurohormones play on aggressive behavior needs more attention. Learning how vasopressin, oxytocin, testosterone, serotonin, cortisol, and dopamine affect aggressive behavior will provide owners with solution based methods for their pets.
