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Canine Aggression Part 1: The Brain

Updated: Aug 11, 2020

The following is an excerpt from my final thesis written for my Canine Behaviour Professional Diploma.


The reasons people choose to bring a dog into their home are varied. Some may wish to simply have a four-legged companion to share daily walks and chesterfield snuggle time with, others may have a desire to compete in any number of available dog sports, such as agility, obedience, scent detection work or man-trailing. Dogs may be brought into the family as a working dog who herds sheep or protects the flock or be raised to be police/military dogs. Regardless of the reason or the dog’s intended purpose when first brought home, the dog is meant to serve a purpose for the people who are committing to care for it. The excitement surrounding bring a new dog into the family is fairly universal…dreams of how time will be spent with the new companion, and what life will be like over the coming years are full of hope, joy and promise. While it can be generally expected that not many dogs will go their entire life without some kind of behavioural hic-up, having to live with and support a dog who winds up being labelled as ‘aggressive’ is not normally part of that initial dream.

Of all the ‘behavioural problems’ that dogs can end up suffering through, canine aggression may be one of the most difficult to live with and work through, even with the support of a trained canine behavioural professional. The wide variety of beliefs as to the motivation behind aggressive displays, the numerous training methods and tools that are available to utilize and the myriad of additional contributing factors that could be making things worse can make the process of helping a dog with this struggle quite difficult.

This paper will discuss canine aggression in detail. It will explore what exactly canine aggression is and how it may and may not serve a purpose from an evolutionary perspective. The function of the different areas of the brain, along with the roles of different neurotransmitters and hormones in the body will be explained and linked to a dog’s emotions and behavioural displays. Factors that may result in a dog suffering with aggression with be explained – factors such as their genetics, early socialization, and experiences as well as subsequent lifestyle and environmental factors, training methods, health and physiology, and nutrition. The numerous types of aggression will be discussed as will how a person could best respond in a situation where they find themselves in the presence of a dog either displaying aggressive behaviours towards them, or another person/animal. Finally, strategies for helping a dog who struggles with aggression, and their family, will be outlined along with an explanation of different pharmacological and holistic treatments, as well as when and if euthanasia should ever be considered.

Canine Aggression – What is it?

Behaviour can be defined as an outward expression of an internal emotional state. It can be influenced by a variety of factors, either singly or in combination. Some of these factors include: the environment, health and physiology (illness, pain), training methods, biology (genes), breed specific behaviour, medication (side-effects), accidental reinforcement and learning from others. One could say that behaviours displayed by dogs that humans consider to be problems are really just normal dog behaviour. They are only problems because they pose some sort of inconvenience to us. To some extent, this may be true. However, a dog displaying aggressive behaviours is suffering on an emotional level and so the problem really goes beyond being an inconvenience just to people. A dog resorting to aggression is communicating that it is uncomfortable with how things are in its immediate environment and that it needs help.

Aggression is defined as the challenge or intent to do harm. Aggressive behaviours in dogs can be directed at people, other dogs, objects and/situations, and there is on-going debate as to what actually constitutes aggression. Behaviours often labelled as aggressive include barking (a low, throaty bark), growling and snarling (with or without showing the teeth), snapping (often at the air, with no intent to harm, as a warning to ‘back away’), lunging, staring and posturing (in order to increase the distance between the dog and the threat. It can result from feelings of fear, pain, frustration, over arousal, anxiety, stress and occurs when a dog is pushed beyond its natural limits of coping.

In order to better understand how and why a dog may be pushed beyond its ability to cope and resort to aggressive behaviour, an understanding of the brain is needed as there is a link between emotions and physiological responses in the body…and these are controlled by the brain.

The Brain

The brain is divided into three main areas, each of which use information from the body and perform specific functions that in turn affect a dog’s behaviour.

The largest area of the brain goes by many names: forebrain, cerebrum, cerebral cortex or ‘thinking brain’. It is divided into two main parts: the telencephalon, which is comprised of the frontal and temporal lobes and is responsible for memory, learning, temperament and intelligence, and the diencephalon, which is comprised of the parietal and occipital lobes and is responsible for taking in information and translating from the senses. Also in the forebrain is the thalamus which relays sensory information to the diencephalon, the hypothalamus which controls the release of pituitary hormones and regulates behavioural responses, the amygdala which is the survival and defence area, the hippocampus which helps keep rage and aggressive responses in check and the olfactory bulb which is the scent processing centre.

Between the forebrain and the brainstem is the second area of the brain, the midbrain, or mesencephalon. This area of the brain allows information to move back and forth between the forebrain and the hindbrain and allows for environmental associations to be made. It is also involved in regulating vision, hearing and sleep cycles, may play a role in associating arousal and punishment, is important in completing tasks requiring attention and alertness and provides a stable structure for the higher brain structures to develop on.

The last main area of the brain is the hindbrain, or metencephalon. This is also known as the ‘survival brain’ and is where the fight or slight reflex originates. This part of the brain controls fine motor skills, the regulation of blood flow, pulse rate, processes rewards and is where reflexes for reactivity and reactiveness come from. It contains the brain stem and cerebellum that are both important in dealing with the nerves and impulses that flow from all parts of the body to the brain and vice versa, as well as controlling voluntary movements. The medulla oblonglota in the brain stem is the first part of the brain to develop and controls involuntary functions such as the heart beating, respiration, swallowing and pulse rate. The cerebellum, or ‘little brain’, communicates to the cerebral cortex via the pons and controls voluntary movements.

The central nervous system is one of two main divisions of the nervous system (the other being the peripheral nervous system). It is comprised of the brain and the spinal cord. The brain is comprised of brain cells, called neurons, that transmit information from our senses to our brain and from our brain to our body. Theses neurons have a cell body in the middle surrounded by dendrites on one side and an axon on the other that transmits nerve impulses away from the cell body, across a synapse to the dendrites of another neuron. This creates a neural pathway along which nerve impulses travel.


Neurotransmitters are chemicals that are released by the axon of neurons in the brain (the brain cells responsible for transmitting information. These chemicals transmit signals across the synapse from one neuron to another and can change the way another neuron responds by either exciting or inhibiting the message. Three neurotransmitters important in understanding fear, anxiety and reactive responses in dogs are noradrenaline, dopamine and serotonin.

Noradrenaline deals with responses to stress, danger and/or threats. It stimulates the central nervous system to trigger arousal, alertness, vigilance and attention in the presence of a trigger. When a dog is confronted with something that makes it uneasy, this is the chemical that guides its attention to it and puts the dog on alert. When the fight or flight response is triggered and the sympathetic nervous system is activated, noradrenaline is released. The body’s heart rate and breathing rate increases and there is an increase in blood glucose levels (to provide energy to muscles in the case that ‘flight’ needs to occur). Chronic exposure to high levels of noradrenaline can result in gastrointestinal difficulties, anxiety, and muscle tension.

Dopamine works with the prefrontal cortex and helps with behaviour modification as it is a ‘feel good’ chemical that also plays a part in motivation. “Low levels of dopamine make people and other animals less likely to work for things, so it has more to do with motivation and cost/benefit analyses than pleasure itself.” (Buckley, 2012). A dog with low dopamine levels may be very quiet, less inclined to initiate movement, and seem depressed. Increased levels of this neurotransmitter increase the motivation level in a dog and make positive training possible as it drives a dog to repeat rewarded behaviour. Finding rewards that our dogs absolutely love can make modifying behaviour much easier. Using a favourite toy or an especially tasty type of treat or food when working with a stressed, anxious, fearful or reactive dog and help to increase dopamine levels, reduce stress and push the dog to continue trying a task or behaviour.

Serotonin is responsible for emotional states, bonding behaviours and regulates mood balance alongside the hypothalamus. It also plays a role in regulating heart and lung function, sleep-wake cycles, behaviours, awareness of pain, appetite, body temperature and movement. This neurotransmitter can be affected by diet, which has a direct impact on an animal’s mood. Nutritionally derived serotonin is something the body relies heavily on, so a poor diet that lacks tryptophan results in lower serotonin levels and increased mental instability. In many species, dogs included, depletion of serotonin in the body has been linked with aggression. Increasing serotonin in these cases may help to decrease the aggressive behaviour, as it has a strong inhibitory effect on emotional responses and impulsive behaviour, alongside positive behaviour modification techniques. The use of positive behaviour modification techniques is important as serotonin also “directly attenuates the subjective experience of pain occurring during highly emotional displays involving anger or aggression, thereby mitigating against the effectiveness of physical punishment in the control of emotionally charged (affective) aggression.” (Dog Behaviour).

Too much serotonin in the body can result in ‘serotonin syndrome’ which is a potentially life-threatening condition caused by exposure to antidepressant medication given at incorrect levels. Because serotonin impacts so many of the body’s systems, symptoms of serotonin syndrome include: diarrhea, tachycardia (rapid heart rate), rapid breathing, increased body temperature, high blood pressure, trembling, seizures, difficulty walking, rigid muscles, confusion, depression, hyperactivity, lethargy, agitation or aggression, behavioral abnormalities, temporary blindness and comas. If treatment is given quickly (within half an hour of noticing symptoms, activate charcoal can be digested to help absorb the antidepressant and reduce the amount that is taken up by the dog.

It is important that the levels of these neurotransmitters are kept in proper balance to ensure stable behaviour. Keeping a dog’s stress levels low, setting up an environment for them that promotes a general calm state of mind and ensuring that they are receiving proper nutrition can help in optimizing the roles of these neurotransmitters in the body.

Neural Pathways

Our dogs can learn new things because of neural plasticity, which is the ability to form new pathways for neurotransmitters to follow within our brain. The more a behaviour and response occur, the more efficient the synapse between the neurons in the brain will become, and the more this neural pathway will get used in the future. The repetition makes the synapse stronger making it easier for nerve impulses (both positive and negative) to travel along it. Dr. Tom Mitchell, in his book ‘How to Be a Concept Trainer’ describes these neural pathways as tunnels in the dog’s brain that each represent a choice that the dog can make. The more a choice can be made, the more likely it will occur in the future and so ‘dogs will do what they’ve always done’ if the behaviour has led to a desirable outcome for them. Strengthening the neural pathways that represent good choices and positive behavioural outcomes is what we need to help our dogs with in order to decrease the likelihood of poor behaviour. Dogs who repeatedly show aggressive responses to people or other animals have strengthened the neural pathway in their brain that reinforces this behaviour. They will continue to respond in the same way unless a new pathway with a different choice is not only created but made to be deemed more desirable by the dog.

Table from "How to be a Concept Trainer" Mitchell (2019)

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