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Canine Aggression Part 3: Health and Physiology

Health and Physiology

A dog’s physical health has a huge impact on their behaviour. Stress, pain from illness and diseases, irritation from allergies and emotional turmoil stemming from separation related issues can all contribute to canine aggression. It is important to take a whole dog approach when trying to determine the reason behind aggressive behaviour as the cause may not be as simple as it initially appears to be.


Stress is any emotional, physical, social or any other factor that elicits a positive or negative response in an individual or dog. Eustress, or good stress, allows a dog to burn off energy, learn something new and is important for personal growth. It is usually short term and can act as a motivator to push through a challenge. Without some sort of positive stress, a dog’s life would be quite boring and would likely result in frustration that in turn causes behavioural problems. Distress, or bad stress, can be overwhelming, cumulative and have long-lasting, debilitating mental and physical effects. Sometimes bad stress comes from what we might think of as positive work, such as training behaviours repeatedly without enough reward, or a play date with another dog that has gone on just a little too long. It can also come from events that are inherently stressful to an animal, such as fireworks or car rides. Regardless of the type of stress being experienced, its effects will differ from dog to dog and generally results from a basic need not being met.

Stress, health and behavioural issues are all closely interconnected. While stress does serve a purpose in us and in our dogs (it keeps us alive and in tune with dangers around us thanks to the fight or flight response), it also causes an increase in hormones in our bodies and many different physiological changes occur when this happens. Many health issues in our dogs put stress on their bodies, and when our dogs are under constant stress due to dealing with fears and anxiety daily, then health issues can arise. Being able to identify what is occurring when we see behavioural changes in our dogs requires being aware of our dog’s ‘normal’ state, making sure that we are paying attention to what is affecting our dog on a daily basis and ensuring that we seek veterinary support when we cannot identify the possible cause for behavioural changes that we may be seeing.

Positive and negative stress triggers our hypothalamus to release cortico-trophic releasing hormone to the anterior pituitary which then produces adrenocorticotropic hormone (ACTH). ACTH travels in the blood to the adrenal glands, located just above the kidneys. Cortisol, the stress hormone, is produced in the outer shell of the adrenal glands. All the body’s non-essential processes are downgraded when cortisol is released, such as the immune system and digestion. The parasympathetic nervous system, which is responsible for rest, healing and recovery, basically begins to shut down. The sympathetic nervous system takes over, stimulates the medulla and there is an increase of adrenaline and norepinephrine produced. We will see an increased heart rate, breathing changes, dilated pupils, less eating, increased salivation, a rise in blood pressure, hyper-awareness, and hyper-vigilance. The body will also begin producing excess glucose in case the body needs excess energy to flee a situation (as a part of the ‘flight’ response).

While our dogs always need some level of cortisol in their bodies, too much can result in behavioural changes. A dog’s perception of events as the levels of cortisol increases in their bodies changes…they become more pessimistic and may begin displaying behaviour that we haven’t seen before such as growling, barking, snapping etc. Chronic stress affects different organs in different species. We tend to see lots of bladder issues in cats who are under stress. In dogs, stress usually exhibits itself in issues relating to the skin (itchiness, dryness, cracking etc.).

Pain from an undiagnosed health issue can cause an increase in cortisol and in turn affect our dog’s perception of events in a negative way. Stomach upset in dogs due to an imbalance in gut bacteria can result in pain and cause our dogs to act out when touched, be less likely to want to walk or play and may make them more ‘grumpy’ in general. Inflammation anywhere in the body decreases the levels of dopamine which in turn decreases motivation and is then reflected outwardly as lethargy or sluggish behaviour. It is difficult because we cannot see what is occurring internally in our dogs, and so we have to rely on what we can see outwardly as far as behavioural changes go, as well as regular vet check-ups and blood sample work-ups that may signal something is off.. This can be a bit dangerous however, as sometimes we may not notice that there is an internal issue until it is too late and too much damage has already been done.

When should we consider that there may be more to our dog’s behavioral changes than fear, anxiety or something else environmental that we can help with and it is time for a vet visit? Any sudden onset behavioural changes where we cannot easily identify a trigger should be a sign that it is time to visit a vet to rule out issues that we cannot see. Noticing excessive itchiness, chewing, whining when touched, difficulty moving or gait changes, difference in toileting habits or the consistency of still/urine, behaviour changes as a result of exercise, changes to sleep-wake habits, disorientation, excessive pacing/barking/salivating, pupil abnormality, head tilting, dragging of limbs or a seasonality to behaviour problems are all signals that there is likely something going on with our dog’s health that needs some medical attention.

Knowing signs of possible medical conditions that may be affecting our dog’s behaviour could save their life. When we notice changes in what is considered ‘normal’ for our dogs and seek help from a veterinary professional as soon as possible, there is a chance that we can ease their suffering and possible prevent long-term damage internally. Also, by ensuring that we understand how to keep our dogs as calm and as fear-free as possible can help not only their mental health, but their physical health as well. Keeping the body’s cortisol levels at a safe level will ensure that the body’s natural rejuvenation functions are working properly, and that healing can occur when needed.

Trigger Stacking

Trigger stacking is something that occurs in every dog, but in different degrees and with varying outward effects on behaviour. If we are aware of what trigger stacking is and how to minimize it, many unwanted behaviours including aggressive displays, that our dogs may exhibit can be diminished.

Every dog will respond differently to different stressors that occur throughout the day. What bothers one dog, may not bother another. How quickly a dog can ‘brush off’ something that *did* bother it also varies. Dr. Tom Mitchell, in his book “How to Be a Concept Trainer” uses the metaphor of a bucket to help describe trigger stacking and the impacts that it can have on a dog. He explains that we can think of every dog as having a bucket into which the stressors (both positive and negative) that occur throughout the day go into. More optimistic, confident dogs will have bigger buckets than pessimistic, less confident dogs. Also, dogs that have a stronger concept of arousal up-arousal down will have a larger hole in the bottom of their bucket than those who cannot let of go of stressors that easily. Throughout the day, the bucket fills with stress and if the hole in the bucket is not big enough to let it go the bucket overflows (the dog is over their threshold for what they can cope with) and we see reactive, fear or ‘aggressive’ behaviours that we may not normally see as a result. One seemingly inconsequential event can become the bucket overflower, or ‘straw that broke the camel’s back’ and result in our dogs barking, growling and lunging or exhibiting other fearful behaviour such as cowering and hiding.

The default emotional state that is ideal for our dogs is one of calmness. When calm, a dog is in a state of positivity and low arousal. When stressors start to occur, possibly being barked at by another dog, having someone knock on the door or having a play date with an excitable puppy pal (positive stress), the sympathetic nervous system within the dog is activated and starts to take over. Glucose is released into the bloodstream, adrenaline production is increased, blood flow is redistributed, heart rate increases, and synapse functions go into overdrive. Stress hormones are released into the body and their production stops approximately 70-100 minutes after the event. For some dogs, it can take up to 72 hours for these hormones to completely leave the body. So, if a dog is exposed to stressor after stressor throughout the day without appropriate time to recover from the preceding one, the impacts compound and reactivity can result.

Outwardly, there are physical signs that we can see that show that our dog is experiencing higher levels of stress and that it might be time for some quiet, calming behaviours away from the things that are causing upset. Dogs may start to scratch more, lick their lips, yawn, pace, pant and in general seem ‘on edge’ and more responsive to things that would normally not bother them.

There is no reason to ever use forceful training methods when there are so many other positive and reinforcing ones to use that will achieve better results and work to build a stronger relationship between owner and dog. ‘Quick fix’ aversive training methods may seem appealing to some people who do not want to put in the time to work with their dog, but using intimidation, fear, pain and punishment will only serve to escalate the fear and/or reactivity in a dog. A dog who is subjected to these methods when already stressed will just have the stress compounded and find themselves over the threshold much faster. In time, and sometimes in a very short time, a reactive behaviour will occur in response to the negative training method as the dog will have made a negative association that they easily remember.

Ensuring that most of our dog’s day is spent in calmness is one way to help prevent trigger stacking. Lauren Langman and Dr. Tom Mitchell of Absolute Dogs Concept Training discuss how important the ‘Calmness Triad’ is for a dog. Having sufficient periods of active rest, time to engage in passive calming activities (such as having a frozen kong or long-lasting chew) and being rewarded throughout the day for showing calm behaviours helps to reinforce a calm state of mind that is then better able to cope with stressors as they occur and helps to ‘keep the bucket empty’. Playing games to boost the concepts of optimism and confidence in our dogs can also help to increase their ability to cope with stressors as well. Being in tune with what the triggers are for our dogs and being mindful to not put them in situations where they will encounter multiple ones without enough down time in between is also something that we can do to help prevent this.


More and more, nutrition is beginning to be looked at more closely as a factor contributing to aggressive behaviours in dogs. We have known for a long time that the quality of our dog’s food can affect their health in positive and negative ways. The higher the quality of ingredients in the food, and the more balanced the diet is overall, the better chance we give our dogs at living a healthy and disease-free life. Nutrigenomics takes the study of the impact of food on health a little deeper. It is a relatively newer word that is used to describe ‘the science of how diet effects the epigenome, which in turn alters our genetic predisposition toward health or to disease.’ (Dodds, 2015)

The epigenome is a structural layer surrounding our DNA that dictates which genes in the body need to be used at which time. It controls the expression of those genes through chemical reactions within cells and determines what proteins the body will produce. Cells will be either healthy or diseased thanks to the epigenome and this is an innate decision that is based on signals from the environment (including diet). We can change the epigenome over time based on what we expose ourselves to, and this also applies to our dogs.

Nutrigenomics looks at how to get cells to respond in a healthy manner by feeding our dogs a diet that is geared towards their individual needs, age, lifestyle and health…’nutri’ = nutrition and ‘genomics’ = study of genomes. One important aim of nutrigenomics is identifying the early stages of a diet-related disease so that we can make changes to the diet in the hopes of preventing further illness. In addition, nutrigenomics can help in making decisions about what foods are needed in order to maintain a healthy state and prevent illness or disease from happening. There is a genetic component to many nutritional diseases that affect our dogs and nutrition plays a role in managing and treating them. Certain vitamins and minerals directly affect gene expression and other diet components, such as fibre have an indirect effect by changing hormones. Tailoring a diet for a dog based on its genome can optimize their health. When health is optimized, the likelihood of a dog displaying aggressive behaviours related to pain, disease and and/or general discomfort is diminished.

If behaviour is regulated by neurotransmitters and hormones in the brain, then changes in their availability/production levels will impact behaviour both positively and negatively. Feeding a quality diet that has components that will help to balance the levels of neurotransmitters and hormones known to play a role in aggressive behaviour may help in lowering the incidences of these behaviours in dogs. For example, a dog who is being fed a diet that contains tryptophan, which is a precursor to serotonin, may be less likely to display stress related aggressive behaviours as serotonin is associated with feelings of calmness and relaxation. Low levels of serotonin in the body (due to low levels of tryptophan in the diet) have been associated with an increase in defensive aggression in dogs and decreased impulse control. While tryptophan is found in protein, it is actually a diet containing appropriate amounts of carbohydrates that will help the tryptophan make its way to the brain and impact serotonin levels. Amino acids found in some proteins can overtake the tryptophan and prevent it from being useful in the body.

A deficiency in certain nutrients has also been linked to aggressive behaviours in dogs. Diets low in magnesium and niacin may result in a do resorting to more aggressive behaviours, and a lack of calcium is linked to fearfulness and irritability which can quickly turn into aggression under certain circumstances. Many commercial dog foods are full of additives, preservatives and pesticides that compound in the body over time and contribute to unwanted behaviours. Artificial thickeners and colouring has been linked to hyper-activity / increased arousal, which is tied to aggressive responses. The preservatives BHT and BHA, as well as pesticides used on growing grain crops which are then used in dog food are all cause for concern when it comes to the health and behaviour of our dogs. For a dog who is displaying aggressive behaviours it is worth visiting a canine nutritionist who may be able to help provide some insight as to how to best balance the diet for the dog to see if any changes can help decrease the behaviour.

The Five Freedoms

The Five Freedoms are a consolidation of the basic needs that should be met in order to live a happy, comfortable life and directly contribute to a dog’s health and well-being. For dogs who struggle with aggression it can help to review the five freedoms and determine what might be lacking in the life of the dog and how it could be impacting their behaviour.

The first is to be free from hunger, thirst and malnutrition. The quality of food we feed our dogs can directly impact their mood (low tryptophan results in low serotonin) and stress their system. Additionally, the quantity of food must be adequate for the size and activity level of the dog. A dog constantly in a state of hunger or thirst may understandably guard food resources when they are present, or be more aggressive in their attempts to access these resources.

The next is to be free from discomfort, both physical and mental. Knowing what is normal for your dog is important as some animals can be quite stoic and hide their discomfort well. Training methods that cause physical discomfort may result in a dog displaying aggressive behaviour towards it’s handler. Mental discomfort may come in the form of loneliness if locked away for long periods of time, destructive behaviours due to lack of stimulation, or being in an environment full of noises for a dog who is noise sensitive.

The third freedom is to be free from pain, injury and disease. This can be managed by routinely visiting the vet and providing whatever additional care or supplements might be needed depending on the needs of the dog. Pain-related aggression may occur if a dog is in pain and then touched, poked or prodded in an area that is sensitive.

Freedom to express normal behaviours such as barking, jumping, running, playing, digging, chasing is the fourth freedom. Dogs need an outlet for their energy and frustration, and ideally they should be allowed to ‘unload’ in a way that is natural to them. Without this, redirected aggression could occur if a dog spends too much time in an aroused state because of lack of activity.

The last of the five freedoms is to be free from fear and distress. The more we learn about how to communicate with our dogs and recognise signs that might indicate they are uncomfortable, stressed, hurt the better able we will be to help them. Fear-based aggressive displays may be one of the most commons forms of aggression in dogs, and if we can get to the root of the fear and work with the dog to overcome it using positive methods the need to resort to aggression will diminish.

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