The Causes of Aggression in Dogs

Aggression is defined as an overt behaviour with the intent of inflicting harm on another individual (Nelson & Trainor, 2007). It is a threatening behaviour. One that is directed at another animal or human. Ultimately, aggression is a behavioural response and not a character trait - something that an animal does instead of something that an animal is.

There are many reasons a dog might behave aggressively. Most commonly, it is a result of fear. A dog behaving aggressively likely wants to increase distance between itself and whatever the scary thing is. More distance means that the dog feels safer, and safety is a basic need we all have.

Aggression is rarely down to just one reason and as such, not only the dog, but also its history and environment need to be taken into consideration to change or reduce aggressive behaviour.

Nutrition

The amount, digestibility of food and when your dog is fed may influence their behaviour. Certain dietary components may actually improve a dog's tolerance to stress (Bosch et al., 2007). For example, tryptophan - an amino acid - can do so thanks to its crucial role in serotonin synthesis. Thus, supplementation has proven useful in some instances. Another amino acid, Tyrosine, has also been suggested to be helpful during periods of severe stress.

Fat can also influence a dog’s behaviour. The amount of docosahexaenoic acid (DHA) a dog’s mother receives influences a puppy’s neural development which may then affect the puppy throughout its life. Furthermore, the ratio between omega-3 and omega-6 in a dogs diet may also influence behaviour.

Hungry dogs may be more irritable due to low blood sugar levels that could then lead to an increase in aggressive behaviour. Dogs that have gastric upset or irritation due to their diet may also behave in a more aggressive manner.

Neural, Endocrine and Genetic Component

There are many hormones and neurotransmitters that aid in the regulation of mood. Serotonin and dopamine are among these. Factors such as lack of sleep, chronic stress and anxiety, exercise levels, and aging can lead to decreased serotonin levels, which could in turn contribute to aggressive behaviours.

Hypothyroidism is a common cause of aggressive behaviour. Aggression is sometimes the only symptom of hypothyroidism in dogs and many authors have suggested that thyroid screening be included when assessing aggression cases as it can be a primary cause or an aggravating factor.

Pain also affects behaviour, with aggressive behaviours often having a pain component. Among other things, the pain could be due to an underlying disease like hip dysplasia, thus highlighting the importance of thorough yearly check-ups with a knowledgeable veterinarian.

In some cases, aggressive behaviours may also have a genetic - and therefore a hereditary - component (Pérez-Guisado et al., 2006).

Learning History

A dog may have learnt to behave aggressively due to past experiences. They may have learnt that the “scary thing” goes away when they behave aggressively.

Often times dogs may have experienced something traumatic (in their view, perhaps not ours) which increases the likelihood of aggressive behaviours occurring in the future.

Well-meaning owners may also punish their dogs for behaving in an aggressive manor, further perpetuating the cycle and increasing aggressive behaviour.

Arousal levels

Dogs are more likely to behave aggressively when they are aroused (including excitement). This is due to increased hormones levels, such as adrenaline and cortisol, which may be linked to such arousal periods. This in turn means that they are more likely to “make poor judgement calls”.

Early Experiences

A dog’s in vitro experience and their experiences during puppyhood affect their behaviour throughout their lives. The stress hormones a mother’s body produces, passes to puppies in utero. This increases the likelihood of the puppies becoming fearful adults. Being reared by a fearful mother also increases the likelihood of the puppies turning into fearful adults.

Early weaning has also been linked to an increase in the likelihood of aggressive behaviours (Mikkola et al., 2021). Early trauma and incorrect or lack of socialisation can lead to a dog being more likely to act aggressively later in life due to sensitisation and fear responses.

Social Context

Dogs that feel constrained may act aggressively because they know they can’t escape. This can be when a dog is on lead or in a tight space.

Dogs living in households with many other dogs may also behave aggressively due to competition for resources (including attention from owners), but the company of other dogs have actually been associated with a decrease in aggressive behaviours. This is likely due to the fact that these dogs are able to play, leading to decreased frustration and an increase in the “happy feeling” hormones.

What to do if you have an aggressive dog?

Do not punish your dog for their aggressive behaviours. This will likely worsen the behaviours. Avoid and remove your dog from situations in which they are likely to behave in an aggressive way. Be mindful of the environment and your dog. This is very important when starting desensitization/counterconditioning.

If the aggressive behaviour developed suddenly, it is likely due to an underlying health issue that warrants a vet visit.

Only use equipment that is safe for your dog. For the safety of your dog and others, train your dog to love wearing a muzzle and use leashes with multiple points of attachment on a secure harness and collar. Two layers of safety is always better than on. This means that using a muzzle and a leash is safer than using just a leash. If one layer fails, then there is always another. Use safety barriers (baby gates, fences etc.) if applicable to your situation.

Contact a certified behaviourist or behaviour consultant to help you with a behaviour modification plan.

References

Bosch, G., Beerda, B., Hendriks, W.H., Van der Poel, A.F.B. and Verstegen, M.W.A., 2007. Impact of nutrition on canine behaviour: current status and possible mechanisms. Nutrition research reviews, 20(2), pp.180-194.

Mikkola, S., Salonen, M., Puurunen, J., Hakanen, E., Sulkama, S., Araujo, C. and Lohi, H., 2021. Aggressive behaviour is affected by demographic, environmental and behavioural factors in purebred dogs. Scientific reports, 11(1), pp.1-10.

Nelson, R.J. and Trainor, B.C., 2007. Neural mechanisms of aggression. Nature Reviews Neuroscience, 8(7), pp.536-546.

Pérez-Guisado, J., Lopez-Rodríguez, R. and Muñoz-Serrano, A., 2006. Heritability of dominant–aggressive behaviour in English Cocker Spaniels. Applied Animal Behaviour Science, 100(3-4), pp.219-227.