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Why do animals become aggressive?

Updated: Jul 13, 2022

This article outlines the main types of aggressive behavior presented in animals of all kinds. However, this will be most relevant to dogs.


Aggression is a very large topic that means and evokes different emotional responses to different people. Aggressive looking behaviors happen for a variety of reasons and if you are living around an animal you are scared of please seek the help of a professional.


I can not give exact advice on how to fix aggressive behavior because each situation is vastly different. This is for your safety or the safety of your animal’s victim in their aggression bursts.


Let’s first start with this idea of “fixing” behavior. First off as an animal behavior consultant or even a trainer we don’t “fix” animals. To fix something assumes that something is broken in the first place. Just because your animal might behave in a way that causes you distress does not mean they are broken.


In fact they are very whole. Their behavior reflects (in most all cases) their ability to learn, to achieve outcomes, and be a successful member of their species (as far as they are concerned). As part of being an animal is the ability to behave as means to an end. Never lose sight of the idea that all behavior has a purpose--this includes aggressive behavior.


There are a few behaviors that often fall under this label of an aggressive animal that as a behavior consultant we will most likely tell you the solution is terminal management or in extreme cases euthanasia may need to be considered.


  • Predatory aggression - This is when an animal (usually predatory animals, primarily dogs) finds attacking and even killing animals (of any kind) to be reinforcing. This real and very scary type of aggressive behavior frequently relates back to an animal's genetics. They are often a predatory species predisposed to finding attacking, chasing, biting, shaking, and ultimately killing to be intrinsically reinforcing.

  • Brain damage/Mental illness - Yes, animals of all kinds can suffer from mental illnesses. Trauma to the head or a developmental/genetic defect can trigger severe aggression in an animal. I have to bring this up because this category is often forgotten. Just like us they have brains in their skulls that are fragile and subject to illness, trauma, and defects. Unfortunately, aggression related to this frequently goes unnoticed and completely untreated. I suspect that many behavior euthanasias were because of something wrong with the brain. How often do puppies get dropped on their heads?

  • Sick/rabid or animals in pain - The final aggression category that I want to put here is sick animals or animals in pain. Illness and pain can put anyone in a bad mood or make them irritable and more likely to use aggressive behavior to “feel better” or protect oneself. This is why we will frequently ask if pet parents have consulted a veterinarian depending on the type of behavior they are describing. Pretty much all aggression related consultations will be referred to a veterinarian before beginning their behavior modification program.


One last comment about the above reasons for aggressive behavior. I am in no way saying that it is a lost cause to help animals that suffer from these reasons for aggression or that they should all be euthanized! I have had people try to twist my words on the topic before–I am saying that I would support euthanasia of an animal for the safety and welfare of all parties involved. This is not an easy decision but I want those who are currently living in the anxiety an aggressive animal causes to know, I see you and support whatever decision you ultimately come to. If you want to keep on fighting for your animal and are doing everything you can to help them I support you too.


There is no right answer to dealing with an aggressive animal and what is feasible for one family may not be for another.



Alright, so above I mentioned 3 triggers for aggressive behavior that are what I would call management-for-life categories. The rest of this article is about aggressive behavior that can more easily be modified and results are possible.


  • Fear Aggression - Fear related aggression is where an animal feels threatened and they have discovered that aggressive behaviors will help them get the scary thing to go away.

  • Resource Guarding/Aggression - Some aggressive behavior stems from an animal feeling that their livelihood is at risk. They are protective of what they find reinforcing and probably fear it might be taken away or tampered with. They anticipate that there is risk to their valued resources and the aggression is learned as means to maintain possession of the resource.

  • Safety & Security - Most animals become worried when forced to do something they are unfamiliar with. It’s for this reason we push socialization for young puppies so hard. It’s also a huge proponent of behaviors labeled as aggressive behaviors. When an animal feels that they have no choices this very well could be seen as a breach of their safety. Where security is concerned, I like to think about how a human we feel secure when we are with someone we trust to have our back. When an animal doesn’t feel secure or safe they become more irritable and they may learn that aggressive behavior benefits them.

  • Frustration Induced & Redirected Aggression - Sometimes our animals find themselves in a situation where they are prevented from getting to something that they want or find reinforcing. As time goes on they begin to develop a pattern of aggressive behavior on usually innocent beings or items around them. I see this most with dogs but it could happen in rats or birds who are kept in cages and teased. Dogs I faced with the restriction of leashes when walking about highly distracting (and often with lots of possible opportunities for reinforcement) areas. They become outwardly frustrated and in their cognitive inhibited state lash out.


Those are the core types of aggression (if I’m missing something please comment below). It’s apparent that aggression comes in many forms and stems from many different emotional experiences. I should also point out that there can be cross over between any of the aggression categories I have listed. They are by no means black and white. Aggression is typically a very deep multi-level problem and where we might have success in manipulating one form of aggression the dog could still be aggressive in another scenario.


We need to be specific about labeling aggressive behavior and understand the different stems of aggression. These types of aggression, if you will, will be handled in different ways and why a blanket label, “aggression”, and general help can be incredibly harmful.


Professional help from a professional who is well versed in behavior modification is needed. Aggression should rarely (and most cases never) be handled by using aggression. Furthermore, correcting aggressive outbursts will only suppress the behaviors. Correcting pre-cursing aggressive behavior like growling in dogs will most definitely worsen your situation!


Please disregard any advice to scold, punish, correct, or use force as a means of solving aggressive behavior.


What can you do?

  1. First understand what emotions are underlining the aggressive behavior.

  2. Observe and be meticulous about all environmental factors around aggressive outbursts from your animal. Take notes and mention the time of day. What happened leading up to the behavior? Who was involved? What were they doing? What other influencers were in the environment (sights, smells, noises, social interactions, or forcers)?

  3. Ask yourself what is the function of the behavior? What is the animal gaining or what is the outcome for their behavior? Is there something they want and it is now maintaining the aggression?

  4. Now, make a plan. Starting with how can you avoid situations that trigger aggression? Is there a way to provide the same outcome the animal is seeking without them needing to use aggression? Understand your budget both financially and energetically. What resources do you have available and will you utilize throughout the modification process? Part of your plan should be getting your veterinarian and a behavior expert involved.

  5. Avoid and stay safe! Prevention through management is worth its weight in the safety of your loved ones, yourself, and the community. The last thing you want is the animal to continue to rehearse and ultimately strengthen the aggressive behaviors before you can begin your behavior modification program.


I want you to know, I know, that aggression of any kind and to any severity is still frightening. Even if no one has ever been hurt, the potential for the animal to escalate aggression is real and should be avoided at all costs.


Please seek the help of an educated professional to assist you. If hiring someone is outside of your budget then if nothing else join the KAS Supporting membership to get templates and advice through our forums. We also do periodic free public support group sessions for dog-parents. Check out events to find when the next session is happening.


There are free resources out there but I just advise that you stick with a couple trusted groups or platforms. If you try to follow too many different advice avenues it can become confusing and most certainly will lead to failure to achieving your desired outcome.



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