Aggression is a behavior meant to threaten or harm. Aggressive behavior can also be used to increase distance between the aggressor and its recipient. Aggressive behaviors can range in intensity from postural displays such as growls and snarls, to lunges, snaps, and bites.

A dog or cat may behave aggressively for several different reasons. When managing aggressive behaviors, it is important to determine the motivation for the aggression.


There are many ways to classify aggressive behavior. One useful classification considers aggression to be either offensive or defensive. Offensive aggression is an assertive action initiated to achieve a goal or gain access to a resource. Defensive aggression is an action exhibited toward a perceived threat, or a defence response. Other classifications in veterinary behavior are based on the function, stimuli, or context of the aggression.

Common diagnostic categories include:

  • fear-induced aggression
  • conflict aggression
  • possessive, protective, and territorial aggression
  • predatory aggression
  • social/dominance (status-related) aggression
  • inter-male/inter-female aggression
  • pathophysiologic (medical) aggression
  • pain-induced aggression
  • targeted aggression (toward familiar people/animals, or toward unfamiliar people/animals)
  • parental aggression, redirected aggression, play-related aggression

It is important to consider the factors surrounding the initial aggressive event when classifying aggression.

Over time, the animal could change how it displays aggression, as it recalls past responses, learns consequences, or undergoes treatment interventions. These factors could alter the aggressor’s future behavior. Note: there is no one single cause of aggression. An individual pet can display one or more forms of aggression.

Multiple factors and stimuli may combine to push a dog or cat to a point where aggression is displayed. For example, a dog may exhibit territorial behaviors and be fearful of children. This dog may only exhibit aggression if a strange child comes onto the dog’s property when it is cornered or tied up and cannot escape.


Dogs and cats may display aggressive threats within their normal behavioral repertoire. These tactics are used to resolve competitive disputes, escape threatening situations, or increase their reproductive potential. Normally, animals only resort to biting if there has not been an appropriate response to less dangerous aggressive behaviors.

Safety must always be a primary consideration, as dogs and cats can cause physical and emotional harm to people and other animals. Aggression must be assessed and managed. It is estimated that from 2 to 5 million human bite wounds occur annually across North America. Most people are bitten by animals they own or are familiar with. Children are more likely to be bitten by a dog they know than by a stray, unfamiliar dog.

Some aggression in dogs and cats may be abnormal. Abnormal aggression is when the response occurs impulsively or without inhibition, using an intensity that is much greater than would be needed to resolve the actual threat. These excessive responses may arise from a genetic predisposition, a lack of appropriate socialization, insufficient maternal care, and/or exposure to traumatic events.

An underlying medical or behavioral illness can trigger an abnormal aggressive response. Underlying medical conditions, high levels of fear, anxiety or frustration can increase the intensity of an abnormal aggressive response.


High-level aggressive threats such as growls, snaps, or bites are easily recognized. However, dogs routinely communicate using more subtle postures. You can learn to recognize more subtle threats by watching the body posture and facial expressions of dogs interacting with each other. See the handout “Canine Communication – Interpreting Dog Language” for more information.

A direct stare is a dog’s way of saying “stop right there”. A confident dog may maintain eye contact with another dog, waiting for it to look away, which would indicate that the message was received.

Prolonged, mutual eye contact by the other dog would be viewed as a threat. At that point, the interaction may escalate, unless one dog turns away. Escalation might involve standing taller, taking a step forward, or using a snarl or growl. You may notice raised hair along the dog’s neck and back, which creates the appearance of being larger. Raised fur (piloerection) suggests arousal and can indicate the level of threat has increased. If your dog stares at you, and you continue to stare back, your dog may feel threatened.

Note: This situation is different from when you train your dog to look directly at you. In that case, your dog would show a very eager or relaxed posture to earn a reward.

If you notice your dog’s muscles are very stiff, his fur is raised, and/or his eyes look very black (his pupils are dilated), it is important to indicate that you do not intend to pursue a confrontation. You may try to slowly look away. If it is safe, turn away and gently ask your dog to go get a toy or treat to further diffuse the situation.

In some dogs, dark-coloured eyes or hair covering the eyes could make eye contact difficult to determine. In those cases, you may not notice this important warning signal. Therefore, it is important to learn all the subtle communication postures a dog may display. See the handout “Canine Communication – Interpreting Dog Language” for more information.