Aggression is one of the most commonly reported dog behaviour problems. Often a dog demonstrating aggression towards other dogs, species or people is considered an abnormal individual. However, aggression is a normal functional behaviour of the dog. It can be triggered by a variety of situations or conflicts. When the aggression threshold is very low or the aggressive behaviour is excessive or displayed inappropriately, it is considered a behavioural problem. Aggressive behaviour in dogs is also context-dependent. This means that the situation the dog is in and the stimuli it is exposed to strongly influence aggressive responses. For example, a dog that is aggressive at the gate in its territory may not show this behaviour in other situations. Because aggression is a complex behaviour and has many causes, it is helpful to categorise its functional types. The most common forms of aggression are dominance, fear, territorialism and possessiveness. Other forms include maternal, intersexual, playful and transferred aggression. Predatory behaviour is often erroneously described as a type of aggressive behaviour, whereas it should be classified as a separate behavioural pattern.
A dog displays dominance and aggression in response to an apparent attack on its social status. Also, food, toys, bones and a favourite bed can trigger competition and subsequent dominance-subordination interactions. Dominance aggression between dogs can also be triggered when two dogs compete for their handler’s attention. According to the dominant model of social behaviour, the more dominant dog in a relationship is more confident and easily obtains, for example, the desired object. However, if an animal lower in the hierarchy tries to improve its status or competes for a specific object or place, this may result in aggressive behaviour. For dogs that have a well-established relationship, this may be limited to direct eye contact, growling and snapping of the muzzle. However, in some cases, competition between dogs that display dominant behaviour or compete for a favourite toy or bone can escalate into fighting.
Dominance aggression, directed against humans, is usually triggered by situations in which the dog perceives the human as a threat to its social status. It is generally in response to gestures or body postures that the dog reads as dominant. These include standing over the dog, and physically punishing or restraining the dog. Attempting to take property can also cause aggression in a dominant dog, as controlling access to desired objects is an important component of dominance behaviour. However, possessive aggression in a dog, especially when it only shows possessiveness towards one or two specific items, may not in itself be significant in the display of dominance aggression.
Aggression from fear
When a dog is nervous or fearful in the presence of strangers or new situations, it may display aggression from fear. The dog usually adopts body postures consistent with nervousness (pacing, panting, restlessness, lowered body position) or fear (lowered body position, ears pointed back, laid back, tail wagging, hair ruffled). When a human (or another dog) approaches, the dog first growls a low, throaty growl, then attempts to walk away, retreating or running away. Alternatively, the dog may remain motionless, with ears placed at the back of the head and a lowered body position. A dog that has frozen motionless out of fear may suddenly snap in the air or bite when an attempt is made to make contact with it.
Fear aggression may also be a learned behaviour, in response to experiencing painful restraint (e.g. at the vet’s surgery) or when the dog has been inappropriately physically punished. The animal associates the previously experienced pain with a person (e.g. a veterinarian) and reacts aggressively to avoid repeating the unpleasant situation. Similarly, dogs may show aggression out of fear towards other dogs if they have previously been attacked by unknown dogs or members of their pack.
Like dominance and submission, territorial behaviour is normal canine behaviour. A dog’s territory includes all valuables and places that must be protected. The territories that a dog usually defends consist of its home, farmyard, beds and places where it spends the most time, and the owner’s car. Territorial behaviour manifests itself in barking and excessive excitement, decreasing when the dog accepts an intruder entering the territory or when the intruder leaves. However, a dog that barks incessantly and displays aggression towards an intruder, shows territorial aggression. It can be directed against humans, other dogs or other animal species.
Protective aggression is a type of aggression towards a human who approaches or interacts with the dog’s handler. Essentially a form of territorial or possessive aggression, dogs that defend their owners are either securing the 'territory’ of their human handler or recognising them as valuable property (see below).
Dogs that are more confident by nature or belong to breeds selected for protection/watching may exhibit enhanced territorial and defensive behaviour. Without special training or experience, some may react aggressively to intruders entering the house or yard. In other cases, handlers may inadvertently or deliberately reinforce territorial aggression in their dogs. As with dominance, there are significant differences in territorial behaviour between breeds and individuals. Slight displays of aggression can be controlled with training. However, if a dog is very aggressive or is encouraged to behave in this way by its owner, this type of aggression can be very difficult to modify once it has been established.
In some cases, it is fear or nervousness rather than confidence that increases territorial aggression. A dog that feels vulnerable or unable to escape will act aggressively to chase the intruder out of the established territory. This type of territorial defence is often seen in poorly socialised dogs that have only learned a small area or are tethered to a kennel. In such cases, treatment focuses on lowering the dog’s fear and getting him used to visitors.
This is often classified as a subcategory of dominance aggression as it refers to aggression displayed when competing for toys, food or access to a handler that is valuable to the dog. However, some dogs will focus on unusual objects such as a piece of paper/tissue, a piece of clothing or even the TV remote control. When the carer tries to take the object away from the dog, the dog holds it tightly, growls and may snap its snout or bite. Often the dog will present the item to the handler but then growl when the human reaches for it.
These situations may reflect a learned behaviour and are related to the dog’s history of sneaking things. When the handler chases the dog or severely reprimands it, the dog learns to respond by defending the stolen item. Possessive aggression can also occur in dogs that have been forced to defend their food from other animals or have learned to distrust humans approaching their bowls. These dogs usually defend their bowl defensively but show no other signs of possessive aggression. In these cases, the behaviour may be learned, in response to restricted access to food or competition for it with other animals. Some carers will repeatedly take away a dog’s bowl while it is eating, believing that this will assert their dominance. However, such practices only make the dog distrust the handler and start defending the bowl whenever the human approaches. Dogs generally accept that someone approaches their bowl, but the repeated and arbitrary taking away of the bowl means for them that access to food is unpredictable and therefore must always be defended.
Other types of aggression
The dog exhibits two types of aggressive behaviour specific to gender. Maternal aggression occurs in females with young, who perceive a human or animal as a threat to the puppies. This type of aggression is not observed in all bitches and usually decreases when the puppies reach several weeks of age. Some males display inter-male aggression. This is probably a form of dominance aggression and is usually shown by immature males who greet other males with dominant body postures and stares. Neutering reduces but does not eliminate inter-male aggression. Some dogs that develop intersexual aggression when they reach social maturity display aggression towards all unfamiliar dogs, not just males.
Pain-induced aggression occurs when a dog is injured and reacts aggressively to being held. Play aggression is seen when playing with people or other dogs, most commonly in young dogs or puppies that are still learning the correct play routines. It can also be displayed by adult dogs that are poorly socialised and misinterpret play signals in other dogs. The last type of aggression, redirected aggression, occurs when a dog is not allowed to discharge its aggression on another dog or human. In this situation, the dog „redirects” its aggression to a person, a dog, or even an inanimate object that is nearby. This is illustrated by the following example: the dog defends the vestibule and bites its handler when the handler pulls it away.