thumb|German Shepherd displaying threat behavior
Dog aggression expressed by dogs is considered to be normal behaviour and various types of aggression are influenced by a dog's environment and genetic predisposition. Dogs commonly display possessive aggression when defending resources or themselves.
Canine aggression may be influenced by a dog's age, sex, health and reproductive status. Canine aggression is one of the most serious behavior problems in animal behavioral medical science. Aggression in canines is particularly important because a dog's sharp teeth can result in serious injuries, which may be ritualized aggression, not actual aggression, if the violation includes a threat. Aggressive behavior in scared dogs is typically not true aggression but rather ritualized behavior intended to avoid physical altercation by way of faking aggression rather than actual aggression. This ritualized behavior can be influenced by anxiety disorder, diet, gut microbiome or genetic background. Some aggression can be treated through animal behaviour therapy or avoided by proper socialization during puppyhood.
Types of aggression
Dominance aggression
1) Punishing dogs has been associated with a strong likelihood of new or increased aggression and other behavior problems; 2) dominance in pet dogs is not a character trait of a dog but rather a power agreement between dogs regarding who has best access to particular resources; and 3) the behavior of dogs controlling access to resources is fluid, not static, depending on context. There is also data suggesting that the concept of spoiling a dog is often a misnomer, that this perceived spoiling is often a matter of meeting the dog's emotional and physical needs.
Defensive aggression
Defensive aggression, also known as fear aggression or avoidance-motivated aggression, occurs when an individual approaches and interacts with a human-avoidant dog. The dog might first try to flee, but may resort to aggressive behavior if cornered, as a means to try to defend itself from a real or perceived threat. The dog displays aggressive behavior in an attempt to avoid a real or perceived negative consequence, such as to avoid pain. Dogs may display a multitude of behaviors leading up to an attack including fear and stress signals, defensive posturing, facial expressions, or no signals at all. Signals are generally different for self-defensive dogs versus those who are truly aggressive, as in competitive aggression.
Predatory aggression
thumb|Predatory aggression is impacted by anxious influences and it includes behaviors such as biting.
Predatory aggression is usually seen as part of the prey drive sequence starting with a visual or auditory trigger followed by chase, capture and kill. This type of aggression is caused by sympathetic system activation and is non-affective.
thumb|Maternal aggression is displayed by mothers that are protecting their offspring. Maternal aggression decreases as their offspring grow up and they are able to defend themselves.
Maternal aggression
Maternal aggression is displayed by mothers when they are approached and particularly when their offspring are young. This type of aggression is linked to pain such as in cases of mastitis. Oxytocin plays an important role in the early bonding between the mother and her offspring.
Leash reactivity is not always synonymous with aggression, but repeated reactive episodes may escalate into more severe aggression if not addressed. Owners often manage this condition by keeping the dog below its reaction threshold, applying desensitization and counterconditioning protocols, using marker-based reinforcement for disengagement, and avoiding aversive corrections, which can worsen stress and pain sensitivity.
In some cases, underlying pain or medical conditions may lower a dog's tolerance for stimuli and trigger overreactions; thus veterinarians are advised to rule out musculoskeletal or neurological causes before or alongside behavior therapy.
Causes
Gut microbiome
thumb|Lactobacillus are present in the guts of dogs that exhibit aggressive behavior.
The gut microbiome is linked to numerous health problems such as diarrhea and bowel disease, which results in pain and can possibly lead to aggression. The gut microbiome affects the affective disorders such as anxiety, which contribute to canine aggression. Aggressive behavior in dogs is often associated with fear, the separation from their owner, or noise sensitivity, all of which may result in or contribute to anxiety disorder. The owners of 673 dogs noted that their dogs express fear by barking, and growling. The presence of aggression in dogs is associated with defense mechanism. and HSUS explains that barking and growling are normal communicative behaviors for dogs not necessarily indicative of aggression.
Breed, socialization, and health status
thumb|Dogs that suffer from injuries, disorders or illnesses are more likely to display aggressive behavior, especially when located in frontal cortex, the hypothalamus, the thalamus, the amygdaloid body, the medial mammillary, the nucleus, the habenular nuclei, the hippocampus and the caudate nucleus.|alt=
It is important to recognize that aggression is displayed more in certain breeds, which indicates the influence of genetic background on aggression in dogs. According to the data collected, gonadectomized dogs between the ages of 7 and 12 months were more likely to be aggressive to strangers. Guy and colleagues found that environmental factors directly contributed to aggression in dogs. For instance, a dog that lives in a home with an adolescent or has a skin disorder is more likely to exhibit aggressive behavior to familiar people and strangers. They also argued that the relationship between the owner and dog determined whether or not the dog would exhibit aggressive behavior.
Treatment
thumb|A dog is being walked by its owner with a leash, which is used to establish and maintain control. It is an owner's responsibility to modify aggressive behavior of their dog.
Treatment differs for each circumstance, and treatment plans are based heavily on the type of aggression. Based on Judith Blackshaw's study, dogs exhibited dominance aggression towards familiar people and therefore suggested obedience training as the proper treatment. However, it is still imperative for an owner to train their dog every day for a minimum of 10 minutes.