A dog's bark may be worse than his bite, but most of us would rather not find out one way or the other. Growling, baring teeth, snarling, snapping, and biting are all aggressive behaviors--but dog aggression includes any behavior meant to intimidate or harm a person or another animal. Although these messages are among the handful of communication tools available to dogs, they're generally unacceptable to humans. Because humans and dogs have different communication systems, misunderstandings can occur between the two species.
But from a dog's perspective, there's always a reason for aggressive behavior. A person may intend to be friendly, but a dog may perceive that person's behavior as threatening or intimidating. Dogs aren't being schizophrenic, psychotic, crazy, or necessarily "vicious" when displaying aggressive behavior.
Because aggression is so complex, and because the potential consequences are so serious, we recommend that you get professional in-home help from an animal behavior specialist if your dog is displaying aggressive behavior.
Dominance Aggression: Dominance aggression is motivated by a challenge to a dog's social status or to his control of a social interaction. Dogs are social animals and view their human families as their social group or "pack." Based on the outcomes of social challenges among group members, a dominance hierarchy or "pecking order" is established.
If your dog perceives his own ranking in the hierarchy to be higher than yours, he'll probably challenge you in certain situations. Because people don't always understand canine communication, you may inadvertently challenge your dog's social position. A dominantly aggressive dog may growl if he is disturbed when resting or sleeping or if he is asked to give up a favorite spot, such as the couch or the bed. Physical restraint, even when done in a friendly manner (like hugging), may also cause your dog to respond aggressively. Reaching for your dog's collar, or reaching over his head to pet him, could also be interpreted as a challenge for dominance. Dominantly aggressive dogs are often described as "Jekyll and Hydes" because they can be very friendly when not challenged. Dominance aggression may be directed at people or at other animals. The most common reason for fights among dogs in the same family is instability in the dominance hierarchy.
Fear-Motivated Aggression: Fear-motivated aggression is a defensive reaction and occurs when a dog believes he is in danger of being harmed. Remember that it's your dog's perception of the situation, not your actual intent, which determines your dog's response. For example, you may raise your arm to throw a ball, but your dog may bite you because he believes he's protecting himself from being hit. A dog may also be fearfully aggressive when approached by other dogs.
Protective, Territorial, and Possessive Aggression: Protective, territorial, and possessive aggression are all very similar, and involve the defense of valuable resources. Territorial aggression is usually associated with defense of property, and that "territory" may extend well past the boundaries of your yard. For example, if you regularly walk your dog around the neighborhood and allow him to urine-mark, he may think his territory includes the entire block. Protective aggression usually refers to aggression directed toward people or animals whom a dog perceives as threats to his family, or pack. Dogs become possessively aggressive when defending their food, toys, or other valued objects, including items as peculiar as tissues stolen from the trash.
Redirected Aggression: This is a relatively common type of aggression but one that is often misunderstood by pet owners. If a dog is somehow provoked by a person or animal he is unable to attack, he may redirect this aggression onto someone else. For example, two family dogs may become excited, and bark and growl in response to another dog passing through the front yard; or two dogs confined behind a fence may turn and attack each other because they can't attack an intruder. Predation is usually considered to be a unique kind of aggressive behavior because it's motivated by the intent to obtain food, and not primarily by the intent to harm or intimidate.
The likelihood of a dog to show aggressive behavior in any particular situation varies markedly from dog to dog. Some dogs tend to respond aggressively with very little stimulation. Others may be subjected to all kinds of threatening stimuli and events and yet never attempt to bite.
The difference in the threshold prompting aggressive behavior is influenced by both environmental and genetic factors. If this threshold is low, a dog will be more likely to bite. Raising the threshold makes a dog less likely to respond aggressively. This threshold can be raised using behavior modification techniques, but the potential for change is influenced by a dog's gender, age, breed, general temperament, and the way in which the behavior modification techniques are chosen and implemented.
Because working with aggressive dogs can be potentially dangerous, behavior modification techniques should only be attempted by, or under the guidance of, an experienced animal behavior professional who understands animal learning theory and behavior.