An aggressive dog may growl, show aggression by posturing or snapping and showing teeth. Aggression can lead to a serious dog bite for any human near to a dog that is being aggressive, so humans must always be on guard around any strange dog especially if they are showing any signs of aggression.
Causes for dog aggression: There are many reasons why a dog may turn aggressive. Some reasons have to do with a single event and others are long-term behavioral in nature. Agonistic - There may be a personality conflict between two dogs and one or both dogs may without cause exhibit aggression. Barrier Frustration - When a dog is confronted with a barrier such as when he/she is tied up, the dog can become frustrated. The dog is unable to escape, so it becomes aggressive and ready to attack in retaliation and in frustration over a situation that to him/her seems hopeless. Competitive - Dogs that are housemates may fight, over social status (wanting to be top dog). Displaced - A dog that has been displaced from its home, especially one that is walking the streets alone and frightened may show aggression towards anyone it meets out of fear. Dominance aggression - The dog thinks that he/she is the leader of the household including over the humans and must show aggression in order to keep his/her pack in line, especially if he/she feels that a situation is a conflict one in which his/her dominance is being challenged. Drug-induced aggression - Certain medications can have aggressive behavior as a side-effect because the dog's perceptions have been affected. Encephalopathic - A medical condition in the brain can cause aggression. Fear-induced aggression - A dog that snaps or growls when afraid thinks that he/she is fighting for his life because of a threatening situation and should be avoided and only handled by authorities trained in situations like this. A dog that has been abused, has genetic tendencies towards aggression, or has been cornered and feels that it cannot escape will attack. Any dog that is growling, snapping or shows fear posturing such as crouching with its tail between its legs should be avoided. Food Guarding - A dog that has been abused or who has spent time on the streets, fending for food may not realize that humans give food and do not take it away and may become aggressive if it thinks that humans that are too close while he/she is eating may take food away from it. Food aggression may also be directed at other dogs or other pets during feeding times for the same reasons. Improper socialization - A dog that has been isolated and not socialized as a puppy or has been socially stunted due to medical reasons or abandonment reasons can become aggressive. Intra-sex - This is aggression behavior that female dogs have only towards other female dogs or male dogs have only towards other male dogs. Irritability - Aggressive behavior that is due to a medical problem that lowers a dog's aggression threshold like when the dog is in pain. Maternal protective - This aggression can be witnessed when a female dog is protecting her young. Owner protective - This type of aggression can be displayed when it is appropriate as in when the owner is being attacked by an intruder or it can be inappropriate as when a dog growls at a parent who is leaning over a child to kiss goodnight and the dog is inappropriately protecting the child.