Inter-dog aggression (aggression between dogs) can be highly variable, but it generally appears between 1 and 3 years of age and is more common between dogs of the same sex.  Inter-dog aggression often focuses on issues of social status and control, which become apparent at social maturity (approximately 18 to 36 months of age in dogs). Inter-dog aggression can occur between dogs that are either unknown or known to each other.  In the latter case, it can be initiated by a young dog that is becoming socially mature or by the older dog that perceives the changing status of the younger dog. 

Very few dogs are aggressive to other dogs because they never learned how to interact with them when they were young.  Dogs focus primarily on the parent(s) and littermates until they are 5 to 8 weeks of age, when they become very receptive to interaction with people.  Puppies form hierarchies within the litter, and these social orders are maintained both by some agonistic behavior (posturing, vocalizing, and snapping) and by active and passive deference by the other pups (rolling on the back and urinating or looking away).  The few studies of this issue indicate that these puppyhood hierarchies appear to have no association with the relative status of the animals when they reach social maturity.  It is conceivable (but probably rare) that some dogs that never see other dogs when they are puppies might have some problems relating to other dogs; however, these problems are often related to fear.  

Inter-dog aggression is not associated with sexual maturity (approximately 6 to 9 months of age), although there is a role for testosterone in inter-dog aggression.  Testosterone facilitates fighting: intact (non-castrated) male dogs react quickly, react to a higher level overall, and take longer to calm down. Castration greatly decreases roaming, urine marking, and fighting between dogs and appears effective in diminishing all of these behaviors in about 60% of all dogs with such problems.  It is important to remember that all behaviors have learned components.  The fact that a hormone facilitates the development of a form of aggression does not mean that diminishing the hormone will simply “fix” the problem.  If a dog has exhibited a series of behaviors for a long time, that dog has learned about the behavioral patterns and his response. Accordingly, simply removing the hormones that help that response does nothing about the learned component. That is a role for behavioral modification.

The situation with intact (non-spayed) female dogs is not so simple.  Hormonal cycling does not appear to facilitate aggression between female dogs in the same sense that testosterone facilitates aggression between male dogs; however, clients often report that many intact bitches experience “mood” changes before and during estrus (heat).  If there were some mild status-related problems between the female dogs in the household, they might be exaggerated at such times.  If there is an intact male dog in the house, he might become highly motivated to pursue on e of the female dogs and further disrupt the social order.  There are few in-depth studies of any association between female hormones and aggression, but available studies indicate that female hormones do not play a large role in inter-dog aggression.

Both male and female dogs are healthier if they are neutered. Not only do fewer infectious and cancerous problems related to the reproductive tract develop, but females, if spayed early, have a lower risk of mammary cancer.  Furthermore, roaming can be fatal to a dog.  No dog is a match for a car, and in many areas, dogs are driven off private property by guns.  Most communities have “dog at large” ordinances that prohibit free-ranging, wandering dogs. This is safer for the dog and for people

Dogs that react to unknown dogs generally do so for two reasons: they are either afraid or they perceive, with or without cause, that the other dog represents a threat to their property or self.  Dogs that fear other dogs can be spontaneously afraid of them or be afraid because they have been attacked.  

Dogs that react as if there is a challenge about social status when there is none are reacting inappropriately, and out of context.  If there is a challenge (starring, hackling up, placing a paw on shoulders, growling, snarling, snapping) of any kind, a reaction might be appropriate, but it is important to remember that, as for people, rules apply to many normal social behaviors in dogs.  If the approaching dog just stares at your dog and your dog lunges for the other dog’s throat, refusing to back down even when the other dog has rolled over, your dog is not behaving appropriately.

Inter-dog aggression between housemates occurs more commonly between same-sex housemates.  It is not unusual for two dogs to have lived together harmoniously for years before problems occur.  These problems are not related to “inappropriate or incomplete early socialization” or because someone did something to the dogs.  The development of these problems reflects the intrinsic change that all social animals experience when they become socially mature.  The more common scenario for inter-dog aggression within a household involves the younger dog that was fine as a puppy but, now that it is becoming socially mature, challenges the older dog.  Challenges to the other dog can include blocking access to a bed or crate, lying on the other dog on a couch, stealing the other dog’s biscuits, rawhides, or toys, blocking the other dog’s access to food, shoving past the other to get out or in a door or car first, and posturing in a ritualized display where the challenger approaches. 

Challenges can involve staring, vocalizing, or outright aggression.  Challenges may start with staring and escalate to aggression.  Regardless, it is important to treat the problem as soon as it becomes apparent.  The longer that it is allowed to persist, the worse the dogs’ behavior will become.

A similar scenario can occur even if the younger dog is not challenging the older animal.  In such cases the older dog begins to sense the change in the younger dog and spontaneously starts to react.  Alternatively, the younger dog may try out some of the behaviors that develop with age (pushing on another dog) and not be the least bit aggressive, but the older dog perceives the younger dog as a serious problem and becomes aggressive.

In general, the dog that is challenged responds in one of three ways: (1) it acts absolutely deferential and shows the other dog that it is not interested in holding a higher rank (rolling on its back and urinating, looking away, waiting for the other dog to be first at everything), (2) the dog fights back and wins or loses and that outcome is accepted by both dogs, or (3) both dogs jockey for status and each is unwilling to concede status to the other.  In the last situation the aggression continues and may be prolonged, confusing, and dangerous.  In the second situation the situation resolves, but the process of resolution is still potentially dangerous.  Most behavior modification recommended for dogs with inter-dog aggression is derived from the first situation.

Much has been written about ranking dogs numerically and determining the “alpha” dog.  Such paradigms usually fail in profound cases of inter-dog aggression because the situation is not that simple.  Inter-dog aggression is associated with status relationships between dogs.  These relationships are not absolute.  They change with age and health status.  The manifestations of the relationships can be affected by the people who are present and by how those people interact with the animals.  Some relationships apply only to feeding and sleeping orders.  Because dog hierarchies, like those of people, are not linear, the amount of aggression exhibited may depend on which dogs were where and when they were there.  A dog that challenges one dog may not care about another dog in the household that, to all outward appearances, seems to act the same and be the same age and sex.  Chances are they are not acting identically, and it is in the subtleties that the problems with the relationship occur.

Treatment of inter-dog aggression focuses on setting and maintaining a new social order.  In general, reinforcement is given to the dog that is best able to maintain social status when this is contested in a fight.  (Generally, but not always, the younger, the larger, and the more physically fit the dog is, the more confident it is).  Preferential treatment or attention reinforces that dog as the higher-ranking dog.  In some households in which the problem dog (C) attacks one dog (A) but mildly pushes around another dog (B), the dogs might respond to all being reinforced in a linear manner (A over B and C, B over C).  How this is best done warrants some discussion, but two important cautions must be issued.

First, never physically punish these dogs.  Doing so only raises their level of distress, and they may feel that they have to fight you.  This reaction could manifest itself as fear, pain, or redirected aggression.  None of these choices is good, and you could make a bad situation worse. 

Second, if possible, never reach between two fighting dogs.  Most people have good intentions and want to separate fighting dogs to prevent injury to them.  If you place your body parts between the dogs, the dogs might accidentally mistake you for the other dog and injure you.  When this happens, the dogs usually withdraw, but the damage is already done.  Instead, if you know your dogs have a problem, watch them closely whenever they are together and keep cardboard, a broom, a bucket of water, a hose, or a blanket handy.  These are all “remote-control” items that can be used to separate the dogs safely.  In general, once the dogs are apart, they start to calm down and you can remove the aggressor.  Removing the victim if the aggressor is unrestrained may enhance the helplessness of the victim to the aggressor.  If no small children, high-strung people, or nervous animals are in the house, a loud noise such as that generated by a foghorn can also help separate the animals.  Remember that any animal that is injured is in pain and is frightened.  These animals can bite without being malicious.  Avoid this by transporting them by means of blankets and loose muzzles.

1) First, separate all dogs involved in the inter-dog aggression at all times when unsupervised.  This will not be difficult for inter-dog aggression involving dogs outside the household, but can be difficult within a household.  If the aggressor can be identified, that dog should be confined to the less desirable room (a spare bedroom, rather than your bedroom; a pen in the heated, well-lit basement, rather than the kitchen where the dogs are fed).  All other dogs should have free range.  If more than one dog is actively problematic, the problem dogs should be confined and the non-problem dogs can be left loose.  If every dog is a problem, they should all be kept in crates where they cannot see each other or threaten each other.

If your dog reacts only to other dogs on the street, avoid them until you have completed the designated behavior modification program.  Always walk your dog on a head collar.  At the first sign of any inappropriate behavior, ask the dog to sit and relax.  Close the dog’s mouth with the head collar.  If the dog still reacts, turn it around immediately and ask it to sit and relax.  If the dog still reacts, remove the dog from the situation as quickly as possible.  Use the head collar to close the dog’s mouth and lead it to a place where it can sit and relax.

Reinforcing the chosen dog has active and passive components.  First, separate them as discussed previously.  Second, enforce higher status by feeding one dog first, letting it outside before the other dog, giving it a treat or toy first, walking it first, playing with it first, grooming it first, and so on.  You can also have the dog sleep in a crate or on a bed in your room or on your bed (if you like this and the dog never growls at you while you are sleeping), whereas the other dog sleeps in another room or crate outside your room.  Each dog needs daily individual attention.  The dog that is being reinforced should always get the attention first in the presence of the other dog if this can be done quietly.  If necessary, restrain the other dog with a harness.  If you are walking the dogs as a group, make sure that the dog that is “out in front” is the dog that you are trying to reinforce.

Fit all dogs with Gentle Leader Head Collars or a harness, and gradually reintroduce them to each other when there is no attention being given.  For example, watch television while they both sit quietly, secured at a distance where they can see each other but cannot lunge and connect.  If the problem dog stares at the dog you are trying to reinforce, squirt it with a water pistol or a compressed air canister.  If the dog that you are reinforcing stares at the other, ignore it if the other dog does not growl.  If the other dog growls, use the air canister or water pistol.  If the aggression intensifies, remove that dog and banish it.  If the dog that you are reinforcing stares at the other dog and the other dog looks away, reward them both with food treats - that is exactly the behavior you are trying to reinforce