Domestic dogs are social animals. This means that they naturally live together in groups which have social structures called dominance hierarchies. This structure is often called a "pecking order." Dogs will also establish territories which they often defend against entry by intruders or rivals. The territorial and social nature of dogs need to be considered when a new dog comes into the household.
CHOOSE A NEUTRAL LOCATION. Introduce the dogs in a location which is not a part of either dog's territory. This will minimize the chances of either dog viewing the other as a territorial intruder. Each dog should have their own handler. With both dogs on leash, take them to an area such as a park with which neither is familiar. If you frequently walk the resident dog in this park, your dog may view it as his territory, which would not make it a neutral location. Choose another site.
USE POSITIVE REINFORCEMENT. From the first meeting, you want both dogs to expect "good things" to happen when they are in each other's presence. Let the dogs sniff each other. This is a normal canine greeting behavior. As they do so, talk to them in a happy, friendly tone of voice "BE GOOD, FIDO. TAKE IT EASY." Do not allow the dogs to sniff and investigate each other for a prolonged period of time. This can sometimes escalate to an aggressive response from one or both dogs. After a short time, get both dogs' attention and give them both a simple command such as "sit." Reward the dogs with a small tidbit of food for obeying the command. Take the dogs for a walk and let them sniff and investigate each other at intervals. Continue with the "happy talk",simple commands and food rewards.
BE AWARE OF BODY POSTURES. A body posture which indicates that things are going well is a play-bow. The dog will crouch with his front legs on the ground and his rear in the air. This is an invitation to play which usually elicits friendly behavior from the other dog. Watch carefully for body postures which indicate an aggressive response may occur. This may include stiff body posture or a stiff legged gait, a prolonged, intense stare, baring of teeth or deep growls. If you see any of these postures, interrupt the interaction immediately. DO THIS in a calm and positive way. DO NOT interrupt the dogs in a threatening manner. For example, both handlers can call their dogs to them (give a little jerk on the leash if necessary), have them sit or lay down and reward them each with a treat. The dogs should become interested in and excited about their treats which should prevent the situation from escalating into aggression. Try letting the dogs interact again, but this time for a shorter period of time and/or at a greater distance for from each other.
TAKING THE DOGS HOME. When the dogs seem to be tolerating each other's presence without fearful or aggressive responses and the investigative greeting behaviors have tapered off, you can take the dogs home. Whether you choose to take them in the same or different vehicles will depend on their size, how well they ride in the car, how trouble-free the initial introduction has been and how many dogs are involved. Please use a crate if necessary. If you already have more than one dog, it is best to introduce the resident dogs to the new dog one at a time. Multiple resident dogs may "gang up" on the newcomer.
The Importance of Dominance Behavior and Social Structure
Whenever more than one dog lives in a household, the dogs will establish a dominance hierarchy. IT IS EXTREMELY IMPORTANT that you understand the next line. THE SOCIAL HIERARCHY WILL BE DETERMINED BY THE OUTCOMES OR THE INTERACTIONS BETWEEN THE DOGS AND THE OWNERS CANNOT CHOOSE WHICH DOG THEY WANT TO BE DOMINANT. When a new dog is brought into the family, the hierarchy is upset because the newcomer's and the resident's place in that hierarchy is unclear.
WHO WILL BE DOMINANT? It is difficult to predict which dog will ultimately be dominant. It is again extremely important that you realize it ultimately may not be the existing dog. Individual personality traits as well as breed characteristics are important factors. Initially the resident dog may be dominant over the newcomer, but this may change rapidly. Generalizations, such as males being dominant over females, older dogs being dominant over younger dogs and larger dogs being dominant over smaller ones, may not apply to your dogs.
HOW IS DOMINANCE ESTABLISHED? Dogs usually establish their dominance hierarchies through a series of interactions involving ritualized behaviors which include body postures and vocalizations which do not usually cause injury. One dog may "stand over" another by placing her paw and/or draping her neck on the shoulders of the other dog. However, because of past experiences, inadequate socialization or genetic temperament tendencies, some dogs may instead, with very little warning, escalate dominance displays into aggression. If this occurs, you may need to seek the assistance of an experienced dog trainer/behaviorist.
SUPPORT THE DOMINANCE HIERARCHY. You need to support whatever dominance hierarchy or "pecking order" the dogs establish for themselves. DO NOT UNDERMINE THE NATURAL HIERARCHY BY ATTEMPTING TO TREAT THE DOGS EQUALLY OR SCOLDING THE DOMINANT DOG WHEN HE/SHE ASSERTS HER STATUS. Dominant dogs can and should be allowed to take toys away from more subordinate dogs, to push in and receive attention from the owner, to control favorite sleeping places, toys and other valuable (from the dog's point of view) resources. YOU MUST support the dominance hierarchy by allowing this to occur. Although this can be difficult for owners to do if the resident dog becomes the subordinate, again YOU MUST allow this to occur and support the dominant dog. Dogs do not understand equality. One dog must be dominant and you cannot decide for them which one it will be.
KEEP THE ROUTINE. Keep the resident dog(s)' routine the same as much as possible by keeping feeding, exercise, play, sleeping times and locations the same as before the new dog arrived. You can give each dog some time alone with you - perhaps the subordinate dog has a difficult time playing with toys because the dominant dog takes his toys away. Put the dominant dog outside with a really good chewy while you play with the subordinate dog inside (or vise versa). When structuring these individual play sessions, the dogs that are not receiving attention should be kept busy doing something else they enjoy. If the dominant dog thinks that the subordinate dog is receiving special attention that he is not, it may undermine the dominance hierarchy and cause a fighting problem.
INTRODUCING PUPPIES TO ADULT DOGS. Puppies will usually pester adult dogs unmercifully. Before the age of 4 months, puppies may not recognize subtle adult body postures which signal that the adult has had enough. Well socialized adult dogs with good temperaments can set limits with puppies with a growl or a snarl. These behaviors should be allowed, even if the puppy overreacts and "screams bloody murder!" Adults which are not well socialized or have a history of fighting with other dogs, may attempt to set limits with more aggressive behaviors, such as biting, which may harm the puppy. Be sure to give your adult dog plenty of quiet time away from the puppy and some individual attention as described above. PUPPIES SHOULD NEVER BE LEFT ALONE WITH AN ADULT DOG UNTIL YOU ARE ABSOLUTELY CONFIDENT THE PUPPY IS NOT IN ANY DANGER.
IF PROBLEMS DEVELOP. If the introduction of a new dog into your household does not go smoothly, do not allow the conflicts to continue. The more often they occur, the more difficult it will become to get the dogs to coexist peacefully and both the dogs and their owners can be severely injured in fights. Punishing one or both dogs is NOT the answer and is likely to make the problem worse. Talk to your veterinarian or your local spay and neuter clinic about spaying/neutering any dogs which are still intact and contact an experienced dog trainer/animal behaviorist for information about behavior modification.