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 also establish territories which they often defend against entry by intruders or rivals. The territorial and social natures 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 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 his or her 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 the park several blocks from your house, your dog may view that park as her 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, which is normal canine greeting behavior. As they do so, talk to them; in a happy, friendly tone of voice "FIDO, BE GOOD, TAKE IT EASY". Do not allow the dogs to investigate and sniff each other for a prolonged time, as sometimes this escalates to an aggressive response from one or both dogs. After a short time, get both dogs’ attention, and give each dog a tidbit of good in return for obeying a simple command such as "sit". Take the dogs for a walk and let them sniff and investigate each other at intervals. Continue with the "happy talk", food rewards, and simple commands.
Be aware of body postures
A body posture which indicates 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. These would include hair standing up on the dog’s back, (piloerection), baring of teeth, deep growls, a stiff legged gait, or a prolonged stare. If you see any of these postures, interrupt the interaction immediately. But DON’T do so in a threatening way, but rather calmly and positively get each dog interested in something else. For example, both handlers can call their dogs to them (give a little jerk on the leash if necessary), have them sit or lie down, and reward each with a tidbit. The dogs will become interested in and excited about their tidbits which will prevent the situation from escalating into aggression. Try letting the dogs interact again, but this time for a shorter time period and/or at a greater distance 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. If you have more than one dog in your household, it may be 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 When ever more than one dog lives in a household, the dogs will establish a dominance hierarchy. This social hierarchy is determined by the outcomes of interactions between the dogs. Owners cannot choose which dog they want to be dominant. When a new dog is brought into he family, the hierarchy is upset because the newcomer’s place is unclear.
Who will be dominant?
It is difficult to predict which dog will ultimately be dominant. Individual personality traits as well as breed characteristics are important factors. Males are generally dominant over females, young adult dogs are generally dominant over puppies and aging dogs, and larger dogs are generally dominant over smaller one. Initially, the resident dog is likely to be dominant over the new comer, but this may change rapidly. Remember that these are generalizations and 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 cause injury. One dog may "stand over" another by placing her paws on the shoulders of the other dog, or by draping her neck across his shoulders. 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 a certified animal 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 asserts his status. Dominant dogs can and should be allowed to take toys away from more subordinate dogs, to push in and receive attention and petting from the owner, to control favorite sleeping places, toys, and other valuable (from the dog’s point of view) resources. Support the dominance hierarchy by allowing this to occur. This can be difficult for owners to do if the resident dog becomes the subordinate dog in the hierarchy.
Keep the routine
Keep the resident dog(s)’ routine as much the same as possible by keeping feeding, exercise, play and sleeping times and locations the same as before the new dog arrived. You can also 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 chewy while you play with the subordinate dog inside (or vice versa). When structuring these individual sessions, the dogs which are not receiving attention should be kept busy doing something else they enjoy. If the dominant dog thinks the subordinate dog is receiving special attention which he is not, it may undermine the dominance hierarchy and contribute to a fighting problem.
Introducing puppies to adult dogs
Puppies usually pester adult dogs unmercifully. Before the age of 4 months, puppies may not recognize subtle body postures from adult dogs which are signals they have had enough. Well socialized adult dogs with good temperaments can set limits with puppies with a growl or snarl. These behaviors should be allowed, even if the puppy overreacts and "screams bloody murder!" Adults which are not well socialized, or who have a history of fighting with or challenging other dogs may attempt to set limits with more aggressive behaviors such as biting which can harm the puppy. For this reason, new puppies should not be left alone with an adult dog until you are confident the puppy is not in any danger. Be sure to give an adult dog some quiet time away from the puppy and some individual attention as described above.
If problems develop
If the introduction of a new dog to your household does not go smoothly, do not allow the conflicts to continue. The more often they occur, the more difficult it will be to get the dogs to coexist peacefully. Dogs can also be severely injured in fights, as can owners if they attempt to intervene. Punishing one or both dogs in NOT the answer, and is likely to make the problem worse. Talk to your veterinarian about spaying and/or neutering any dogs which are intact, and contact a certified animal behaviorist for information about behavior modification.