Aggression from one dog to another is one of the more common behavior problems facing pet owners in multi-dog households. Behaviorists nod knowingly when confronted with problems involving aggression between dogs. Iinterspecies aggression of this type can make living with the feuding pair a positive nightmare.
I always surmise that I have an interactive pet behavior problem to deal with when I see two or more case records for my attention ... and only one owner. Then I notice that the owner has a brace of dogs in tow and my suspicions are confirmed. Sibling Rivalry
Aggression between or amongst dogs in the home, whether the dogs are siblings or not, is a hierarchical struggle for dominance or "top dog" status. Overt aggression among members of a stable pack, either in the wild or at home, is relatively rare. The cohesiveness of the group is normally maintained through postures and gestures that signal a prearranged order of seniority.
The logic that underlies this process is similar to that employed in a bakery when customers are required to take a number and wait to be called. This "know where you stand" state-of-affairs avoids conflicts and arguments among people over loaves of bread. The same principal applies among members of a pack of dogs. There is a number one dog (the "alpha"), a number two dog (the "beta"), and so on, right through to the last or "omega" dog. This order is established over time by canine measures of seniority or rank, including age, length of tenure in the pack, personality, biological sex, and size.
But dog owners are constantly destabilizing their packs by adding new members. They also mess things up by intervening in matters about which they know very little and understand even less well.
In the wild dog pack, new members are added at birth and are helpless at first. A totally different situation exists in homes when well-meaning pet owners add a juvenile or adolescent dog with different customs and different background from the incumbents. We wouldn't like if someone added an unknown person to our household without our permission, and dogs seem to feel the same way. To make matters worse, the newcomer may not have been properly socialized and may be gauche in terms of dog etiquette. Owners may strive to make the newcomer "feel better" in his new home by offering privileges that the residents themselves may not always enjoy, thus adding to the problem. Incumbents can only look on in horror as the new member usurps their owner's attention, occupies their territory, and gobbles up their food.
Prevention is better than cure. Owners must first guess their own dogs' reactions before bringing a newcomer into their home and should "vet" the would-be addition to the pack for sociability, personality, and compatibility. Good findings include a dog-savvy dog that has been raised with its original pack, its littermates, until a decent age (say, 8 weeks).
Next, you can assess personality, with or without professional help, as being outgoing, dominant, sensitive, shy, or whatever, and intuition should help guide you as to the wisdom of a particular mix. For example, bringing a hyperactive, in-your-face type of dog into a home where a quiet, elderly dog lives may not be the smartest move. Finally, be sure to introduce the soon-to-be housemates on neutral territory and under pleasant circumstances. If the two dogs seem to get on well, institute a "trial marriage" for a while, being prepared to reconsider your options if things don't work out.
Sometimes, interdog aggression occurs in a household arises when a younger dog reaches maturity or when an older "leader dog" becomes infirm and unable to maintain its authority. Under these circumstances, within the bounds of humanity and fairness, it is best to let dogs sort out their own affairs, as they would do in nature. Interventions by well meaning persons, who invariably try to "make things equal" by supporting the "loser" only serve to ensure that fighting will continue in order to establish the hierarchy.
When problems of this nature have been allowed to develop, there are procedures you can engage to turn things around. The leader dog must be singled out and supported over the next in command and thus down the pack order so that the hierarchy can restabilize. Leader dogs need (and expect) to be fed first, played with first, petted first, and so on, and they must get their way or the consequences may well be dire. See Interdog Dominance Aggression
Dogs don't expect or even appreciate equality; they just need to know where they stand in the pack order. Ensuring and supporting the correct order goes a long way toward helping resolve negative interactions among cohabiting dogs.