Dogs are social animals whose evolutionary history makes them willing and able to live in groups. Group living enabled wolves to work together to obtain food, raise their young and defend their territory. It would be counter-productive for members of a group to fight with each other and risk injury. That would prevent them from working with the group.
Although domestic dogs are not wolves, they do have a social structure in which each dog is either dominant (leader) or subordinate in its relationship with each pack member. This is a “dominance hierarchy.” The leader or “alpha” dog is the one that has first access to all the “critical” resources. These resources include food, resting places, mates, territory and favored possessions. Assertion of dominance by the alpha is generally communicated through facial expressions, body postures and actions. Fighting is rare, since as soon as the subordinate submits or defers to the alpha animal and the alpha gets its way, he or she gives up the challenge.
Fights between dogs in the household are often about dominance or social status. Social status aggression most often occurs when dogs reach social maturity at 12-36 months of age. Fights will be about those resources that are considered important to dogs. Therefore fights may occur over treats, owner attention, greeting the owner upon return, sleeping positions near the owner, entering or exiting the home, high arousal situations such as fence running or movement through tight spaces. These fights occur most often between dogs of near equal status and often, but not always, dogs of the same sex, and seem to be most severe between female dogs.
Trying to treat two dogs as equals will only serve to counter the natural tendency toward a hierarchy. The dog that is the more dominant in a relationship needs to be supported in its position and the more subordinate must be taught to accept the relationship. When you support or encourage the subordinate dog as it tries to gain access to resources such as your attention, the dominant dog may begin to challenge and fight, in an effort to keep the lower ranking dog in its “ place.” If you then discipline the dominant dog, or pull the dominant dog away, you have favored, supported and come to the aid of the subordinate dog.
Conflicts may occur between dogs when the dominance status is ambiguous or when they are particularly close in rank. After the decline, illness or death of an older dog, fighting may begin in the remaining dogs even when one is clearly dominant. This is because the older dog may have been dominant to both dogs, and now they are trying to establish new positions. In any case the fighting can be severe and injurious.
Although you should generally attempt to allow dogs to resolve their differences on their own, you will need to intervene if there is the potential for injury. Under no circumstances should the dogs be allowed to “fight it out.” You could be injured due to redirected aggressive attacks, or when you attempt to break up the fight.