No matter what breed of dog you own, if you have two of the same sex, they're more likely to fight. Even two sisters, born and raised in the same litter, won't hesitate to duke it out just as viciously as two brothers. Of course, that doesn't mean that two sister dogs can't get along. Prevention is the best policy for both them and you, so be diplomatic in your day-to-day interactions with the pack.
Spay your dogs -- the earlier, the better. Two intact females are substantially more likely to fight each other than two spayed females, as they are less aggressive and aren't motivated by the need to compete for a mate. The older a dog is before she is spayed, the likelier she is to retain her hormone-influenced aggression afterward, so ask your vet about the procedure while your dogs are young.
Separate the dogs before you feed them. Don't settle for feeding them on opposite sides of a room, or simply from two different dishes. Actually sequester them from one another, either by keeping them in separate, closed-off rooms or by feeding them in their crates.
Give your dogs treats only as rewards for following commands. Simply tossing them out on the floor can lead to fighting over the treats, while giving them away without being earned teaches your dogs that they don't necessarily have to obey you to get a reward.
Distribute rawhides, marrow bones and other high-quality treats equally, and only when the dogs are in separate rooms. Treats like these are likely to make dogs aggressive, competitive and territorial.
Treat the dogs equally, no matter what. If they fight, for example, don't reprimand just the instigator, and don't coddle the dog who lost the fight. Dogs establish a pecking order on their own, and when you appear to favor one dog or the other, it confuses them and destabilizes that sense of order.
Supervise your dogs when they play together. If you recognize the behavioral cues that anticipate a fight, like stiff movements, hard stares and bared teeth, intervene and separate the dogs.
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