It sounds as though it is a dominance issue. Your new pup is coming into maturity, and your existing female may be concerned that the new girl is going to make a play for alpha bitch.

It sounds as if the new pup does not start altercations, but merely defends herself. If this is the case, then you may be able to calm the situation by recognising the pack structure and giving some positive reinforcement. For example, always greet your old girl first when you walk into the room. Put her bowl down first when you feed them. If you give them titbits, always give her titbits first. And so on and so forth - I'm sure you get the idea. If she begins to realise that you (ie the pack leaders) recognise that she is still above the other dogs in the pack structure, then she may calm down and stop behaving aggressively.

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