Aggressive Territorial Behavior

Question
Is there any hope of modifying aggressive territorial behavior once a cat has begun exhibiting it? I have two Abyssinian cats: a neutered male and a spayed female, ages 4 and 5, respectively. They are half brother and sister and have been wonderfully companionable for many years, though the older female has always been the "boss." Recently the female has become increasingly more aggressive toward the male and suddenly, last week, she became totally aggressive: stalking, chasing then attacking him. I am deeply attached to both cats but dividing my open-area house into 2 separate living spaces would be difficult and very expensive. I am now keeping the cats separate except when I have the female on a harness. I can stop her from attacking the male, but she is still exhibiting aggressive behavior and I'm afraid if her aggression is thwarted she may redirect the behavior. All 3 of us are traumatized by the situation. Is there any way I can modify the female's behavior and so keep both cats?

Answer
Paula,

I am assuming they are both altered and are indoor only cats.

When this kind of sudden behavior change takes place, it is usually a medical issue. I would have the two of them into the vet for a check up. If a cat is feeling some discomfort, it is not unusual for it to exhibit defensive behavior (e.g. hissing & spitting at the sight of another cat). You could also have hormone levels checked and thyroid levels checked.

There are some cats who seem to go off for no apparent reason and the symptoms get worse and worse.  Our vet calls these aggressive psycho kitties and whatever is going on causes them such extreme frightening behavior, euthanasia may be the best option, but is still a very last resort!

There can be behavioral causes like a change in the number or types of family members, e.g. a new baby or a new pet, but you did not mention it.  Sometimes, just seeing a new cat in the vicinity outside a window, may be enough to cause some of this behavior. If you think it is behavioral, you can check:

http://jacksongalaxy.com/category/cat-behavior-and-care/

look up aggression and behavior issues.  Jackson is an excellent feline behaviorist and has excellent video tapes to demonstrate his suggestions.

Please let me know how you make out.

Concerned regards... Norm.