Social Aggression Between Cats Sharing A House
This type of aggression is commonly referred to as "territorial" aggression; however, it is not always territorial concerns that motivate the attacks. The aggressor may attack only one cat in a multi-cat household and, while the victim is usually a newcomer, it may occasionally be a cat with which the aggressor has previously had a good relationship. These disputes generally arise when either cat reaches maturity at about 8 - 24 months of age. The aggressor is not necessarily the first cat that was introduced into the household, nor the eldest.
This type of aggression usually develops gradually (unlike redirected or fear-induced aggression). It begins with hissing and growling; progresses to swatting and chasing; and finally involves attacking and fighting. The victim may become progressively more afraid of the aggressor and may begin to hide in remote areas of the house, coming out only when the other cat is not around. Occasionally, litterbox problems occur because the fearful cat is too afraid to leave the hiding place. It is very important to provide the victim with a safe haven in the house to protect it from injury and stress.
While many cat owners who experience this problem opt to find a new home for one of the cats, those who have a strong attachment to both cats, may prefer to try systematically desensitizing the aggressor to his victim. This is the same procedure that we recommend for introducing a new cat into the household or reintroducing two suddenly hostile cats (refer to "Re-Directed Aggression Towards Other Cats"). After desensitization and counterconditioning steps have been taken, it is essential that the owner be prepared to carefully monitor the next very important step--their interactions once they are allowed to be together.
Any sign that an aggressive encounter may be brewing should be heeded and the cats should be separated immediately. Further altercations will only cement their hostility toward each other. Have a large squirt bottle handy to douse the aggressor if he makes a threatening advance. (Do not shout or scold, as this will frighten the victim as well.) If your timing is perfect, he will get the message.
If you do not see progress with any of the suggestions above, you may want to read the chapter "Give Peace a Chance" in Dr. Nicholas Dodman's book, The Cat Who Cried for Help, before consulting with your veterinarian regarding drug therapy.