Inter-cat Aggression


PetMD Editorial

Updated Aug. 24, 2022

Aggression between Cats

Bringing a new cat into a home where there is already a cat, or cats, can cause some problems -- both passive and active aggression. Females will usually live peaceably with each other. However, if aggression and conflict rear their ugly heads in a household of formerly nonviolent female cats, it’s probably because one has reached the status of social maturity, which occurs at around four years of age. In addition, non-neutered males will aggressively compete for mates.

When the cat is introduced to your other pets, the most typical behavior is active aggression -- the resident cat goes after the intruder, biting, hissing, and scratching. Some cats are very confident and will not back down, regardless of the size of the other cat. Also, very confident cats will often use passive aggression. In this case, a stare and a lowering of the head are all that are required to bring the other cat into submission.

Symptoms and Types 

Overt Aggression  

Overt aggression is any hostile behavior that is obvious. The most clear sign of aggression is attacking and fighting, but before that there are other signs that can be observed; clear indications that there is going to be trouble. If you become aware of this behavior, you can be sure that without intervention, there is going to be a fight.

Behavior typical of the aggressor includes staring the other cat down, hissing and swiping at the other cat, raising its back and hackles (the erectile hairs on the back are raised), and thrashing its tail back and forth. Involuntary physical reactions to aggression will also show. The ears will be flattened back against the head, and the eyes will be dilated. The aggressor will mark areas of the home to establish its dominance, either by using the scent glands under its chin, or by urinating.

Covert Aggression  

Covert aggression is less apparent. The aggressor will use different methods to keep its rival away from resources like food and water, the litter box, and sleeping areas. Human attention will also be an opportunity for the aggressor cat to show its dominance, as it will block the other cat from receiving attention or even getting near to people. Favored areas of the victim cat may be marked by the aggressor, causing the victim to avoid those areas. The victim will urinate and defecate in other places in the house if it is kept away from the litter box.


Inter-cat aggression can be caused by a decrease in individual space, usually due to overcrowding, or because a new cat has been brought into the household. It can also be due an illness or a disruption in the household, such as moving or hospitalization of one of the cats.


Your veterinarian will want to rule out a physical cause for your cat’s behavior before making recommendations for solving the problem. If it is determined to be a physical cause, appropriate treatment will be prescribed. For instance, blood tests may reveal hyperthyroidism. Other physical conditions that can cause your cat to behave unusually are brain lesions, and urinary tract disorders.

If there is no evidence showing that the behavior is rooted in a physical problem, your doctor will consider behavioral conditions. This can range from incomplete socialization with other cats; fear aggression, or not having the psychological resources to deal with situations it is not accustomed to; and territorial aggression, where the cat has decided that it is going to settle a piece of turf as its own and will defend it.

If the behavior is not due to any physical ailments, anti-anxiety medication or behavioral modification may be recommended.

Living and Management 

Now it is up to you. Begin by putting the aggressor in a less-desirable room behind a locked door with food, water, and a litter box. The victim cat, on the other hand, should be allowed to roam freely and have access to the more favorable areas. If there are more than two cats, group them by personality, but keep the "bullies" alone.

The aggressors should only be allowed out when they can be supervised. At the first sign of aggression, it needs to return to the less-desirable room. You may want to try leashes and harnesses to gain control of the animals. In addition, put a bell on the aggressor so the other cats always know where it is. Use rewards to condition cats to each other, such as treats, massage, and grooming. Do not use physical punishment...ever. It will only make things worse.

In most cases, behavior-modification methods work, however, it may some time to see a difference in the cat and there may be relapses. Establishing dominance over your pets is important. Your cats may never form close bonds, but they should be able to share a living space.

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