Cats can react aggressively when they feel another cat is invading their personal space or is otherwise threatening something they want to protect, from a litter of kittens to a favorite toy or resting spot. This can happen when a new cat enters the household, when one cat reaches maturity, or with cats outside the family. 

Protective aggression isn’t exclusive to male or female cats, and it can happen between cats of any age. With multiple cats in a household the aggressor may have a problem with one cat and not others. Protective aggression usually begins with growling and hissing, escalating to chasing and swatting, then actual fighting or attacking. The victim of protective aggression may become fearful and should be given somewhere safe to get away from the aggressor. 

This type of aggression can appear suddenly between cats who previously have gotten along well together.

Often a form of fear-induced aggression, it can happen when something frightening happens that startles both cats; they see one another reacting, and both become defensive. Sometimes, one cat sees something, such as another cat outside the window, that provokes offensive or defensive aggression and unleashes their agitation on the other resident cat. Once this pattern begins, it can continue if you don’t intervene. 

When cats feel threatened, they can be provoked to attack in their own defense. A cat could fear attack from another cat, punishment from a person, or something else entirely. The important thing is how the cat feels — even if the threat from the other cat or person isn’t as severe as the cat thinks, it’s the cat’s experience of fear that matters. Defensive postures from a cat include crouching, tucking their tail under their body, putting their ears back, vocalizing, and rolling slightly to one side. 

This is most likely to occur with unaltered adult males; it usually includes a lot of posturing, yowling, staring, growling, and stalking. If one cat backs down, an attack is usually avoided. Even when an attack occurs, the actual fighting may be quick, with the cats returning quickly to posturing and vocalizing. While this type of aggression often ends without injury, you should still always check both cats to make sure no one was hurt. 

• First, call or visit to your veterinarian. Any sudden behavior changes in your cat may be a sign of a serious health problem; cats are very good at masking symptoms until they’re very sick. 

• Have all cats in your home neutered or spayed; even one intact cat can influence the behavior of all the pets in your home. 

• Separate your cats and reintroduce them slowly and carefully to let them learn to spend time with each other happily and without fear or aggression. The process of reintroduction may take anywhere from a few days to a few weeks, but if you go slowly and let the cats set the pace, you’ll have the best results. See “Introducing Cats.” 

• If your intercat aggression problem is severe, contact a veterinary behaviorist for advice on using medication along with your behavior modification. Only a veterinarian can prescribe the appropriate medication for your cat; don’t use any medications without the guidance of your veterinarian. 

• Try products available on the market for reducing stress in cats, which ultimately can be very effective in reducing aggressive behavior. Of particular interest is the product Feliway, which simulates feline facial pheromones and sends an “everything’s all right” message to cats. It comes in a spray as well as a plug-in diffuser. You can use the spray on vertical surfaces in areas where your cat likes to hang out including doorways, corners of furniture, and corners of walls. Feliway comes in two different formulas; one for single cats and one for multi-cat households. It doesn’t work with all cats, but many cat owners report excellent results. Look for suppliers on the Web, or check with your local pet store or with your vet. 

• Don’t allow the cats to continue to fight or intimidate one another. Cats generally won’t work things out for themselves; once a problem starts, it will get worse if the cats continue to behave aggressively. The stress can cause other behavior issues as well, such as litter box problems. If you need to stop a fight between cats, you can use a loud noise, a squirt of water, or something soft tossed between them. If you need to, you can toss a towel over one cat to separate it from the other; don’t reach in with your hands or you may be hurt yourself. 

• Don’t punish the cat — punishment will make cats more fearful and is likely to escalate problems or make you the target of defensive aggression.