Why are my cats suddenly fighting with each other?

There are many possible causes for aggressive behavior among household cats. Sometimes, a specific trigger or inciting event can cause one cat to suddenly behave aggressively toward another (see the handout “Cat Behavior Problems – Aggression Redirected”). More often, aggression develops gradually. Since early aggressive communications may be subtle, you may not notice or even recognize one cat threatening another.

Besides redirected aggression, the leading causes of aggressive behavior between household cats are territorial aggression, fear-based aggression, and incompatible personalities. There may be more than one type of aggression occurring simultaneously. If you are concerned that your cats are exhibiting signs of fear or aggression when together, have your veterinarian examine them both, as underlying medical conditions can contribute to fear and aggression. If there is no underlying condition, consult a veterinary behaviorist to determine the cause(s) and create a treatment plan.

What is territorial aggression?

Territorial aggression is directed toward people or animals approaching the cat’s property. Cats who live indoors may treat specific indoor spaces as property and exhibit territorial aggression when other household cats approach them or when a new cat is introduced to the home. As in other species, territorial behavior is more likely to occur once cats mature socially, typically between two and three years of age.

What does territorial aggression look like?

Cats displaying territorial aggression exhibit behaviors intended to prevent other cats from gaining access to certain areas of the home. Blocking may be done subtly, for instance, with just a stare, or it may be more overt, with a growl or lunge. The cat may stalk or even chase other cats that attempt to approach their areas. Cats often use their position along a pathway to effectively block another cat’s access—they may sprawl across a doorway, for example. The posture of the territorially aggressive cat is usually upright, with ears forward.

Why does territorial aggression escalate?

Each cat’s personality can affect the progression of territorial aggression. When the cat that is the recipient of the aggression is calm, social, and able to recognize a subtle threat, then by walking quietly away from a challenge, the aggressor’s territory can be maintained without any physical conflict. If the recipient cat instead runs away, perhaps due to fear, the aggressor can become excited and respond with a chase that can result in injury.

In other cases, the aggressor’s personality drives the escalation. For instance, some cats that exhibit territorial aggression do not use subtle postures but instead behave impulsively when approached and immediately lunge at or chase any cat that approaches, leaving no opportunity for a peaceful resolution.

How can territorial aggression be prevented?

The risk of territorial aggression between two resident cats may be reduced for cats living in an enriched environment with adequate resources spread throughout the living space. Multiple litter boxes should be placed so that each cat can gain easy access. Provide perches and cat beds in several desirable locations to reduce conflict overviews and resting spots. Multiple feeding stations should be provided as well. Be sure to provide plenty of toys in assorted areas and schedule regular play sessions. If the cats can share toys, create games that allow the cats to take turns batting at a shared toy. (See the handout “Cat Behavior and Training – Enrichment for Indoor Cats”.)

To prevent territorial aggression when a new cat joins the home, introduce the newcomer gradually. Feliway MultiCat® (Feliway Friends® in Canada) and Feliway Classic® pheromone diffusers can be placed in several locations as these products may reduce stress and facilitate harmony. (See the handout “Introducing New Adult Cats”.)

What is fear-based aggression?

Fear-based aggression is an aggressive response that occurs in response to a perceived threat. A frightened cat may either freeze, flee, or fight. Sometimes, the threat is genuine—such as when one cat offensively approaches another. Some cats are innately fearful and exhibit postures reflecting fear in response to stimuli that are not actually dangerous. An abnormally fearful cat may initiate an aggressive response toward a friendly cat.

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Initially, a fearful cat may avoid the other household cat. Her posture may be low, and her ears may be held back as she retreats. If the other cat is very social and continues to approach, the fearful cat may freeze and hiss or growl with her ears held back. However, if freezing does not afford relief, then over time, the fearful cat may put her ears forward and lunge. Even though the cat is fearful, the overtly aggressive response is motivated by fear and is described as fear-based aggression.

Why does fear-based aggression escalate?

Cats naturally chase things that move away. A cat that was initially friendly or neutral will be inclined to pursue a fearful cat that flees and can become aroused and aggressive in the process. This dynamic increases the recipient’s fear while reinforcing the chaser’s aggressive response.

How can fear-based aggression be prevented?

It is important to recognize subtle signs of fear. If you notice that one of your cats is starting to spend time alone or in unusual resting places, such as the top of the refrigerator or under a bed, it is time to look closer at the interactions between your cats. Notice whether one of your cats is overly eager to play or join the fearful cat; if so, provide plenty of activities to keep her busy. Providing an enriched environment with a wide range of toys and opportunities for social play is particularly important for social cats. Create multiple comfortable resting spots in the main living spaces, with easy access to all resources, so your fearful cat does not find herself alone under a bed or isolated on a high piece of furniture. Seek professional help early, as fear can increase over time.

Is there a way to treat aggression between cats?

If your cats are exhibiting aggressive behavior toward each other, whether or not there are physical conflicts, they would benefit from an assessment from a professional such as a veterinary behaviorist. Once the motivation for the behavior is determined, a treatment program can be designed to improve the relationship between the cats. Behavior modification, and in some cases, medication, may be recommended to reduce both fear and aggression. A professional can also create an enrichment program to reduce stress. Be sure to have your veterinarian examine both cats.

If you are concerned for the safety of your cats, it is best to separate them except when you are available to carefully supervise and prevent them from staring, stalking, or otherwise being frightened until they can be assessed. The confinement space should be cozy, with a litter box, food and water, toys, and, if possible, a seat with a view of the outdoors. When the cats are together, keep a heavy blanket handy to toss over one that might be about to pounce. (See the handout “Treating Aggression Towards Other Household Cats”.)

— Update: 16-02-2023 — cohaitungchi.com found an additional article Treating Aggression Towards Other Household Cats from the website vcahospitals.com for the keyword aggressive cat behavior toward other cats.

What is the best way to safely introduce (or reintroduce) a cat into the household? 

In order to ensure that there are no injuries and that all introductions are positive, a desensitization and counterconditioning program is the best way to ease or re-introduce a cat into a household (See our handouts on behavior modification; ‘Behavior Modification – Implementing Desensitization and Counterconditioning’, ‘Aggression in Cats – Territorial and Fear Aggression To Other Household Cats’).

Begin by confining the “new” cat to a separate room or portion of the home with its own litter box, food, bedding, perching area, play toys and water. Allow the existing cat to continue to have access to the rest of the home. This arrangement provides a separate territory within the home for each cat, and allows both cats an opportunity to adapt to the smell and sounds of each other, without the possibility of direct contact or physical confrontation. If the new cat is housed in a screened-in porch or a room with a glass door, it may also be possible to allow the cats to see each other through a safe partition. Be certain to provide sufficient play, social interactions and even a little reward training with each of the cats. If you are planning to use a crate or a harness and leash for exposure exercises, be certain to spend some time training the cats to accept the harness and to get accustomed to their crates. A Feliway^TM diffuser or Feliway^TM spray on the cat’s bedding or in its crate may help each cat to adapt and settle more quickly. Offer small meal portions (rather than free choice feeding) and treats on opposite sides of the common doorway to increase the proximity of the cats. Remember that, by separating the cats, you are not only providing an opportunity for them to adapt to the presence of the other cat, but also to prevent threats, fearful displays and attacks which would only add to the fear and anxiety. When the cats show no fear, anxiety, or threat toward each other behind closed doors, it may be useful to switch positions, with the other cat confined while the new cat is allowed household access. The next step is to progress to controlled exposure exercises.

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Training should occur when the cats can be occupied in a highly “rewarding” activity such as feeding, play, or treats. Provided both cats are far enough apart to minimize the possibility of aggression, and the reward is sufficiently appealing, the cats will focus on the rewards rather than each other. In addition, if the rewards are saved exclusively for these introduction times, the cats will quickly learn to expect “good things to happen” in the presence of each other (counter-conditioning). In addition to ensuring that the cats are at a safe enough distance to minimize fear, both cats (or at least the one that is likely to be the aggressor) can be confined to an open wire mesh cage or a body harness and leash.

This will ensure that the cats can neither escape nor injure each other, and provides a practical means for controlling the distance between the cats for desensitization and counterconditioning. If the cats have been in cages during the first training session, they can be placed in each other’s cages at the next session (so that each cat is exposed to the other cat’s odor). By using cage confinement of one or both cats, or a leash and harness on one or both cats, the cats can be brought progressively closer at each subsequent feeding session, as long as there has been no fear or anxiety and both remain interested in the food. Over time, the cats are fed closer together until a point where the cats can eat or take treats in each other’s presence.

Another way to integrate cats is with play therapy. Some cats are more interested in play, toys or catnip than they are in food. One of the best toys is a wand or fishing rod handle with a stimulating play toy such as a catnip mouse or feathers on the end for chasing and pouncing. Begin by having both cats play at a distance from each other. Or, keep one cat in a crate with food while the other is out with play therapy. Over time, put the toys between the cats and let them play with the toys together.

Since odor may help cats learn about each other and enhance harmony, rubbing each cat with a towel and then bringing the towel to the other cat may help them get used to the smell of each other. Counter-conditioning with favored food rewards when presenting the towel or rubbing the cat with the towel may also help facilitate the introduction. Be sure to wipe the towel gently over the areas where scent glands are most prominent (i.e. the face, back and tail) first on resident cat, then using the same towel rub the new cat and leave that towel there. Repeat the process in reverse, rubbing the towel on the new cat and then on the resident cat and leave the towel with the resident cat.

What if neither cat seems to get comfortable with the other cat? 

Introductions must proceed slowly. The cats need to be far enough apart that they are relaxed and will take food or a treat while in the presence of the other cat. If the cats will not eat then they are too anxious and probably too close together, and the introductions are not accomplishing the goal of learning to associate the other cat with pleasant things. If the cats will not eat in each other’s presence, try moving the dishes further apart. If one or both of the cats still will not eat, separate the cats, do not give any food, and repeat introductions with food in a couple of hours. If the cats eat at that time, repeat using the same distance at the next feeding. If things go well, you can move the dishes a little closer together at the next session.

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If introductions where the cats can see one another are not successful, you will have to start with a much milder level of the stimulus. You might begin by keeping the cats in their own rooms and feeding on opposite sides of the door. A glass or screen door would allow you to add the stimulus of sight while you feed the cats on opposite sides of the door. While they are usually aware of the other cat, the fear or anxiety might be diminished and the cat will eat. If you don’t have a glass or screen door on the room, the next step would be to prop the door open a few inches so that the cats can see each other while they eat.

This is a slow process; you cannot rush things. Allowing either cat to interact in an aggressive manner sets the program back. The cats must remain separated except during times such as feeding when the cats are distracted, occupied, and engaged in an enjoyable act. In other words, good things are associated with the presence of the other cat. If the cats are doing well, you might want to increase their time together. However, if there are specific times, specific resources or specific areas of the home where threats or aggression are likely to recur, long term or permanent separation at all times except for supervised social play, feeding and training might be necessary. Although preventing recurrence is the goal, there may be occasions where aggression begins to recur. At the earliest signs of fear or anxiety, it would be preferable to distract the cats with a “come” command (if the command has been reward trained), or by pulling on the leash and harness, if one has been left attached.

In this way the aggressor is inhibited and the fearful cat (which might be stimulating further chase and attack by the aggressor) may begin to feel more confident when the owner is around to supervise and inhibit the aggressor. Counter-condition and reward the fearful cat for not running away. Reward and countercondition the aggressor cat if it comes to you, settles down and leaves the other cat alone. With enough different litter boxes, climbing areas and places to hide, it should be possible for many cats to adapt and remain together (or perhaps be separated while the owners cannot supervise).

Another option is to install an electronic cat door through which one cat can escape to its own room. These cat doors will only open for the cat wearing the activation collar, and some types are designed to be activated by the cat’s microchip.

Despite slow and careful progression, some cats may continue to display aggression, and it may be necessary to accept that they may never be compatible housemates. Although cats do live in social groups, they also have the opportunity to leave them if they do not feel welcome. The social groups we create in the home do not provide that opportunity. The only way to avoid territorial competition in these cats may be to find a new home for one of the cats, or to provide separate living quarters for each cat within the home. If the cats get along at certain times of the day, they can then be allowed limited exposure and interaction at these times. A leash and harness or wrapping a blanket or comforter over the more aggressive of the cats are options that might be useful for separating cats should aggressive displays emerge. If the problem is too severe, it may be helpful to medicate one or both cats. The option of drug therapy should be discussed with your veterinarian.