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Compendium September 2007 (Vol 29, No 9)

Understanding Behavior: Intercat Aggression

by Sharon L. Crowell-Davis, DVM, PhD, DACVB

    Aggression between cats that live in the same household can be a very perplexing problem for owners. As with human-directed aggression, intercat aggression is not a phenomenon with a single cause but can result from a variety of issues.

    New Versus Established Cats

    A very common problem is the introduction of a new cat to the household. In general, this is more easily accomplished if at least one of the cats is a kitten or juvenile. If all the cats are adults, assimilation is facilitated if they are all well socialized to their own species. Serious problems are most likely to arise when one or more adults that have not been socialized to their own species are involved. Because these cats lack appropriate experiences, they do not understand normal feline communication and etiquette and are likely to have intense and inappropriate responses to the sight of another cat: they may run away and hide in fear, or they may attack in an attempt to drive the other cat away from their personal space.

    Nevertheless, introduction of a new cat is not simple or easy, even with well socialized cats. This is because cats tend to have very insular social groups that do not readily accept strangers. Therefore, introduction of a new cat, even a kitten, to an established group of cats with a stable social organization needs to be gradual. A new cat should never be simply brought into the house and put in the same room with the established cats in the hope that things will work out. Fighting is likely to ensue, generating classically conditioned fear and avoidance responses to each other, and may even trigger conflict between cats that formerly got along well together.

    Gradual introduction involves progressive exposure through the modalities of sound, scent, sight, and touch. For most owners, it is easiest to start by placing the new cat in a specific room with its own resources (i.e., food, water, litterbox, toys, resting site). If the cat is disturbed by the relocation from its former habitation, or if there are health concerns that require a quarantine period, this step may need to last several days. During this time, the cats will hear each other through the closed door and can gradually become habituated to the sounds of each other. Eventually, the locations of the resident cat(s) and the new cat should be periodically switched, allowing them to become familiar with each other's scent. Other means of familiarizing the cats with each other's scent and exchanging scent can also be used. For example, bedding can be moved from one section of the house to another, or the owner can use small cloths to gently rub each cat on the neck, around the mouth, and under the ears, a process that cats appear to enjoy if it is done correctly. Scent glands are located in these areas, and both species-specific pheromones and individual chemical scent will accumulate on the cloths. The cloths can then be left in various locations around the house. Using the same cloth to rub every cat in the house may facilitate the development of a group-specific odor among the individual cats.

    The process of introduction is facilitated if any interior doors have windows or screens. Glass allows the cats to see each other, while screens allow them to see, smell, and hear each other. Thus, if possible, the cats should be repositioned so that a glass or screen door, rather than a solid door, is between them. At this point, interactions at the door can be encouraged by placing trails of treats up to the door on both sides. However, interactions should not be forced—for example, two people should not pick up the cats on either side of the door and force them to face each other. It is important to allow the cats to choose when they are ready to interact, recalling that integration of an immigrant into a group of feral cats typically requires weeks.

    If interior glass or screened doors are not available, alternatives can accomplish the same objective of gradual exposure. Most cats can readily leap a single baby gate; however, double-stacked baby gates will keep all but the most athletic cats separated. Limiting the amount a solid door can be opened is another option; owners can place a narrow (1- to 2-inch-wide) wooden block in the doorway and use a doorstop or heavy furniture to keep the door from opening any wider than the space produced by the block. It may also be possible to tie the door closed. Again, the narrow opening allows the cats to see, smell, and hear each other. They can even touch each other to a small extent (e.g., nose to nose, paw to paw), but fights are not possible.

    Sometimes, crates or cages are useful. However, they should be used only for cats that are comfortable with being in a crate. If a cat is stressed or distressed by being confined in a crate, development of amicable relationships with cats on the outside of the crate or in nearby crates will not be facilitated. In fact, hostile relationships can be produced as the other cats become associated with distressing circumstances. If a cat is comfortable in a crate, it should be provided with necessary resources (i.e., food, water, litterbox, treats, toys) while confined. If one cat is more self-confident and outgoing while another cat is timid, the timid cat should generally be left loose while the more confident cat's movements are restricted. This allows the timid cat to be the one that decides to initiate interaction.

    In the final stages of introduction, the use of harnesses, leashes, and human handlers can be useful. There must be one human for each cat involved in the introduction. As with crates, the cats must be familiar with and comfortable in harnesses (see the box ). The harnesses essentially provide a tool to initiate systematic desensitization and counterconditioning (see "Classical Conditioning: Learning by Association," June 2006). The leashes allow the cats to be in the same room but give the human handlers the mechanism to restrain their movement and pull them apart if sudden, unexpected conflict should erupt. Each cat should be placed in a harness and moved to a position where it can see the other cat but is far enough away that fear or aggression is not triggered. From this distance, each handler can countercondition his or her cat by whatever means works best for that cat (e.g., play, treats, petting). In successive sessions, the cats are gradually moved closer and closer together until they are allowed to directly interact. Curious or friendly greetings, especially nose touches, should be reinforced, while the distance between the cats should be increased if any fear or hostility is exhibited. Eventually, the harnesses can be removed and the cats allowed to interact freely.

    Dominance Problems

    Cats in a socially stable household develop dominance relationships. If the cats are very friendly and sociable, these interactions may be so subtle that the typical owner cannot identify them. Sometimes they are obvious but not problematic, as when all cats give way to the alpha cat, but the alpha cat is not aggressive unless challenged and only goes to desirable resources (e.g., food, favorite resting sites) when it wants them.

    Sometimes dominance relationships become a problem in multicat households. If a high-ranking cat starts guarding resources for long periods even when it is not using them—for example, sitting for hours beside the food bowl or at the entrance to the one room in the house with litterboxes—the simplest solution is often to ensure that those specific resources are available at multiple locations throughout the house. Some high-ranking cats are bullies; that is, they regularly exhibit intense dominance displays and aggression to lower-ranking cats even when the low-ranking cat is clearly signaling submission and is attempting to avoid interaction. This problem appears to be most common with cats that were not well socialized to their own species when young. Treatment of the bully with a selective serotonin reuptake inhibitor, such as fluoxetine (0.5 to 1.5 mg/kg/day), along with close monitoring of social interactions and persistent disruption or prevention of bullying incidents can often be beneficial. If one or more of the cats being bullied is extremely timid, treatment of those cats with buspirone (0.5 to 1.0 mg/kg q12h) often produces a decrease in fear and increase in confidence that can improve the overall social dynamic (see "Selecting Psychoactive Medications for Behavior Problems," August 2006, for further information). In cases of significant dominance-motivated aggression, the cats involved in aggressive interactions should be kept totally separated when not supervised.

    Fear Agression

    Fear aggression sometimes develops in households of cats that have historically gotten along well together. Triggering events leading to a classically conditioned fear response may or may not be witnessed by the owner. Fear aggression can be readily distinguished from dominance-motivated aggression by signaling. Whereas a cat motivated by dominance regularly stares at the cat it is aggressive toward, often with its ears held erect and rotated to the side, and walks with stiffly extended limbs toward the cat it will attack, the fear aggressor will lower its ears and head and lower its tail to the side of its limbs (see "Understanding Cats," April 2007). The fear aggressor often tries to avoid the cat it is afraid of, resulting in attacks only when that cat persists in attempting interaction; however, it will sometimes approach and attack the feared cat, apparently trying to drive that cat out of its personal space. In some of the worst cases, the fear aggressor will defecate or urinate as it pursues and attacks the cat it is afraid of, further illustrating that the underlying motivation is fear. The main components of treatment are as follows:

    • Keep the cats separated when they are not being supervised
    • Give the fearful cat medication (e.g., fluoxetine) that may have anxiolytic and antiaggressive effects
    • Gradually reintroduce the cats using the techniques already discussed for introducing new cats

    Play Agression

    Play aggression, which is sometimes directed at humans, can also occur in intercat relationships. As with dominance, inadequate or inappropriate early socialization can be a contributing factor. Normal cat play is fairly rough, with pouncing, batting, and chasing. However, appropriately socialized cats will deescalate when the play becomes too intense. Problems develop when the arousal of intense play escalates into a fight. When presented with cats that may have this problem, taking a detailed history of exactly what the cats do in these situations is essential. Owners who are not familiar with normal cat play may be interpreting normal play as "fights" even though neither cat is being hurt and there is no growling or hissing. If the play does sometimes escalate into true fights, the cats should be kept separated when not supervised. When the cats can be supervised, play should be allowed but must be disrupted in the very early stages of escalation. Disruption can be accomplished in a variety of ways, depending on the individual cats, such as rolling multiple balls past the cats, calling them, going to the kitchen, running an electric can opener, or having several humans distract the cats in different directions toward human-interactive play.

    Redirected Aggression

    Arousal-induced and redirected intercat aggression can also occur. For example, if an unfamiliar cat comes to an open window, the household cats may become aroused by its presence but, being unable to reach it, redirect their aggression onto each other. As with human-directed aggression in this situation, the cause of the arousal, not the aggression between cats, needs to be addressed.


    Intercat aggression occurs for a variety of reasons. A complete behavioral history and medical evaluation is necessary before diagnosing the cause of the problem and devising a treatment plan.

    See the Key Points box.

    Downloadable PDF

    Dr. Crowell-Davis discloses that she has received financial support from CEVA Animal Health and Merial.

    NEXT: Abstract Thoughts — Chronic Nasal Discharge in Cats


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