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Intoducing a New Cat to the Old

Peter L. Borchelt, PhD


|October 2003|Peer Reviewed

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Our clients frequently ask what the best way is to introduce a new cat into a home with other cats. Should the cats just be put together in a room? Should a dominant or territorial cat be treated differently?

It is not surprising that introducing a new cat into a household with existing cats is confusing, because so many techniques have been described in popular books and magazines. Most suggestions have been based on the concepts of dominance and territoriality. The common-sense understanding of these terms is that dominance (being no. 1, or top dog or cat) is good and is a highly sought position, and that one of its benefits is holding an exclusive territory. Dominance and territoriality are commonly believed to be the primary basis of social behavior.

Unlike dogs, cats are not obligatorily social, and they can live well outside of feline social groups. Many feral cats hunt and live alone, and many single cats do well living in a household with only one person. Moreover, in contrast to dogs, cats do not have specific dominant and submissive postures.1,2 Cats signal offensive behavior (slow side to side head turning, staring, slow approach), defensive behavior (ears back, hissing, crouch), and various combinations of offensive and defensive behavior (e.g., arched back, tail up, piloerection, and hissing). Defensive behavior is not the same as submissive behavior-it does not signal, "I give up," but more like, "I may be afraid, but if you come closer, I may scratch and bite."

Space, Time, and Context

The social structure of domestic cats is best described as highly dependent on a combination of space, time, and context. Thus, in a group of domestic cats, one cat may displace another from a resting place at night but not during the day or from a different place at any time. Most problems in the home relating to feline social interactions are less a product of dominance and territoriality and more related to defensive behavior.

When a new cat is being introduced to one or more cats in a home, it is impossible to know in advance how the social structure will develop. It is unlikely that all cats will immediately act friendly to one another. More likely, one or more cats will display some degree of defensive behavior for a period. Occasionally, defensive behavior is short-lived and the cats soon get along. Other times, mutual defensiveness escalates to more aggressive behavior in which one cat runs and the other chases.


Rarely, a cat will, from the beginning, "relentlessly pursue" another, constantly looking for and rapidly chasing it.1 This behavior most clearly fits the description of a territorial cat and usually indicates that the prognosis for successful introduction is poor. Fortunately, most cases in which cats display defensive behavior eventually lead to the successful formation of a social group.

Given that an aggressive cat can seriously injure another cat or the owners if the cat redirects its aggression toward them,3 it is not prudent to simply put the cats together and hope for the best or, even worse, to counsel the owner to let the animals "fight it out." When cats are highly aroused and display moderate to high levels of defensiveness, they very quickly (in one or a few episodes) associate aggression with anything that moves-people, animals, or even inanimate objects.

Separate = Safe

The safest and fastest method in the long run is to assume the worst and separate the cats, until the situation can be assessed. If the new and resident cats are not afraid and seem to be playful and social, then the introduction can take place quickly. The owners must supervise the session and provide interactive toys to maintain play behavior. If more than slight or short-term defensiveness is displayed, then one cat should be confined by itself and additional sessions conducted. This method of introduction often only requires a few sessions per day for a few days.

If one cat is very frightened and hides, then that cat must be confined in a room by itself with food, water, and a litter box until it explores, plays, and exhibits social behavior with people. In this situation, the introduction between cats must be much more gradual to avoid even moderate levels of defensiveness. The gradual introduction is accomplished along a visual dimension, starting with almost no visual contact and slowly increasing it.

Bridging the Gap

A closed door provides zero visual contact. Removing the saddle beneath the door to make a gap of about 1/2 to 1 inch provides a bit of visual contact. In easy cases, the cats will soon play with each others' paws under the gap, after which they can be supervised during play with the door partially open, then in the room together. If the cats do not play, interactive cat toys such as string, feathers, or yarn can be used to attract the cats to the gap and then encourage them to play through the gap. Food may also be used to attract the cats to the gap, but once there the food may distract the cats rather than eliciting interaction between them. The goal is to associate play with the sight of "furry paws" and then proceed as above.

In some cases, the visual contact cannot progress directly from "feet" to "cat," but must be more gradual. A screen door or stacked gates can be used to separate the cats, with a cloth or cardboard screen used to reduce visual contact. Small holes in the screen provide gradually increasing visual contact during play or even during the day when the cats happen to pass by. Visual contact is maintained at a low enough level to elicit curiosity and can be increased gradually when there is some evidence of social behavior, especially play.

Full-View Friendliness

If the cats seem defensive or aggressive at any point, visual contact must be reduced. Only after the cats are social and playful in full view of each other are they allowed to interact together, with supervision, without the barrier.

Full integration into the household is usually accomplished within a week or two to a month or two, but some cases take much longer. As long as progress is observed, no matter how slowly, the procedure should be continued. Occasionally, cats will show very gradual progress for long periods of time and then improve dramatically in a few days.

Under normal circumstances:

• Assume the worst-separate the cats.
• Supervise introductory session.
• Provide interactive toys to maintain play behavior.
• If more than slight/short-term defensiveness is displayed, confine one cat and conduct additional sessions.

If one cat is very frightened/hides:

• Confine frightened animal in room by itself until it explores/plays with people.
• Conduct very gradual visual introduction to avoid any defensiveness.
• If cats do not play through visual/access gaps provided, encourage play with interactive toys provided through gap.

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