These groups are generally made up of related females and their offspring, as well as several unrelated males. The ability for so many cats to live in relative harmony in the wild is due to the fact that each group has a large home range and each individual has an established territory within that home range. When we bring cats into our homes, we have to realize that our house and yard basically comprise their home range; home range size may be greater if the cat spends more time outdoors than inside. In multi-cat households, each individual cat will stake out a territory within this home range. It just follows then that in a multi-cat household, each individual cat will have a fairly small territory, especially if all of the individuals are living strictly indoors. 

So, how many cats is too many? This is a very difficult question to answer since this depends on the size of the home range, the number of territories available, whether any of the cats are related, and the individual personalities of the cats involved. In some cases, it only takes the addition of one, unfamiliar cat to completely disrupt a previously peaceful multi-cat household.  

There are steps you can take to increase the chances of successfully introducing a new cat to your household. First, you will need to determine how your current resident cats define their territories. Do they share food dishes, water bowls, litter boxes, and resting areas? Does one cat spend all of his time in your bedroom while the others are content to sleep on the back of the sofa? Once you have figured out which territories are already in use, you can bring in the new cat and establish a separate territory for him. This territory should include his own food and water dishes as well as a litter box. Once the new cat is comfortable in his new territory, then you can begin gradual, and controlled introductions to your other cats. Reward your cats with food treats if they behave in a curious or friendly manner. Never directly punish your cats if they are unfriendly to the newcomer. Instead, allow the newcomer to escape back into the safety of it's new territory and try again later. It may be the case that the new cat will never be accepted into the group. While this may be difficult for you to understand, you should try to accept the fact and go on as long as full-scale war is not erupting in your home.  

Although many owners believe that fights will be the obvious result of adding a new cat to their household, many are surprised to find that this is not the case. Other more subtle forms of aggression are more common. In addition, other behavior problems may arise among your established group of resident cats that seem unrelated to the new feline member of your household. First, if aggression is seen, it may not be directed toward the new cat. Instead, it may be directed toward other familiar cats in the house or the owners themselves. There has been a disruption in the social hierarchy and the confusion that is felt by the resident cats may be expressed in outward aggression.  

More commonly, however, owners notice more subtle forms of aggression such as guarding food bowls, blocking access to litter boxes, or "hogging" of favorite resting areas. Although no direct conflict occurs, it is obvious that there is tension among the resident cats. Resident cats may begin urine spraying to mark their territories, urinating on upright objects such as stereo speakers, furniture, door jams, or curtains. If one cat in a multi-cat household is spraying, chances are that others are too. For example, in a household with 10 resident cats, the chance of an owner seeing spraying at some point is 100%. Some cats may be even more sneaky in their show of aggression. These individuals will urinate on your bed, in your laundry basket, or on your jacket as a response to social stress. These are more passive ways of asserting dominance.

So, what can you do if this is happening in your multi-cat household? First, you will want to enlist the help of your veterinarian. You need to make sure there is nothing physically wrong with any of your cats. Cats suffering from stress can develop bladder infections and other medical conditions which may lead to behavioral changes such as inappropriate urination. Even urine marking may be a sign of a physical problem.  

If your veterinarian indicates that your cats are in good health, then you may want to seek the help of a qualified behaviorist who can help you and your cats sort out their new territories and relationships. There are many techniques that can be used to discourage a cat from spraying or urinating inappropriately. It is very important to begin working on these techniques immediately before your cats become accustomed to using their alternate toilet areas.  

You will also want to clean up any soiled areas well using an enzymatic-style cleaner that not only removes the stain, but removes the odor, without simply masking the smell. If you have a cat who is blocking access to the litter box, you will want to add more litter boxes. Most of the experts on cat behavior believe that you should have two litter boxes in your house per cat! The same is true if one or more of your cats is monopolizing the food and water bowls. You can add additional bowls in each cat's individual territory in order to make sure no one is being edged out. It may even be helpful to allow one or more cats additional time outdoors if your cats are indoor-outdoor.

If you have a happy, peaceful multi-cat household, you may be tempted to add another cat--the more the merrier! However, the social dynamics of multi-cat households can be very complex and may not be readily evident until you stress the members with the addition of a new individual. What we all have to remember is this: while cats can live happily in large groups, this may not directly apply to our household situations. Our homes are artificial social situations in which we ask our cats to live; home ranges are small and individual territories are even smaller. It is important for all cat owners to understand normal cat behavior and appreciate the complexity of their social structure in order to better keep the peace in their multi-cat households.