Cats are a bit like people when it comes to friendships. Cats living in a group have "preferred associates" ("friends," if you will), and other cats from which they actively distance themselves (definitely not preferred). Why would this be, you might ask?
Background probably plays some role. Many species, including cats, learn a lot about their inter- and extra-species relationships during the so-called "sensitive period" of their development. For cats, this is between 2 and 7 weeks of age. Pleasurable exposure to whomever or whatever during this period can lead to lifelong acceptance.
The corollary to this is also true: Unpleasant experiences in early kittenhood can lead to lifelong mistrust or even hatred. Perhaps some of cats' preferences are imbued during the critical period. Extreme examples of how this works are provided by reference to the feral and orphan cat situations. Feral cats that have not been exposed to people during the first 7 weeks of their life will never be entirely comfortable around people. Hand-raised orphan cats that have not had a chance to interact with their own species will likely never be at ease with their kind and instead often become "over-attached' to their human caregivers. Which other felines a cat will tolerate is also likely shaped by early learning, or lack thereof.
This is not to say that social learning cannot occur later, as well. One really bad experience down the road of life can also have profound and long-lasting negative effects on a cat's perception of others, and such fears can generalize.
Other reasons why cats may not get along include dominance, sexuality, and territoriality. As with humans, such factors seem to make the cat's world go round and are so important to some individuals as to border on obsession. A cat that is extremely attached to his owner may not appreciate having to share this valued resource with a new cat, a total stranger. Certainly, a red-blooded male will not appreciate sharing his quarters with another of the same persuasion (thank heavens for neutering). Finally, a despotic leader cat that has his house in order will often not appreciate the addition of another cat, especially if the newcomer isn't fully respectful of the laws that he (or she) has layed down.
When cats are brought together under the same roof they often squabble for a while. This squabbling often takes the form of a few hisses, one cat charging the other, or a few well-directed swats. Spats of aggression are likely before (hopefully) peace breaks out. It has been shown that the frequency of minor spats of aggression tends to decline over 4 months until it reaches baseline. However, peace is not a guaranteed outcome, even with careful engineering of cats' exposure to each other to prevent serious "meltdowns." Hostilities can and sometimes do escalate until the situation is untenable for the owner or one of the cats. That's when owners call in the veterinary "fire-engine service."
It's preferable to avoid the development of such entrenched negativity and the following program is designed to do just that:
The Goals To prevent, when possible (and within the bounds of a particular cat's personal limitations) serious altercations between cats when they are introduced.
To foster pleasant experiences for the cats so that they can build some positive history together.
The Program Bring the newcomer into a room that you have prepared as a comprehensive living area for a cat complete with a food bowl, water dish, cat toys, litter tray, cat bed, and climbing frame.
Spend quality time with your new arrival, petting her, offering food treats, and speaking softly.
Subsequently, spend some time with your resident cat giving her the same red carpet treatment.
At set times each day, refill the food bowls on either side of the closed door that separates the cats.
Spend alternate feeding times on different sides of the door, observing the cats' reactions to each other while separated by this barrier. The first step is to ensure that the two cats do not react aggressively toward each other by hissing or batting under the door. Rather, they should remain focused on the pleasurable work of eating or playing, and should show only mild curiosity with respect to the cat on the other side of the door.
If the cats will not come to the door at the prescribed times, or if they display any hostility to each other, the food bowls should be moved back to a distance at which all cats are comfortable and able to eat without distraction. It may sometimes be necessary to arrange to have the cats more hungry at feeding time by feeding slightly less food for a few days.
Change the new cat's environment every 24 to 36 hours so that it spends time in parts of the environment that have recently been occupied by the other cat(s). The other cat(s) should be confined to the area previously habited by the newcomer. This way the cats will have a chance for olfactory investigation of each others' scents without any risk of conflict. The sense of smell is very important to cats and is one of the ways they recognize each other.
If things are going well, crack the door an inch or two at feeding time allowing the cats to catch glimpses of each other. Ideally they should show interest but no aggression to each other at this level of exposure.
If there is no adverse reaction on the part of any of the cats at the "cracked door stage," further visual access can be permitted by way of a screen. Sometimes it is necessary to progress more slowly by scotch-taping newspaper to the screen to limit the visual access to a 4-inch slot (rather than using the full screen). If necessary, visual access can be increased incrementally.
Once the cats are acting non-chalantly across the screen it is time to progress to the next level – having them in the same room together.
Rather than risk losing all gains in one fell swoop, have the cats restrained on harnesses or in see-through crates. Position them on opposite side of the room initially and feed them in this situation. If the cats eat, this is a good sign.
Over hours or days the cats can be moved closer to each other and/or can be allowed to spend more time in each other' presence (still with physical restraint in place).
Then free one of the cats and note her response. A curious but friendly interest in the other cat is good news.
On the next feeding, free the other cat (the first one is now restrained). Make similar observations.
Finally, free both cats simultaneously and hope for the best (but be ready to intervene, if necessary).
If at any stage of the proceedings there is a negative consequence, then simply return to the previous "safe" level of exposure and hang for a while, days if necessary, until the cats have regained their composure and can be brought closer together once more.
Some people might consider this program to be overkill, but it minimizes all risk of an acute behavioral meltdown and prevents the establishment of any permanent malevolence. Also, if the cats are slated to be friends, it is possible to move through the program more quickly, progressing as fast as the cats' reaction to each other permits. True to the old maxim, it's better to be safe than sorry. Otherwise put: An ounce of caution can save an awful lot of grief - and an awful lot of behaviorist's bills!