By nature cats are solitary animals. They are also very adaptable animals, and as a result they can adapt to life in groups and tolerate the close proximity of other cats, but that does not mean that they particularly like it. When cats fall out, the underlying cause is generally a range dispute, because cats are very territorial. If one cat moves into territory which another cat has claimed for itself some kind of confrontation is almost inevitable. Since cats like to keep their ears and fur intact, the first responseis generally non-physical intimidation. Expect a lot of hissing, growling and aggressive posing. In many cases that is enough to end the dispute, and one cat walks away. The cat that stands its ground wins.
Not all turf wars end that amicably. If two cats have their hearts set on the same chunk of territory, fur is going to fly. With single cats, a bout or two is generally enough to settle the point beyond dispute, and after the initial turmoil peace is restored. (Remember, if there has been a physical fight, check your cat/cats for any bites. These can develop into nasty abscesses, often concealed beneath fur. If your cat has a tender spot or swelling it is time for a check-up with the vet.)
Because a losing cat's natural response it to get out of the area at high speed, it can be seen that the issue is more complex in a multicat household, where the humans have decreed that the cats are stuck with each other. And territorial disputes between house-sharing cats will happen. The extent of the damage will largely depend on you and how well you handle the initial introductions. It is a cardinal sin to bring a new cat home and drop her straight into the household expecting love at first sight. This virtually guarantees disaster and months of border warfare.
So it is important to follow the initial step of slow introduction (details in our article on 'Bringing the new cat home'). But even with the best will in the world, chances are high that territorial disputes will flare up. Territorial aggression doesn't have to be a yowling, fur-flying spectacular. It may be so subtle that you don't notice it at first. For example, let say your cats are using the same litter tray and there is a dominant male in the pack. This male may block access to the litter tray by sitting in front of it or quietly stepping in front of a cat that is heading for the litter tray. The other cat may well start urinating in odd places of the house to avoid a confrontation. You can lessen the bully's chances of succeeding with this behaviour by providing multiple litter trays for the inmates. Do the same with food bowls, so that the less dominant cats are not deprived of their food.
The aggressor cat will not necessarily be the oldest cat or the longest resident. Indeed, most of the time it is the newcomer who has to muscle her way into the existing household/outdoor structure. In our area there are numerous outdoor cats and when our previous cat got older and slower, she preferred to spend most of her time enjoying the comfort of the sofa. As a result our garden became a battleground between two rival cats. When our old cat died, we got a young muscular moggie. I was interested to see how she would fit in. The rival cats spotted the newcomer and were ready to defend what they thought was their territory. However, by watching the areas that her cospecifics marked out (that's us - from the cat's point of view marking our territory by gardening, weeding etc.) our cat formed a rough idea of what she felt was 'hers'. The first couple of times we stayed with her in the garden, but once she got familiar with the surroundings we left her to it, watching her through the window. In the first month there were lots of hostile encounters, hissing and growling. We only intervened a couple of times when it seemed that a physical encounter was inevitable. After about two months our cat had won her territory. Now a couple years later, we never see any other cats in our garden.
We have included this to show that by and large the cats will sort out their own territorial disputess with minimal intervention. Serious problems usually occur with two dominant cats living close to each other, or even in the same house, confined to an indoor environment. If they cannot sort out a territorial heirarchy the cats will fight. Because there is nowhere for the loser to back off to, once aggressive behavior starts it may escalate to full-scale warfare. You will need to stop fights as soon as they develop. Use a water pistol or loud bang to distract the sparring partners. Never try to separate the cats physically since they are likely to join forces and turn against you. Once the initial fight is over, put the cats in separate rooms till they calm down. Punishment will only elicit further aggression, and keep adrenaline levels high all round. If tensions remain hight for a prolonged period, talk to the vet who may prescribe some calming medication.
If the cats have not been spayed or castrated yet, and peace and harmony is a priority, get it done ASAP. Neutered cats have much more laid-back characters. But sometimes you just have to accept that despite all your efforts some cats will just not get on. In that case you have two choice: if you have enough space, separate them permanently; if that is not an option, you may have to resign yourself to re-homing one of the cats. Remember the first in last out rule.
Sudden hostility may also develop between cats, which, until then, had been getting along amicably together. When this happens the first thing is to take the cats to the vet to check for health problems. If both cats are healthy, then the increased weakness of one has not changed the balance of power, and some other trigger is to blame. For example, it has happened that when a cat has returned from a sojourn at a veterinary clinic, her housemate become aggressive. This, again, is territorial aggression. Cats are very quick to redefine territory as 'theirs' once another cat has gone off-site, and the return means reluctantly having to share living space again. For the same reason, moving to a new house may be very disruptive. The cats lose their territory and have to re-establish the household hierarchy all over again. Under those conditions, if cats become aggressive towards each other, split them up and re-introduce them slowly, as you would do if bringing a newcomer into the household.
There are many factors that determine how cats get along with one another, and even animal-behavior experts don't fully understand all of them. We do know that cats, which have a lot of experience in being around other animals in general are more likely to be sociable to other cats than cats who were brought up on their own. Also certain breeds of cats are better at adjusting to life in indoor multicat households. Nevertheless, cats are by nature solitary animals and when pairing cats there is always a risk that they just will not get on.