Adopters should have realistic expectations when introducing a new pet to a resident pet. This process can take time and patience. Every cat is different and acclimates to other animals in his or her own way. For example, an eight-year-old cat that has never been around other animals may require a long acclimation period as she learns to share her territory with other pets. However, a kitten newly-separated from her mom and littermates might prefer to have a cat or dog companion. Cats are territorial and need to be introduced to other animals very slowly. Slow introductions help prevent behavior problems from developing.
Confine your new cat to one room with her the following items.
We recommend that the new cat be confined for a minimum of seven to ten days. Feed your resident pets and the newcomer on each side of the door to this room. This will help all of them to associate something enjoyable (eating!) with each other's smells. Do not put the food so close to the door that the animals are too upset by each other’s presence to eat. Gradually move the dishes closer to the door until your pets can eat calmly, directly on either side of the door.
Once both cats are comfortable with this, then the next step is to use two doorstops to prop open the door just enough to allow the animals to see each other, and repeat the whole process. If your resident cat is on a regular feeding schedule, don’t change that schedule. S/he will associate any negative changes with the arrival of the new cat and it will impact their relationship.
If your new arrival is a kitten, or if you are introducing a new cat to another pet, the bathroom is a great place to start. For kittens, it provides safety. There is the least amount of wires to chew on or get tangled in, sharp corners to injure, and small places to get stuck in (but don’t forget to keep the toilet lid down to prevent drowning). The smaller space will also comfort the smaller cat.
Take all of the items listed above and make sure they are placed in every available corner of base camp. Food dish should have ample space away from water; both dishes should be located in the furthest area from the litterbox. If you can provide a cat condo or tree, place it where the cat can get to a window. Scatter the toys around the room (although chances are that the cat will do that job for you!). Make sure to provide articles of clothing, blankets, or towels, anything that carries your and your family’s scent—to give the cat an immediate sense of belonging.
Spend as much time with the new cat as you can during the first crucial days of confinement. Among the toys, you should have at least one interactive toy (a toy in which you are attached to one end and the cat to the other), to play with while you spend quality time. This will set up a routine of play to dispel stress. Also, take this time to set up a trusting relationship. Talk to your cat. Don’t necessarily try to pick him or even pet him, if he is acting fearful. Give it time.
Don’t crowd the room with every family member either. Cats on edge have a heightened sense of their already keen fight/flight response. Give everyone a turn, but let all family members know that quality time can be playing, or simply sitting and reading the newspaper aloud in a soft voice. Be aware of the small things like how your legs might be blocking what the cat perceives as an “escape route.” Make yourself as small as possible when sitting on the floor with the cat, especially in a cramped base camp like the bathroom.
Switch sleeping blankets or beds between your new cat and your resident animals so they have a chance to become accustomed to each other's scent. Rub a towel on one animal and put it underneath the food dish of another animal.
Once your new cat is using her litter box and eating regularly while confined, let her have free time in the house while confining your other animals to the new cat’s room. This switch provides another way for the animals to experience each other's scents without a face-to-face meeting. It also allows the newcomer to become familiar with her new surroundings without being frightened by the other animals.
Avoid any interactions between your pets that result in either fearful or aggressive behaviour. If these responses are allowed to become a habit, they can be difficult to change. It's better to introduce your pets to each other so gradually that neither animal becomes afraid or aggressive. You can expect mild forms of these behaviors, but don't give them the opportunity to intensify. If either animal becomes fearful or aggressive, separate them, and start over with the introduction process with the same small, gradual steps outlined above.