Wouldn't it be nice if all it took to introduce a new cat to your resident pet were a brief handshake and a couple of "HELLO, My Name Is...." nametags? Unfortunately, it's not quite that simple, which means you'll need to have some realistic expectations from the outset. What are realistic expectations? First, it's recognizing and accepting that your pets may never be best buddies but will usually come to at least tolerate each other. Second, it's understanding the need to move slowly during the introduction process to increase your chances for success.

Of course, some cats are more social than other cats. For example, an eight-year-old cat who has never been around other animals may never learn to share her territory (and her people) with other pets in the household. But an eight-week-old kitten separated from her mom and littermates for the first time might be glad to have a cat or dog companion.

Cats are territorial, and they need to be introduced to other animals very slowly so they can get used to each other before a face-to-face confrontation. Slow introductions help prevent fearful and aggressive problems from developing. Here are some guidelines to help make the introductions go smoothly:


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Confinement

Confine your new cat to one medium-sized room with her litter box, food, water, and a bed. Feed your resident pets and the newcomer on each side of the door to this room, so that they associate something enjoyable (eating!) with each other's smells. Don't 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 while standing directly on either side of the door.

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The Old Switcheroo

Swap the sleeping blankets or beds used by the cats so they each have a chance to become accustomed to the other's scent. You can even rub a towel on one animal and put it underneath the food dish of another animal. If there are more than two animals in the house, do the same for each 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.

Next, after the animals have been returned to their original designated parts of the house, use two doorstops to prop open the dividing door just enough to allow the animals to see each other, and repeat the whole process over a period of days—supervised, of course.

(more info at http://www.cats-tell.com)


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Slow and Steady Wins the Race

Avoid any interactions between your pets that result in either fearful or aggressive behavior. 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 a mild protest from either cat from time to time, but don't allow these behaviors to intensify. If either animal becomes fearful or aggressive, separate them, and start the introduction process once again with a series of very small, gradual steps, as outlined above.

PLEASE NOTE: When you introduce pets to each other, one of them may send "play" signals which can be misinterpreted by the other pet as signs of aggression. If that's the case, always handle the situation as "aggression" and seek professional help from a veterinarian or animal behaviorist right away.

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Precautionary Measures

If one of your pets has a medical problem or is injured, the introduction process might be stalled a bit. Check with your veterinarian to be sure all your pets are healthy. You'll also want to have at least one litter box per cat, and you'll probably need to clean all of the litter boxes more frequently. Make sure that none of the cats is being "ambushed" by another while trying to use the litter box, and be sure each cat has a safe hiding place.

Try to keep your resident pets' schedule close to what it was before the newcomer's arrival. Cats can make a lot of noise, pull each other's hair, and roll around quite dramatically without any injuries. If small spats do occur between your cats, you shouldn't attempt to intervene directly to separate the cats. Instead, make a loud noise, throw a pillow, or use a squirt bottle with water and vinegar to separate the cats. Give them a chance to calm down before re-introducing them to each other.

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(More at http://www.cats-tell.com)

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I hope this helps everyone!
Good Luck