Introducing cats should be a slow process. In fact, the slower the process, the better chance of a successful introduction. "Slow" means at the pace of the cat who is showing the most signs of stress. This may be the new cat and/or the resident cat(s). This introduction may be a matter of days, weeks or months—it's up to the cat. It may be tempting to rush the process, but patience will make everyone in the household (people and cats) much happier in the long run.
Separate the cats
Your new cat should be kept in a single room during the initial stages of this introduction process. This is important for both the new cat (so they can begin to feel comfortable in their new territory) and for the resident cat (so they can adjust to the new cat’s presence in the home). To set everyone up for success, ideally the new cat is kept in a room that the resident cat doesn’t spend all that much time in. (This may not be possible in a small apartment.) For example, if your resident cat enjoys sleeping in your bedroom, ideally the new cat is kept in a room other than your bedroom.
Provide the new cat with a new litter box, food/water dishes, toys, scratching post and bed, blankets or towels to help them leave their “scent” on.
Do not move on to any further steps until both the new and the resident cat are showing calm and relaxed behavior. The new cat should be confidently exploring their room and showing social behavior with the people in the home. The resident cat should be acting as they always did prior to the new cat’s arrival.
Site Swapping
Carry the newcomer out of his room, put him in the bathroom, and shut the door.
Allow the resident cat to walk into the newcomer’s room, then shut that door.
Allow the newcomer to explore the rest of the home.
Do this process several times to give the new cat time to explore the whole house and feel comfortable.
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.
The “Other Side of the Door” Feeding Ritual:
This feeding ritual, which is all about creating a positive association between the newcomer and the resident cat.
Mealtime will consist of bowls set up on either side of a closed door. These bowls should start out far enough apart so the cats will walk up, eat, and walk away without incident, but close enough that they sense there’s another cat on the other side of the door. From there, we gradually move the bowls closer.
Allow the two cats to sniff each other under the door, and monitor for signs that either cat is uncomfortable. Since there is a door between them, you will be listening for any unhappy vocalizations, such as growling and hissing. If you see or hear either cat show these signs of stress, go back to the previous step.
Play near the door
If you don’t witness signs of upset during feedings, it’s time to initiate contact—but don’t open the door just yet. Use a string toy to encourage one cat to bat at it near the door. Use another string toy under the door to lure the other cat over. Allow the cats to bat at their respective strings, gradually bringing them closer together at the base of the door. Encourage the cats to play “patty cake” with each other’s paws beneath the door, if space allows. When both cats seem comfortable and show no signs of stress, move to the next stage.
Limited view
The cats should not be directly interacting at this stage. Instead they should be separated by a tall, sturdy baby gate (at least 36” high) in the doorway of the new cat’s room.
Cover the baby gate with a sheet so the cats can’t see each other. Raise the sheet for a moment or two until the cats see each other and toss them both a treat. Then immediately cover the baby gate with the sheet. Repeat this activity five to 10 times in a row a few times a day.
The key to this process is keeping the interactions very brief and positive. Don’t wait for one of the cats to show stress; instead end the interaction on a positive note before any signs of stress.
When you see positive indications that show the cats are developing comfort with each other, you can increase the amount of time the cats see each other as well as decrease the distance that you are tossing the treats.
Positive signs include:
Eating the treats in the presence of the other cat.
Playing with a toy in the presence of the other cat.
Ignoring each other and going about their own business on opposite sides of the baby gate.
Touching noses through the gate, playing footsies under the gate and/or rubbing their bodies against the gate.
When you see indications that one or more of the cats is stressed, go slower with the process. If one or more cats shows a particularly high level of stress, then go back a step.
Signs of stress include:
Consistent hissing or growling towards the other cat. A hiss here or there is not a concern, so long as the cat is otherwise showing some positive indications as well.
Distancing themselves so they don’t see the other. For the new cat, this may mean hiding in their room. For the resident cat, this may mean running to another room.
Stressed body language. In particular, look for a swishing tail, ears flattened back against the head or turned sideways, hair on their back is raised, crouching or slinking away.
Meeting in Person
Start by playing with the new cat in the room, make sure they are engaged, and keep them moving
Casually bring the other cat into the room and immediately engage them.
The most important component when you bring the cats together is establishing and maintaining a rhythm of play once they are in the room together.
Let the cats interact in person for 5-10 minute sessions, 3-5 times each day. If one of the cats appears overwhelmed or fearful, shorten their periods of interaction. If aggression rears its ugly head, separate the cats and revert to the previous step.
Even if all goes well during these brief sessions, keep the cats living in separate areas for a minimum of 5-7 days and supervise all interactions. Gradually extend the length of each session in 15 minute increments.
When the cats have repeatedly, over at least several days if not weeks, had positive or neutral interactions without showing signs of stress, they are ready for unsupervised time together.
