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Recommendations for introducing kittens to adult cats covers a lot of good information about introducing new kittens to a cat household.

However, we've just adopted 2 older cats (11 and 13) into our household of cats ranging from 2 to 8 years of age.

Is there anything different that should be done for new adult cats?

This answer on a different question mentions:

It's usually recommended to confine new cats to a small room (like a bathroom) for the first couple of days anyway

I have to admit I'd never heard that before; we'd done it in the past when introducing kittens, but that was almost incidental, since the small rooms were the most convenient (and a single kitten fit fairly well in there).

We've given the new cats free access to the whole basement (the bathroom seemed too small, particularly if we were going to put in two sets of bedding, food, water, and a litter pan). Was this a mistake?

They seem to be adjusting very well so far, and have explored the basement thoroughly, as well as the door at the top of the stairs (sniffing at our other cats through the crack). Both of the new cats seem comfortable in the new environment, with one of them seeming 100% at home, and the other showing some minor skittishness (but he still can easily be coaxed into a lap for some petting and purring).

Does their age allow us to do the transition more quickly? Or is it more likely to cause territoriality/dominance issues with the other cats?

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The reason you confine cats to a small room initially is to not overwhelm them with new (new objects, new scents, new movement, etc). A smaller room has less new and so is less overwhelming. Ultimately, the choice of where to confine a new cat will depend on

  • number of new cats
  • if there are multiple cats, how well bonded they are
  • any potential health issues (a cat who is ill may be easier to care for on a hard surface floor)
  • previous behavior problems (a cat who has previously had litter problems should probably be started in the room you want to have a litter box in)
  • layout of home (the room should be in a quiet area, but there should still be some traffic so the cat(s) get used to the sound of your steps and existing pets)
  • contents of the room (a skittish cat should have a place to hide, my basement would be unsafe because of tools/sharp objects, etc).

Territorial/dominance issues are most likely when cats are whole (i.e. not desexed), and about the same age. Integration is usually relatively easy when cats are at least 5-7 years apart. That said, your new cats have just experienced a lot of upheaval in their lives, so I would be cautious and slow with introductions, and let your cats tell you how well it's going. If there's hissing/fighting, then continue isolation for another week or so. If everyone just kinda sniffs each other and ignores the other, then you should be fine.

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You can easily use the same rules as with a kitten, although a bathroom might be fairly small for an adult cat for too long.

I use a spare room that doesn't have a bed. While being under things is comforting, I would rather be able to get access to a nervous cat. There is a closet where the cat can 'hide' but I can pet him. I put food and water in one corner, litter box in another and let them decompress for a few hours.

Then you start splitting time, staying with the new cat some until they seem comfortable in the room, and reassuring the old cats in the other times. After a while (hours to days) you can start the process of introductions, but the new cat at least has a base station to return to and his own private litterbox.

The only problem with a bigger space like a basement is that you might end up with the new cats just staying down there all the time rather than blending in and eventually merging with the others. As for the kitten versus adult, it often is a wash. A kitten is given more slack and is less 'dominant' than an adult, but on the other hand is often a pest because of the high energy. Adults might hiss at each other more at first, but usually divide up the house spaces fairly well in the end.

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One interesting thing I noticed in my cat rescue days is how well two strange cats will get along is based on how much of a threat the new cat is perceived as.

This follows a fairly intuitive model that often applies to introducing humans as well:

  • Older cats are threatened by new younger cats.
  • Cats are more threatened by same sex cats.
  • Kittens, sickly, small, and injured cats are less threatening.
  • Aggressive or meaner cats are more threatening.
  • Of the existing cats, the one newest to the household is often the one most threatened by a new cat.

I rescued a really badly mauled cat that had to have its leg amputated. After the cat healed and the normal "keep them in separate rooms and let them get used to each other's scent" procedure I introduced the new cat.

There was very little fuss made over the 6 pound three-legged cat, presumably because she wasn't perceived as a threat at all. What was really remarkable was my very aggressive and grumpy 16 year old queen, who hates all other cats, immediately took a liking to her and would clean her head and curl up and sleep with her.

Some authors say cats can be cliquish and prefer certain cats to other cats for no discernible reason. I have noticed this as well. Even in litters that grow up together you will notice that a cat may have a "buddy" or two then there may be a sibling they don't care for much.

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I used desensitization to introduce a 9 month old to my 5 year old.

Cats rely heavily on their sense of smell and so this takes the form of progressive introduction through "site swapping" allowing them both to get used to the other's smell while not letting them directly interact. After the new cat got used to being in the smaller room, I would open and close different parts of the home to allow the new cat to explore and the older cat to smell the new cat.

Another recommendation is to feed them on opposite sides of a door so that they get positive reinforcement. You want to gradually move the plates closer together over time, they can hear and smell each other so they know the other is there. Another key is to make sure all the animals are getting attention and played with. The play is important to get any aggression out.

The next stage was to put in a screen door so that they could see and smell each other but not fight. Then they were introduced without the door for progressively longer periods of time.

It is not always a smooth process and the amount of time used for each stage depends on the age and sex of the cat. The general recommendation is to go about a week through a solid door, a week site swapping, a week with a screen. It depends on what you notice in your particular cats. If things start to get heated, go a step back and start again (for example, I originally skipped the screen door, but I had to go back and install it and it made a world of difference).

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