"I don't like you - go away!"
Introducing New Cats
Introducing a new cat into a household that already has one or more cats requires some time and effort on the owner's part. Initially, house the new cat in a separate room such as a spare bedroom with food, water, toys and a litter box. DO NOT house the new cat in a room favored by the original cat such as the owner's bedroom. The original cat will feel extremely stressed and in fact displaced if this occurs.
Have the new cat evaluated by your veterinarian prior to introducing the new cat to the original cat(s). Many parasites and infections are transmittable from one cat to another, and should be treated before having the cats directly contact each other.
After the veterinary evaluation, exchange bedding between cats and share cloths or an old piece of your clothing that has been rubbed on the fur or around the mouths of each cat. This helps them acclimate to each other's scent. Often the cats will start reaching under the door with their paws or scratch at the door. Reward this positive behavior with praise and treats. The door can then be propped slightly open so the cats can see but not reach each other. Continue to praise the cats when they interact calmly with each other.
The next step is putting one cat in a carrier with food, allowing the other cat to approach the carrier. Give the cat outside the carrier food as well. Feeding both increases the positive associations.
The next step is to allow supervised interactions where the cats are allowed contact. Praise and give special food rewards when they interact calmly. Gradually increase the supervised time together until they appear to be doing well consistently, then allow short periods of unsupervised time together. As long as no set backs occur, gradually increase the unsupervised time together as well. It usually takes longer to introduce older cats than younger cats, with 2-3 weeks being fairly typical.
Introducing Kittens to older Cats
Many older cats are tolerant of kittens up to a point, and then need a break. Have a "retreat room" that the kitten is not allowed in at first, such as the master bedroom if the older cat likes to sleep there. Once the kitten is older and less intent on energetic interactions, the retreat room may be opened to the kitten as well.Adopting two kittens, preferably from the same litter, tends to be much less stressful on the older cat than adopting one kitten alone. The two kittens will seek each other out to play the majority of the time, leaving the older cat to interact as much as it prefers.Make sure you give extra attention to the older cat during the integration process. Cute new kittens can be very demanding on your attention and the older cat may hide more and be less likely to seek out your attention. Giving extra food treats and continuing favorite play activities are very helpful at calming the older cat. In particular, make sure the older cat retains its favorite sleeping spot. Put out an extra litter box if necessary so the older cat has access to it without having to interact with the kitten.
Real Fighting Versus Play Fighting
If the ears on the cat are upright, that shows a lack of hostility.
If the ears on the cat are starting to lay back, that indicates a more aggressive demeanor.
If the cat has flattened ears, then the play fighting is escalating into real fighting.
Cats are definitely not playing if their ears are held flat and eyes are slitted.