Indoor Cats Fighting

Question
I have 3 male indoor cats that have been living together for 6 years.  They have all been fixed, but the two younger ones tend to pick on the older one.  They are not allowed outside except to occasionally sit on the front porch.  Every now and then one gets out and we don't see them, so they spend the night outside.  Yesterday the oldest cat got out.  I saw and let him back in the next morning.  Immediatly the other two started hissing and attacking him like he was a stranger.  I though that it would stop, but it is day two and still they are attacking him.  We've seperated them when we are gone, and watch them when we are home.  What is causing this behavior?  How can we make it stop?
Thanks,
Sommer

Answer
Sommer,

The reason the cats are behaving aggressively towards the cat that got outside may be because he smells like other animals or people that the other cat's come into contact with which means that he doesn't smell like himself to the other cats. The simplest and safest way to help resolve this issue is to perform a mini reintroduction and use Rescue Remedy to help keep everyone calm and comfortable. I'd suggest that you check out my previous answers on new cat introductions for further details about properly introducing a new cat into the household. For the moment it's best to isolate the cat that escaped to the great outdoors in a bedroom, large bathroom or spare bedroom for a few days to a week with all of the usual kitty amenities. This will minimize stress levels for all of the cats while they each become accustomed to the other's scent again. While Houdini kitty relaxes in a room of his own I'd suggest that you give everyone a homeopathic remedy called Bach's Rescue Remedy, it's a blend of flower essences designed to calm and reassure. I've used Rescue Remedy for years to treat cats with a variety of fear or anxiety based issues and I've found it to be gentle, safe and effective. You'll want to add 7-10 drops of Rescue Remedy into each clean bowl of fresh water each morning. While your escapee is in the room of his own I'd suggest taking one or more articles of dirty clothing from the laundry hamper before you wash them and completely rub the kitty who escaped with a dirty shirt or towel - the dirtier and sweatier the better because that will intensify your scent on the cat rather than the other cats being able to smell other cats he may have come into contact with. When you've rubbed your escape artist down daily with dirty clothing for several days to a week then I'd recommend gently rubbing some Rescue Remedy onto the outside of each cat's ear flaps (don't get it inside the ears, it stings) before reintroducing everyone to minimize the risks that things will get ugly. It's not a bad idea to have a special treat planned since eating always seems to make the cats associate something positive (food) with each other's presence which is a good thing. You may also want to keep a heavy towel or broom handy in case things do get nasty, you can toss a heavy towel over an aggressive cat and gently wrap it around him to minimize your risk of being bitten or scratched, a broom can be used to gently guide fighting cats away from each other to minimize the risk of injury to you, the cats, and other family members. If you have any further questions, concerns or you'd like me to clarify anything in this answer I'd be more than happy to help you out any way that I can.