I have two male cats of over a year. They used to play well with each other, up to the age of 8 months, but slowly they became enemies and now they need to be kept separate. I've seen more than a dozen cats live peacefully, but why can't these two be peaceful?
Has anything changed in the home? Is there anything that could be making them stressed? Animals can turn on each other when stressed.
If you can't find an environmental cause then they may have just had a falling-out. Cats, like people, can be cliquish. In a clowder (a pack of cats) you see cats that are good friends, cats that get along OK, and cats that don't like each other at all.
Cat relationships change over time. If there are too many negative interactions a cat may start to hold a grudge against another cat and they may not tolerate each other at all, which seems to be where you are now. The cat that you see as fighting may be being picked on by the other cat and may just be defending itself. You would have to watch them closely to see what is going on exactly.
A common cause of cat conflict is jealousy. If one cat is getting more attention than another, or is attention is in short supply they may start resenting each other, and may hiss and fight. Cats can be sneaky and secretive about this so the behavior is difficult to see in its entirety.
Make sure you/everyone give both cats plenty of affection and praise.
If you want them to get a long again you need to repair their relationship. I have repaired a few. The key is for them to have positive memories associated with each other. Ways you can do this is to praise and pet them both a lot while they are around each other.
Also pet their heads and sides of their face then pet the other cat the same way, back and forth a few times. This transfers their scent to each other so they start to associate the other cat's smell with praise and affection. Give them treats when they are behaving around each other. This is positive reinforcement - the most powerful tool in behavior modification.
Feed them in each other's presence and pet them as they eat.
All of these things will help to build good memories of the other cat.
Your role should be parental here: stop any fights before they escalate. If one is picking on the other (prior to any fighting) shoo the aggressor away and scold him. Note: the one making all the noise or swiping may not be the aggressor.
You didn't write details, so I will present the most likely options:
- if the female is not neutered, it may still fight off males; neutered or not (will last until they both grow up fully at around 1,5 year), if the male is not neutered it might be attempting to mate with rgw female, neutered or not (or if you neutered one of them, but one attempted to mate pre-castration, the other will remember and will react negatively to the other for a long time)
- they ascertain territory (fighting will decrease with time)
- there is another problem which you did not observe, most often sensory in nature; lost sight or hearing often causes irritability in cats, and the loss might be big enough to cause it to act thus, but not enough that you will notice the issue.
In any case all of it is passing. Try to play with them both using stick-toys, etc. If within two weeks you do not achieve the desired effect of reconciliation, you might want to consider permanent separation as the problem is likely to be in psyche (some cats-rarely though- are psychically scarred, for example, after neutering and will not abide the presence of another cat).