It's impossible to estimate how well any particular pair or group of cats will ultimately tolerate each other. Some cats are unusually territorial, may never adjust to sharing their house, and may do best in a one-cat family.
However, many aggressive problems between cats can be successfully resolved. To do this, you may need help, both from your veterinarian and from an animal behavior specialist who is knowledgeable in cat behavior. Cats with aggression problems may never be best friends, but can often learn to mutually tolerate each other with a minimum of conflict. Working with aggression problems between family cats will take time and commitment from you. Don't give up without consulting the appropriate experts.
Cats are very territorial, much more so than dogs. Territorial aggression occurs when a cat feels that his territory has been invaded by an intruder. Depending on where your cat spends his time, he may view your whole neighborhood as his territory. Female cats can be just as territorial as males.
The behavior patterns in this type of aggression include chasing and ambushing the intruder, as well as hissing and swatting when contact occurs. Territorial problems often occur when a new cat is brought into a household, when a young kitten reaches maturity, or when a cat encounters neighborhood cats outside. It's not uncommon for a cat to be territorially aggressive toward one cat in a family, and friendly and tolerant to another.
Adult male cats normally tend to threaten, and sometimes fight with, other males. These behaviors can occur as sexual challenges over a female, or to achieve a relatively high position in the cats' loosely organized social dominance hierarchy.
This type of aggression involves much ritualized body posturing, stalking, staring, yowling and howling. Attacks are usually avoided if one cat backs down and walks away. If an attack occurs, the attacker will usually jump forward, directing a bite to the nape of the neck, while the opponent falls to the ground on his back and attempts to bite and scratch the attacker's belly with his hind legs. The cats may roll around biting and screaming, suddenly stop, resume posturing, fight again or walk away. Cats don't usually severely injure one another this way, but you should always check for puncture wounds which are prone to infection. Intact males are much more likely to fight in this way than are neutered males.
Defensive aggression occurs when a cat is attempting to protect himself from an attack he believes he cannot escape. This can occur in response to punishment or the threat of punishment from a person, an attack or attempted attack from another cat, or any time he feels threatened or afraid.
Defensive postures include crouching with the legs pulled in under the body, laying the ears back, tucking the tail, and possibly rolling slightly to the side. This is not the same as the submissive postures dogs show because it's not intended to turn off an attack from another cat. Continuing to approach a cat who is in this posture is likely to precipitate an attack.
This type of aggression is directed toward another animal who didn't initially provoke the behavior. For example, a household cat sitting in the window may see an outdoor cat walk across the front yard. Because he can't attack the outdoor cat, he may instead turn and attack the other family cat who is sitting next to him in the window. Redirected aggression can be either offensive or defensive in nature.
Because their social organization is somewhat flexible, some cats are relatively tolerant of sharing their house and territory with multiple cats. It's not uncommon for a cat to tolerate some cats, but not get along with others in the house. However, the more cats sharing the same territory, the more likely it is that some of your cats will begin fighting with each other.
When you introduce cats to each other, one of them may send "play" signals which can be misinterpreted by the other cat. If those signals are interpreted as aggression by one of the cats, then you should handle the situation as aggressive.
The factors that determine how well cats will get along together are not fully understood. Cats that are well-socialized (they had pleasant experiences with other cats during kittenhood) will likely be more sociable than those that haven't been around many other cats. On the other hand, street cats that are in the habit of fighting with other cats in order to defend their territory and food resources, may not do well in a multi-cat household. Genetic factors also influence a cat's temperament. Friendly parents are more likely to produce friendly offspring.
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