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For the most part, cats get along well with people and other cats. Sometimes they display their natural instinct to be alone, but who doesn't need some "me time" now and again? However, when a cat displays too much introvertive behavior, it can be a sign that something is seriously stressing her out.
Recognize signs of stress
Stress can be more obvious to detect in an extrovertive cat than in one who is normally shy. Some overall clues to look for include litterbox issues, territorial marking, excessive grooming, hiding, aggression toward people or other pets, loss of appetite and restlessness. In a shy, introvertive cat, stress can be displayed by sitting still for very long periods of time. These behaviors can also be a sign that your cat is ill, so take your cat to the vet if you notice them to rule out any serious heath issues.
Soothe your kitty's aggression
Is your cat displaying more aggression toward a particular person? He or she is likely the source of the stress. To help overcome this, have that person feed your cat and give her lots of treats throughout the day. Also, he or she should try talking to your cat in a softer, higher-pitched voice that soothes a cat's sensitive ears.
Relieve stress with fun
Perhaps the situation is more of a playful aggression that seems to go too far. This can be normal given a cat's heritage as a predator. Try sticking to a play schedule for daily fun that your cat can look forward to. Use fishing-pole-style toys that allow you to sit and wave the toy with your arm while your cat chases it until she's exhausted. Give her a challenge but also build confidence by letting her win a few times.
Keep the peace
A multi-cat household will usually have some tension, but there are ways you can help keep the peace between your furry friends. For starters, make sure each cat has her own litterbox, all in different locations. Every cat will appreciate this privacy. Also, make sure each cat has a separate food bowl. A cat with an alpha personality may eat more than her fair share if your cats use the same bowl.
By identifying the signs of stress in your cat, you can take the appropriate actions mentioned here to help alleviate it. It's no Hawaiian vacation, but it should help!