Whether you've startled her by coming around a corner too quickly or frustrated her during play, your kitty's puffed-up tail is her way of displaying mixed emotions, including fear and aggressiveness. Determining whether she's just playing around or really upset can help you decide what to do about it.
A puffed-up tail typically accompanies fur fluffing on the rest of the body, and may also come along with an arched back and a hiss or two. When your fuzzball suddenly fluffs up with her tail high in the air, she's displaying a natural reaction to a surge of adrenaline through her system. Because she has just been startled or frightened, her body isn't sure whether to get defensive or take an aggressive position. By making herself look bigger, she's prepared to scare off a potential enemy, such as another cat invading her territory.
If your cat lowers her fluffed tail straight out or angles it downward, her attitude has likely moved into aggression instead of just playful defensiveness or indecisiveness. You're more likely to see her display this type of tail puffing when she is confronted by a strange cat or another new animal in the neighborhood, and she may go on the attack soon afterward.
If your cat is only puffing her tail because she has been startled, you can safely ignore it and let her calm down on her own. If she is becoming aggressive because of another animal or something else in her environment, separating her from the irritant can help ease her emotions. In some cases, this is as simple as closing the curtains so she can't see animals in the yard. If the object of her aggression is another pet in the household, keep the animals in separate rooms and slowly introduce them during supervised sessions instead of forcing them to interact all the time. Giving your cat a safe place to retreat to, such as a high cat perch or kitty condo, can make her feel safer and reduce aggressive incidents between household pets.
Kittens playing together often enjoy startling each other, and a kitten playing alone may even purposefully put herself in situations that lead to a puffed-up tail. Just as humans sometimes enjoy playing games that end with a sudden fright, kittens may also enjoy experiencing the adrenaline surge that occurs when something unexpected happens during play. If she acts playful after frightening herself into puffy-tailed excitement, you might want to try doing the same thing again.
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