This is a good example of what I would describe as defensive cat body language. Of course, it normally takes place between adult cats who are in a stand-off. They might be trying to avoid a fight by forcing one or the other to walk away (very slowly!). It could also be described as defensive aggression.
In this instance a gorgeous, ginger tabby kitten is practising his defensive cat body language. We know that this happens a lot with kittens. You see it when they notice a cat shadow on a wall (their own shadow) . They think the shadow is another cat and they enter into this form of body language as a defensive measure against that mysterious intruder.
— Cats of Instagram (@catsofinstagram) January 19, 2019
Or kittens might do this in front of a mirror because they believe that the cat in the mirror is a hostile animal that they have to see off. They achieve this by presenting as large a cat as possible to the intruder. Therefore they turn sideways on and arched to enhance the height, their hair stands erect and the tail is erect as well.
To add to this range of movements this kitten jumps up and down sideways on to make him look even larger and more aggressive and dangerous.
It’s amusing to look at because this is all in play and practice and it’s fun for the cat. It is also fun for us because it makes a good video. There are many examples of this form of feline behaviour and body language on the Internet but this is one of the better examples, in my opinion.
A bit more detail
Dr Bradshaw says that “macho behaviour, laced with a generous portion of bluff, is therefore essential to their success.”
He’s referring to male cat behaviour of course. The only way that a male cat can be sure of leaving any descendants is to convince a female to accept him as their mate and to convince any rival males to back off. The defensive cat behaviour shown in this video is a way of forcing rival males to back off.
As domestic cats are very well armed with claws and teeth they try and avoid fighting unless it is unavoidable. They therefore resort to adopting the sort of postures you see in the video which are attempts to persuade the opposing cat that they are bigger than them and that they are bigger than they really are.
The cat’s profile is made as large as possible as shown in the video. In this instance, as mentioned, the cat also jumps in the air. Dr Bradshaw, states that a clue that a cat may be less than confident about winning a fight is when he pulls his ears towards the back of his head in order to protect them. Ears very vulnerable and are often damaged in cat fights. We see lots of street cats with damaged ears particularly on the most successful and dominant males.