This is an uncommon and subtle form of feline body language I believe. I’d expect many cat caregivers to never see it but it is a very noticeable part of my cat’s set of body language behaviours.
What is rather strange about it is that it looks uncomfortable. It can’t be that uncomfortable but it is less comfortable as far as I can tell than the usual curled up sleeping position. It does seem to be a statement or a signal to the ‘other’ (me) that he is slightly unhappy.
I concede that I might be misreading my cat’s emotions as it is easy to do this. Cat and dog caregivers can fall into the trap of projecting their emotions onto their companion animal through assessing the context in which the cat or dog behaviour occurs rather than focusing on the animal.
We know it can be hard to get into the head of cats and dogs. We have to guess what they are feeling. And we often treat them as little humans which encourages us to project our human emotions onto them.
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More on body language: Cat Body Language