Not many cat owners put a harness on the cat but when they do they may discover that their cat’s behaviour changes quite radically. Their cat may become passive. He may rollover, lie down or crawl along the ground. The overriding reaction is one of passivity. Your cat becomes stationary and does not want to move, almost as if he is temporarily paralysed.
The answer must lie in the “kitten response”. To a cat, a harness appears to replicate being held in the mouth of their mother when being carried from one nest to another. We all know that when this happens the kitten becomes passive which assists the mother in transporting her kittens to safety.
What will happen is that if you are patient your cat is likely to gradually begin to behave normally. You will be able to walk with your cat in a harness within several minutes in my opinion. I can’t guarantee that this will happen but it is likely to happen. Also the next time you put the harness on your cat’s reaction will be less dramatic and he’ll eventually get used to it.
What may be happening is that part of the harness is applying pressure to the back of the cat’s neck. What I mean is that the harness may be applying pressure to the exact same point where a mother would grab hold of a kitten in her mouth, namely the scruff of the neck. This then gives a sensation to the cat of being held in the mouth of their mother.
Therefore, if this theory is correct, it may be wise to buy a harness which does not make contact with the neck if such a harness is available.
There is a rather unpleasant short video on this website of a kitten being “deactivated” by placing a clothes peg onto the back of his neck. This is exactly the same process, in my opinion, as placing a harness onto a cat although the harness is less effective which is why the cat will eventually start behaving normally.
There is also a page on this website about a coat that you can put onto a cat which calms him or her. It is called a “Thundershirt”. Once again I believe that the same principle is being applied.