This is the full social media question: “My boy cat just got neutered, and his sister, who he normally cuddles with, is hissing and growling at him? What’s going on?”
I have seen this kind of question quite a lot on social media. This form of feline behavior can be a bit disturbing for the human caregiver. They can foresee a long period of antagonism between cats who were friends.
They need not be concerned as this sudden departure from normal behaviour will be short-lived.
From the sister’s perspective, her brother could no longer be identified on his return from the veterinary clinic. Cats identify or confirm the identity of other cats, even their human caregiver, through their body scent.
It is why your cat smells your hand or leg often. The brother’s body scent was temporarily masked by the smells from the clinic and the site of operation (the area of his testes).
The clinic staff would have handled him and used some sort of disinfectant and topical antiseptic perhaps. These all add up to the boy cat smelling strange. He became an alien to his once loving sister.
For he suddenly became an invading stranger which tells us how reliant on smell domestic cats are. She can see her brother but relies more on how he smells. And the smells told her that he was not her brother.
It is certain that within a few hours as the smells faded and after he washed himself, as best he could with the Elizabethan collar, she recognised him and returned to her loving self.
You get the same problem when you bathe a cat head to tail and remove their entire body scent. Affiliates in multi-cat homes might and probably will act antagonistically towards him until things settle down and they smell of themselves again after they’ve autogroomed.
It is a good reason why bathing a cat in a multi-cat home is inadvisable unless there is a pressing reason.
Why do domestic cats exchange scent with their human caregiver?
We wiped down my Sphynx for her whole life every few days and her sisters never changed their behavior toward her. She smelled from the wipes or like almonds from her self cleaning. They never ever had an adverse reaction to her. Unfortunately our bonded domestic cats have small altercations since she is gone. It is better when we use Feliway but it is distressing to us because it rarely happened prior to her demise.
This is a subtle process. It depends on a lot of things including the attitude of the other cats. And perhaps wiping down does not produce as strong a smell as from an op. We don’t what they put on the neutered cat. The basic theory is correct though as I have experienced it with my cat first hand.