Okay, I know this is anecdotal, but I just brought one of my two cats home from spending two days at the veterinary hospital. The two cats are from the same litter and have never been seperated before. When I brought the cat home, his brother proceeded to follow him around the apartment, aprehensively, and hissed and growled at him as if he had never seen him before. I didn’t think this cat even knew how to hiss before yesterday. Our sick cat has been back for 24 hours now, and his brother is still hissing at him whenever he comes near. So, my only hypothesis is that cats put an awful lot of trust in their sense of smell in individual recognition, and my sick cat smells different from being at the vet, so he isn’t recognized. Is this consistent with what people know on this topic? Is anything known on this topic?
Animals can recognize each other in several ways, and if they have the time they’ll use them all. My cat absolutely freaks out when a stranger comes to our house - she’s very shy. First she hears them coming to the front door which is her first red flag - we always use the back door. Then she hears an unfamilair voice, which usually sends her running. If she happens to be asleep and they get in before she wakes up, she sees them across the room and is gone. Smell would be the last sense she uses.
Cats tend to get into moods like people, and when something pisses them off, it can last a while and it’s obvious. I’m pretty sure your cat knows exactly who his brother is, but doesn’t like something about the situation… either the smell of other animals or medication, even the “sick smell” on his brother, a differing amount of attention paid to him, or even the fact that he was just gone for a couple days. My cat gets mad at me when I’m gone for a while even though she immediatly recognizes me when I get back. Wait a week and everything should be back to normal. His brother knows it’s him, but probably is just a little freaked out about the change in proceedure or a slight difference in smell - he might just be thinking “WTF?”