A cat owner asked the above question on a well-known website (metafilter.com). I’d like to try and answer the question. The person has two five-year-old indoor cats who are siblings. One of them escaped the home and spent two days under the house before he was recovered. His brother now hisses and growls at him and the owner wants to fix the problem urgently.
Feline non-recognition aggression
The problem is described as “feline non-recognition aggression”. I’m sure lots of cat owners have experienced it. Inexplicably, a cat who was friendly towards another cat in the household becomes hostile. The likely event which could have caused this is because the ‘victim’ has lost his natural smell.
It is not the separation of the cats which caused one brother to become hostile to the other. It is the fact that the brother who remained in the house no longer recognises his sibling. He is suddenly confronted with a strange cat and becomes hostile towards him because domestic cats are generally territorial. Although domestic cats have become far more sociable over thousands of years of domestication.
Coat got dirty?
So, in this example, when the sibling was under the house for two days I’d suggest that he picked up some odors on his coat. Perhaps he rested on some dirty ground or items or perhaps there was something in the air which has become embedded in his coat.
Whatever happened, he no longer smells the same and therefore his brother can’t recognise him because domestic cats rely tremendously on their sense of smell to recognise both their owner and other cats. Sight also plays a part but if the nose does not recognise the ‘creature’ opposite him, the fact that he can recognise the cat or person by sight does not change the outcome. The nose takes priority.
Clean coat and let cat self-groom
In this instance, I would recommend that the owner gently wipes down the cat who escaped with a damp cloth. She or he should then keep the cats apart for a while if necessary during which the ‘victim’ cat will groom himself sufficiently to put his natural smell back on his fur. This shouldn’t take too long but it depends upon the cats.
The owner says that he has separated the cats for a few days. This should be more than enough time. Apparently it isn’t in this instance. Separating them may make the matter worse beyond a certain timeframe. I would reunite them because by now the “stranger” cat should have his natural scent back and be recognised. Keeping them physically separate may reinforce the mental separation.