Many years ago when we got our younger cat Troi, we spent weeks listening to the older cat Alia growl as they slowly got acquainted. If you’ve never heard a cat growl, it’s somewhat different than a dog. It’s often sort of a low grrrrr, rather than a full growl. I can report that over time, it becomes truly tiresome to hear.
This past holiday weekend, my sister brought her two cats with her when she came to visit. They lived in the guest bedroom separate from my two cats and the dogs. About two days after the guest cats arrived, apparently Alia noticed and started growling. She was growling at Troi, which makes me wonder a) is Alia taking out her annoyance about the other cats on Troi? b) is Alia even dumber than I thought and thinks Troi IS the other cats or something?
Most people realize that growling isn’t good, but in the hierarchy of feline noises, things like hissing or spitting are generally worse. In the feline world, growling is a warning that means "look out, I’m annoyed." In this case, the cause of all the growling over the weekend was actually probably because of "non-recognition aggression."
If you have more than one cat, you may have experienced the odd situation that occurs when you bring one cat home from the vet and the second cat acts like she has never seen her feline housemate before. One theory is that the cat coming home smells different. That would also explain Alia’s growling. The house smelled like "other cat" so Alia figured it must be Troi. (Okay, she was wrong, but she’s also not a rocket scientist by any stretch of the imagination.)
Depending on the personalities of the cats involved, non-recognition aggression can be a serious problem. The cats may fight and hurt each other. Plus, if the cat returning home from the vet is still groggy from anesthesia, she’s at a disadvantage and could be seriously injured.
If you hear growling or other aggressive behavior, err on the side of caution. Keep the two cats separate behind closed doors, so they can’t get to each other. Ideally, you should put the aggressive cat in a separate room with a litter box and other necessities until he settles down. This process could take hours, so you may want to plan on leaving him there overnight.
When you go to reintroduce the cats, do it carefully as if they really were meeting each other for the first time. And realize that the problem will probably happen again, so plan on keeping the felines separate for a while after any trips to the vet or other journeys to lands that are foreign to your felines.