My two cats are 15 year old litter sisters, extremely alike in looks, apart from the fact that one has always been a little overweight and the other a little underweight, but both otherwise healthy. I got them 7 years ago from a rescue group, badly suffering from mange and scared of their own shadow. I got the mange sorted fairly quickly, and within a few months, their confidence was heaps better. They enjoy life now, and have their own set of routines which they like to follow, but still have that curiosity to explore. In the last couple of months however, the underweight one's appetite has increased a bit, and I can see she's also put some weight on. In every other aspect though, it's business as usual. I'm not unduly concerned as such, I'm just curious why the change in appetite after all this time?