My inside babies are 2 years old. They are littermates and both female. I have never seen any aggression between them. They are inside only and have access to a large enclosed verandah. Tonight I heard cats growling. They were both down the very end of the verandah and it was dark and they are both black so I couldn't see anything. When I turned the light on it was my cats fighting with each other. I broke them up and then they both went to the netting and we're looking out growling with their tails all puffed up. I think/ hope what happened was another neighbourhood cat was in our yard stirring them up and in the dark they got confused and attacked each other. They haven't snuggled up together since but they ate together. I would love to know what you guys think please. There is another cat that hangs around and stirs them up. In fact I had covered over the flap that lets them out to their enclosure - they had escaped anyways. The flap is well and truely blocked now. Thanks for reading.