I have both males and females that will repeatedly thump each other. Just last night one of my females got jealous of a male laying next to me so she hauled off and "thwacked" him multiple times over the head(lightning speed too) but he got in one good thwack of his own that left her with a bloody scratch down her cute little pink nose. Serves her right! Sometimes even i get "thwacked" if i have dared to pick up a kitty that just didn't want to be picked up. To me it's just like siblings pushing and punching each other, but i do have to sometimes break up some knock down drag out hissing, spiting, growling, shrieking, nail extended fur raised cat fights. The older kittens will play fight each other although sometimes one will get carried away and the other will yelp and run off. But mostly we get thumping and quick succession thwacking if someone is irritated.
Mikala