Warning: These pictures not suitable for kittens under the age of 4 months!
These are both female cats. The tabby one is Rori, who just got spayed. She is alpha cat of the house. The grey one is our foster kitten Smokey, who is in heat. This is what happened when Rori got fed up with Smokey's yowling!
Obviously, they are going to be spending time in separate rooms now when the humans are not home, and Smokey will be in with me for sleeping until she gets spayed. I looked it up on the net, and it seems to be a common dominance behavior for females.