You know, most of the time my girls are great friends, but sometimes they chase each other and wrestle; no big deal actually. Usually it's Lily who goes after Luna.

Tonight, I heard Luna hissing, and when I looked, I saw them on the floor, Lily on Luna's back with her arms around her, and her teeth were in the left side of her neck! Obviously Luna was in pain, because she hissed again, before she came free. She's obviously not hurt (she has a thick fur), but it looked scary what Lily did. And I think it's not the first time she did that, because I heard Luna hissing before, and sometimes I find fur on the floor.

What do you think about this? Is this neck biting thing still a normal behavior to test and train their skills, or is it a sign of aggression/domination?