Hello. I have two ten-week old kittens, and the grey tabby has been exhibiting the strangest behavior lately. He will put his mouth to the orange tabby's stomach and attempt to nurse on invisible nipples. The orange tabby just sits there and takes it and doesn't defend himself or anything, and the relationship is never in the reverse. This isn't an annoying behavior, but I don't know if its normal or if my kittens have a mental disability. Any thoughts?