Oh jeez I should'a looked here first, I just posted my own thread on my little nipple sucker.... Ya, my baby does this, but only when she is laying next to someone she knows well. I asked my vet and she said it was just the equivilant of a kid sucking their thumb. She said my kitty Isis may have developed the habit b/c her mom didnt nurse her enough (or didnt get to, I should say) so because she was weaned too early she picked up this kinda OCD habit... I wouldnt be too worried, but I check Isis' regularly, she only sucks at one nipple, so I just do a quick check to make sure it isnt inflamed, or irritated - have you checked the nipple yet? It may be that it does itch as Diane said above from a small cut, or minor infection... Good luck, hope your baby doesnt want to keep this strange habit like mine seems to want to!