My former neighbor and I were discussing tummy sacks once (total cats between us at the time - 11) and decided they must be hereditary. Many of her cats and one of mine were all related, and they all had tummy sacks, from the time they were kittens, the males and the females. My kitties now are pretty tubby, but none of them have pronounced tummy sacks. Have y'all seen the cat food commercial with the gray kitty with his swinging tummy sack, with the Carole King song "I Feel the Earth Move Under my Feet" playing?