Here's what the Hemingway museum says about the cats:
Ernest Hemingway was given a white six-toed cat by a ship's captain and some of the cats who live on the museum grounds are descendants of that original cat, named Snowball. Key West is a small island and it is possible that many of the cats on the island are related. The polydactyl cats are not a particular breed. The trait can appear in any breed, Calico’s, Tabbies, Tortoise Shell. White, Black, etc. They vary in shapes, sizes, colors and personalities.
Routine procedures such as ear mite treatment, flea spraying, and worming are performed here at the museum by our Veterinarian, Dr. Edie Clark. Dr Clark also administers annual vaccinations, and performs routine animal health maintenance."
We looked, but didn't see any litter boxes or cat litter anywhere;) The cats live outdoors, and have colorful cat sized houses built for them on the property to protect them from the elements. The cats were a highlight for us, and as you can see, we made some friends! The cat on the chair in the picture above spent a lot of time with us... We highly recommend visiting these unique cats if you are ever in Key West.