Here's a very interesting post I found off Otterman's blog about the Ernest Hemingway Museum and the Hemingway Cats. Interesting that most of the cats are polydactal.
Even more interesting is that they sterilise the vast majority of the cats but leave some breeding pairs so that the number always stays around 60 (and are the descendants of Hemingway's original cats) . This is a good case for people who ask what will happen if you sterilise till there are no more cats left - you leave just enough unsterilised so that they reproduce to replace the ones that die but not enough to overpopulate the area.
Note this is obviously a small area which is very self-contained - this does not apply to Singapore yet and probably won't be the case here for many, many years.