There's a snake in the grass on the set of ''Survivor: The Amazon'' -- and we're not talking about those anacondas. Creator Mark Burnett didn't decide until well after the 16 Survivors were cast that he would make the show a battle of the sexes. When the women learned of his decision, needless to say they felt blindsided. ''In that kind of harsh environment men have a definite physical advantage,'' says homemaker Janet Koth, 47, the first female castaway to be booted. ''Everybody can starve in similar ways, but when you're required to chop down trees...the only way it would have been a fair fight was to give us a chain saw.'' Burnett has no sympathy. ''A little girl with spirit and drive can do anything,'' he says. ''It's the classic Mars versus Venus. It's the way you would expect men to behave -- juvenile at times and very driven. And the women broke into awful catfights and at other times were very nurturing, washing each other's bodies in a sensual way.'' Okay, now we understand what he was going for.