Fifty-seven million is a large number, right? Especially when it applies to the estimated kangaroo population of Australia. That's an average of just under three 'roos to every human there. So, to the extent that it is accurate, if you're a farmer, how do you scare off a bunch of the critters before they devour your crops and beat your livestock to the water supply? Well, not with the current tactic of choice: taped, high-pitched - but artificial - squealing noises , says a new report by scientists at the University of Melbourne. Kangaroos, it maintains, grow used to those and then tend to ignore them. Instead, the report says, the thing to do is play tapes of a 'roo stomping one of its large, powerful feet. That sound, according to researcher Helena Bender, is what the marsupial makes when it senses danger and flees. No word on whether such tapes are commercially available. Or on how to get close enough at just the right time to record them yourself.