David Attenborough makes a world-wide journey of discovery in search of fascinating mammals to illustrate why they are so incredibly successful and diverse. Watching an arctic fox hunting at 20 degrees below, he observes... 'The only reason that it and I don't freeze solid up here is that we are both mammals and have the ability to use our food to heat our bodies - we're warm blooded. The fox also has more of that other mammalian characteristic, hair - its body is insulated by fur'. In Australia David watches echidnas and platypus, bizarre mammals that share a remarkable link with the reptiles... they still lay eggs! No one had ever seen what happens inside a platypus' breeding burrow before but using the latest optical probe, David catches the astonishing sight of a newly hatched platypus baby with its mother - and it's feeding on that other uniquely mammalian substance - milk. Most of Australia's mammals give birth to tiny, under-developed babies, which crawl into the safety of a pouch and attach to a rich supply of milk to complete their growth. These are the marsupials and they thrive in an amazing variety of forms, from koalas in the trees and wombats in the snow, to red kangaroos in the desert and rock wallabies on the cliffs. Grey kangaroos might be renowned for their hopping speed but big males are also the kick-boxing champions of the animal world! Marsupials are also found in Central and South America - mostly possums living up in the trees - although one, the yapok, is uniquely adapted to a watery lifestyle. Rarely observed in their natural environment, our infra-red cameras record how these strange mammals catch fish in the pitch dark, using only their front paws and whiskers to feel for their prey. And when a mother yapok dives underwater, her baby is saved from drowning by a waterproof pouch! A different kind of mammal, to which we ourselves belong, has come to dominate the rest of the world. Their babies, developing inside the womb, are nurtured through a remarkable organ - the placenta. Giving birth to well-grown babies might be harder on their mothers but does mean that the youngsters are able to look after themselves much sooner - such as running from predators! As David Attenborough concludes ' Whether mammals lay eggs, or give birth to live young. Whether their babies develop in a womb or a pouch, they have managed to live almost everywhere. The warm blooded, furry, mammalian body - in all its multitudinous variations - really is a winning design'.