David Attenborough looks at why mammals are the most successful creatures on the planet. Mammals have adapted to live almost anywhere - from freezing polar regions, to the hottest deserts and from steaming jungles, to the world's vast oceans. They survive on a great variety of different foods and it's what they eat that so often determines their behaviour - and that of course, includes our own.

In Australia, there are two mammals that still lay eggs, one being the bizarre-looking platypus. Before now no one had ever seen inside their breeding burrows but using the latest optical probes, David is able to watch, for the very first time, a platypus mother with her newly hatched baby and sees it feeding on that other uniquely mammalian substance- milk.

Most Australian mammals give birth to tiny, embryo-like babies, which crawl into the safety of their mother's pouch, where attached to a rich milk supply they complete their growth. These are the marsupials - the kangaroos and wombats, possums and numbats, rock-hopping wallabies and more, and they feed on everything from termites and nectar, to the most noxious eucalyptus leaves that would quickly kill any other animal except a koala! Grey kangaroos might be renowned for their hopping speed but big males are also the kick-boxing champions of the world.

There are also a few marsupials in South America, such as the strange yapok which catches fish in the dark purely by feel. But a different group of mammals, whose babies develop inside the womb and are nurtured through a remarkable organ - the placenta - has come to dominate the rest of the world. Show less