The first episode gives a general overview of mammals before moving on to monotremes and marsupials. Attenborough begins in the high Arctic, where he contrasts the Arctic fox's ability to live there all the time (thanks to its dense coat of fur) with his own need for protective clothing, despite them both being mammals. From there, he travels to Australia to illustrate the evolution of the species with the help of the echidna and the platypus. Both creatures, unlike all other mammals, lay eggs — similar to birds and reptiles — and have been around for 100 million years. With an optical probe, the inside of a platypus nest is able to be shown for the first time. The defining characteristic of a marsupial is its pouch, inside which its young develop, having been born externally. Kangaroos and koalas are two examples that inhabit a warm environment, while the wombat demonstrates its ability to withstand a cold climate. Red kangaroos, in particular, are more at home in arid, desert-like conditions, while their grey cousins are sociable and prefer more temperate climes. The mammalian tongue is very adaptable, and those of numbats and honey possums have become greatly extended to enable the gathering of insects and nectar respectively. However, the most successful group of mammals are the placentals. Attenborough witnesses a wildebeest being born and explains both the dangers and advantages of this way of reproduction.
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