The fourth episode examines rodents, which are characterised by strong, sharp, continuously growing incisors. These enable the animals to eat food that others find impossible, such as nuts or wood, and have enabled them to become the most successful and numerous of all mammals. Attenborough visits the forests of Virginia, where the grey squirrels are able to differentiate between the acorns of the red oak and the white oak: eating the latter and storing the former. Seed-eaters can live almost anywhere, and the desert-dwelling kangaroo rat uses its cheek pouches to transport its supply back to its burrow. A family of beavers is shown in Wyoming. Their construction skills have enabled the building of a dam, which has given them a lake so they can safely swim and forage in the nearby woodland. Infrared cameras are installed in their lodge during winter and a pair of muskrats are revealed to be sharing it. Many rodents are nocturnal, and a porcupine is shown warning off a young leopard. The naked mole rat is a burrower that, like bees and ants but unlike any other mammal, lives colonially with castes of individuals. Rats and mice are the largest group of rodents, comprising some 1,300 species. They reproduce rapidly: a female house mouse can become pregnant at five weeks old, and a plague of the creatures is shown exploiting a grain store. The world's largest rodent is the capybara, a semi-aquatic animal from South America.
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