The next instalment looks at herbivorous mammals. The sloth is a leaf-eater, but it has compensated for the lack of nutriment in its diet by doing less (its reactions are a quarter the speed of a human's). This doesn't apply to all herbivores, which rely on bacteria in their stomachs to digest the leaves' cellulose. Plants can be poisonous, but Brazilian tapirs — the largest inhabitant of the South American rainforest — deal with them by eating a little of each species and then supplementing it with kaolin. In East Africa, via infrared cameras, Attenborough observes a herd of elephants squeezing into a pitch black cave and gouging the walls with their tusks to mine salt for their diet. Grazing animals, such as caribou and wildebeest, must migrate at the onset of winter and make long journeys to find new pastures. Despite its spiny fortification, the acacia is favoured by antelope, elephants and giraffes, which all have adaptations to reach its leaves. Smaller grazers are always at risk from carnivores: so they have developed the means to detect and evade them, and do so more often than may be supposed. A herd of wildebeests is shown defending one of their number by charging the lions attacking it. However, the horns of antelope are primarily used for fighting each other to determine rank within their group and to maintain a breeding ground. Topi are shown doing so to the point where they are so exhausted that they easily succumb to a pack of hyenas.
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