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At the poles it is bitterly cold because the sun rays only hit the Earth obliquely. Here, glaciers grind across the land, gouge out deep valleys and flow down into the sea, crashing into the water. At the equator, where the sun hits the Earth full on, the land is baking hot. As the rainfall has gradually diminished here over the centuries, so the forests have dwindled and been replaced by grasslands and even desert. Throughout all these changes, creatures have evolved with a speed that has matched the changing landscape. In the hot deserts, some animals have evolved to survive without drinking any water whatsoever. Around the poles, on the other hand, creatures have evolved with the ability to generate their own internal heat, and grow insulating coats of fur and fat that prevent them freezing to death.