Why do some animals sleep through the winter?

Think of a cold winter morning, when the air nips at your nose and frost sparkles on the grass. Back in summer, the garden buzzed with beetles, and at dusk a little hedgehog snuffled under the bushes. Now the garden is still and quiet, and the hedgehog is nowhere to be seen.

Where did it go? Like bears and bats, the hedgehog sleeps away the cold months and stays out of sight until spring. Let's wonder together: why do some animals sleep through the winter?

Let's start with what a hedgehog eats: beetles, worms, slugs and caterpillars. When winter cold arrives, those little creatures crawl deep into the ground and keep still. The garden that was full of food in summer has almost nothing left to eat.

Cold brings a second problem. A hedgehog's body works like a little stove, turning food into warmth, and on a freezing night it needs lots of food to stay warm. Winter asks for more food just when there is less of it.

That is why, all through autumn, the hedgehog eats and eats and eats. Its body tucks the extra food away as a soft layer of fat under its skin. That fat is food saved for later, like a lunchbox packed inside its own body.

When the nights grow frosty, the hedgehog pushes dry leaves and grass into a heap under a hedge or a woodpile. It burrows inside and curls into a tight, prickly ball. The thick leaves keep out the wind, the rain and the snow.

Then the hedgehog goes into hibernation, which means a long winter rest that is much deeper than your sleep at night. Hibernation lets the hedgehog wait out the hungry months without going out to look for food. What do you think happens to its heartbeat while it rests so deeply?

Its heart slows right down, from almost two hundred beats a minute to about twenty. Its breathing slows too, and its body grows nearly as cold as the leaves around it. A slowed-down body stops working hard to stay warm, so during hibernation it burns only a tiny bit of fat each day.

Because it uses so little, the fat it stored in autumn lasts all winter long. It stays curled up in hibernation until spring brings the beetles and worms back.

Winter cold hides the hedgehog's food and makes staying warm hard work, so the hedgehog stores fat all autumn and then goes into hibernation. In hibernation its body slows right down and lives on that stored fat until spring.

Under the hedge, the heap of leaves sits still in the frost. Deep inside, a small prickly ball breathes slowly and waits for the beetles and worms of spring.