Hedgehogs look a bit like small porcupines, but they're not related. Porcupines have hollow quills covering their bodies to discourage predators. Hedgehogs have solid spines -- up to 8,000 per hedgehog -- for the same reason. The loose skin on a hedgehog's back helps him to control when his spines stick out from his body and when they relax.
Letting Spines Lie
When relaxed and loose, hedgehog skin helps his spines lay flat against his body, all pointed in the same direction -- behind the hedgehog. They bounce and move a bit as the hedgehog walks, but they don't stick out to get caught on grass or brush. Although items such as fruit, grass and twigs occasionally get tangled in the spines, the hedgehog doesn't use the spines to transport materials; it's just an accident.
Sticking Them Out
A hedgehog's skin isn't always loose. When he feels threatened, he contracts his stomach muscles and curls up in a ball. This stretches the skin along his back, making his spines stick out sharply, ready to poke an approaching predator. Although the spines aren't barbed like a porcupines, they still deliver quite a stick.
Covering Loose Ends
Not all of a hedgehog's body is covered with spines. His head, tail, belly and feet don't have spiny protection, and curling up in a ball is a way to add that protection. But it's more than simply hiding his head and tail. The extra skin gives him the room to tuck his head and tail completely under spine-covered skin, making it difficult for predators to find an entry point to uncurl the protected hedgehog.
The loose skin is helpful even before hedgehog babies are born. Delivering babies with spines could be a prickly process for the mothers, but the babies' loose skin fills with fluid, stretching to cover the spines until after the babies are delivered. This fluid evaporates quickly -- typically within a day or two -- allowing the spines to push through the skin. The skin stays loose to allow the spines to lie back and relax.