Why do hedgehogs roll up?

Hedgehogs are able to roll into a ball because of all of their extra skin and powerful back muscles.

When they feel theratened, they contract their back muscles, which causes them to curl into a ball. Because they have all that extra skin, they can easily roll up. The shape of their body allows them to roll up without hurting themselves as well.

The quills don't hurt them, because the quills are only on their back. Their underbellies are covered in very soft fur; when they roll into a ball, the quills protect them on the outside of it and none of them are tucked into the inner portions of the ball the hedgehogs make.