Adaptation

Porcupines have sharp quills on their backs to protect themselves from predators. A porcupine can have over 30,000 quills, and each quill can be up to a foot long, with the length varying between different species. The quills normally lie flat against the porcupine's back, but when it senses danger they stick up, acting as a defense mechanism against potential predators.The porcupine can't actually shoot the quills, but if something tries to attack it, the barbed tips quills will stick in the attacker, easily coming off of the porcupine's skin. The porcupine can then grow new quills. A porcupine's quills help its survival because the removal of a quill from a predator's skin is very difficult and painful, so predators are discouraged from attacking the porcupine.

Information and picture: http://animals.nationalgeographic.com/animals/mammals/porcupine/