The largest rodents in Africa, the African crested porcupines are covered in long quills up to 24 inches long which are erected when they are alarmed to make them appear large and formidable to predators.  The quills are actually modified hairs and are shed regularly.  Specially adapted quills at the end of the tail are hollow in the center and create a hiss-like rattle when vibrated together.  If these warnings do not deter a predator they will charge backwards attempting to drive the thicker shorter quills of the rump into the enemy.