The average porcupine has about 30,000 quills or specialized hairs, which it uses as a defensive weapon. The quills are comparable in hardness and flexibility to slivers of celluloid (a type of plastic, often used to make movie film) and so sharply pointed they can penetrate any animal's skin. In addition, the quill tips consist of tiny barbs (sharp points similar to arrows), which make the quills very hard to remove. The quills that do the most damage are the short ones that line the porcupine's muscular tail.
If threatened, a porcupine attacks by moving backward or sideways into the aggressor. Numerous quills, which easily pull loose from the porcupine, become lodged in the aggressor's skin. Every time the pierced animal moves, the barbs at the end of the quills penetrate further into its body. Often, the quills pierce vital organs and the animal dies. Within a few weeks, the porcupine will grow back the lost quills.
There are two types of porcupines: Old World porcupines, which live throughout Europe, Asia, and Africa; and New World porcupines, which live in Canada and the United States. Porcupines are nocturnal (active at night), slow-footed, and stocky. Porcupines spend much of their time in the trees, sometimes not coming down for days at a time. They are herbivores (plant-eaters), feeding on bark, buds, leaves, fruits, grasses, and vegetables.
Sources: Costello, David F. The World of the Porcupine, p. 13; Smithsonian, vol. 23, no. 2 (May 1992), pp. 56-67; Travers, Bridget, ed. The Gale Encyclopedia of Science, vol. 5, pp. 2892-94.