Porcupines are rodents with a coat of sharp spines, or quills, that defend them from predators. They are endemic in both the Old World and the New World. After the capybara and the beaver, Porcupines are the third largest of the rodents. Most porcupines are about 2536 in (630910 mm) long, with an 810 in (200250 mm) long tail. Weighing between 1235 lb (5.416 kg), they are rounded, large and slow. Porcupines come in various shades of brown, grey, and the unusual white. A porcupine is any of 27 species of rodent belonging to the families Erethizontidae or Hystricidae. All defend themselves with hair modified into sharp spines. Porcupines vary in size considerably: Rothschild's Porcupine of South America weighs less than a kilogram (2.2 lb (1.00 kg)); the African Porcupine can grow to well over 20 kg (44 lb). The two families of porcupines are quite different and although both belong to the Hystricognathi branch of the vast order Rodentia, they are not closely related. The eleven Old World porcupines are almost exclusively terrestrial, tend to be fairly large, and have quills that are grouped in clusters. They are believed to have separated from the other hystricognaths about 30 million years ago, much earlier than the New World porcupines. The twelve New World porcupines are mostly smaller (although the North American Porcupine reaches about 85 cm/33 in in length and 18 kg/40 lb), have their quills attached singly rather than grouped in clusters, and are excellent climbers, spending much of their time in trees. The New World porcupines evolved their spines independently (through convergent evolution) and are more closely related to several other families of rodent than they are to the Old World porcupines.
Porcupines' quills or spines take on various forms, depending on the species, but all are modified hairs coated with thick plates of keratin, and they are embedded in the skin musculature. Old World porcupines (Hystricidae) have quills embedded in clusters, whereas in New World porcupines (Erethizontidae) single quills are interspersed with bristles, underfur, and hair. Porcupine quills are as sharp as needles, detach very easily, and will remain embedded in an attacker. Unlike needles, however, the quills of New World porcupines have microscopic, backwards-facing barbs on the tip that catch on the skin making them difficult and painful to extract, though they must be removed. Quills are about 75 millimetres (3.0 in) long and 2 millimetres (0.079 in) wide. If a quill becomes lodged in the tissues of a would-be attacker, the barbs act to pull the quill further into the tissues with the normal muscle movements of the attacker, moving up to several millimeters in a day. Predators have been known to die as a result of quill penetration and infection. Quills are still capable of penetrating animals and humans even after death. Quills are released by contact with them or they may drop out when the porcupine shakes them, they are not sprayed out.
Porcupines occupy a wide range of habitats in tropical and temperate parts of Asia, Italy, Africa, and North and South America. Porcupines live in forests, deserts, rocky outcrops, hillsides and grasslands. Some new world porcupines live in trees, but old world porcupines stay on the ground. Porcupines can be found on rocky areas up to 3,500 m (11,000 ft) high. Porcupines in search of salt sometimes encroach on human habitats, eating plywood cured with sodium nitrate, certain paints, and tool handles, footwear, clothes and other items that have been coated in salty sweat. Porcupines are attracted to roads in areas where rock salt is used to melt ice and snow, and are known to gnaw on vehicle tires or wiring coated in road salt. Salt licks placed nearby can prevent porcupines from injuring themselves. Natural sources of salt consumed by porcupines include varieties of salt-rich plants (such as yellow water lily and aquatic liverwort), fresh animal bones, outer tree bark, mud in salt-rich soils, and objects imbued with urine.