Porcupines are large rodents with coats of sharp spines, or quills, that protect them against predators. The term covers two families of animals, the Old World porcupines in the family Hystricidae, and the New World porcupines in the family Erethizontidae. Both families display superficially similar coats of rigid or semirigid quills, which are modified hairs composed of keratin, and belong to the infraorder Hystricognathi within the diverse order Rodentia. The two groups are distinct and are not closely related to each other within the Hystricognathi. The largest species of porcupine is the third-largest living rodent in the world, after the capybara and beaver.