Wildlife biologists have estimated that up to half of all adult females go unmated each year, and females that do become impregnated almost always produce just one offspring. The survival rate of young porcupines is high. After a gestation period of 205-217 days, the female gives birth in April, May or June. Birth may occur in a ground den, although the female doesn't generally select a particular site. The quills are soft and hairlike, about a quarter-inch long, but they become hard and functional as they dry.
Pups are able to climb trees and eat solid food within a few days.
They nurse for about 50 days. Young are sexually mature at 15 to 16 months and breed in their second autumn. Porcupines den in caves, rock crevices, hollow logs and trees, deserted fox dens, brushpiles and abandoned buildings. They have a habit of defecating at their den's entrance, and the resulting pile of droppings is a good indicator of the animal's presence. In winter, porcupines take to their dens for protection from snow, wind and predators.