In North American porcupines the weight of the precocial newborn is 21 ounces. The newborn animals feed on their own after the first few days, although lactation continues for 56 days. Porcupine young are born able to walk with their eyes open. They display typical defense reactions and within a few days are able to climb trees. These characteristics may be the reason why infant mortality is very low.
Porcupines grow for three or four years before they reach adult body size.
North American porcupines feed on conifer needles and on the bark of a variety of trees during the winter season. During the summer they feed more often on the ground and choose roots, stems, leaves, berries, seeds, nuts, and flowers. In the spring they often come out into meadows to feed on grasses in the evening hours. At all times of the year porcupines will eat bark, and can be destructive to forest plantations. North American porcupines are semi-arboreal and spend much of their time in trees, although they are known to come down to the ground to feed and to move from one tree to another.