When climbing, the porcupine uses the stiff bristles on the undersurface of the tail as support. The animal has long, curved front claws which also aid in climbing. In the spring and summer, buds and young green leaves of birch, aspen, and willow are eaten until the tannin levels build too high for the porcupine to tolerate. Because they are vegetarians and most vegetable matter is very low in sodium, porcupines need additional sodium in the blood to balance cell potassium levels. As a result, porcupines seek out salt sources such as natural licks, glue which bonds plywood together, human perspiration on tools, road salt, and some paints.
Porcupines also feed on shed antlers and the bones of dead animals to obtain sodium.
However, an encounter between a young inexperienced predator and a porcupine can be a very painful experience. Some unfortunate carnivores have starved to death because a mouthful of quills has prevented them from eating. In an effort to remove the quills, the predator can cause the barbed quills to work into the deep tissues. However, the quills are coated with a natural antibiotic and rarely cause infection. Predators have different means of killing and eating porcupines.