Crested porcupines have back quills that can stand up into a crest. The crest starts from the top of the head and goes down to the shoulders. Each quill is marked with black and white bands. Some quills can be up to 20 inches long. These long, pliable quills act as guard hairs and form a "skirt." When threatened, the skirt can be erected, making the porcupine appear two to three times bigger. Smaller, rigid quills that are 8 inches long are densely packed over the porcupine’s rump and back. These can stab any potential threat. At the base of the tail, the porcupine has blunt, hollow quills that rattle when shaken, serving as a warning to potential predators. If the noise doesn't work, the porcupine may try to charge backward.