The term 'porcupine feature' refers to the quills of a porcupine, which are its most distinctive feature. Quills are modified hairs that are sharp and stiff, and are used by porcupines as a defense mechanism against predators. They are made of keratin, the same material that makes up human hair and nails. Porcupines have around 30, 000 quills, which can be up to 10 inches long. The quills are hollow and contain air, making them lightweight and allowing them to float in water.