Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff with keratin.[5] Their spines are not poisonous or barbed and, unlike the quills of a porcupine, cannot easily be removed from the hedgehog. However, spines normally come out when a hedgehog sheds baby spines and replaces them with adult spines. This is called "quilling". When under extreme stress or during sickness, a hedgehog can also lose spines.