A hedgehog is the small nocturnal mammal that rolls into a spiky ball when threatened. This defensive behavior hides its vulnerable face, feet, and belly while turning its stiff spines outward toward a predator.
Hedgehog spines are modified hairs made rigid with keratin. Unlike porcupine quills, they are not barbed, poisonous, or designed to detach easily. Two large muscles in the back help direct the spines, while loose skin and a surrounding muscle allow the animal to curl tightly into a ball.
Most hedgehogs forage at night, searching for insects and other small invertebrates. Their spines are a passive armor system, not an offensive weapon, although a predator may be discouraged by the painful, prickly surface. Some desert species are more likely to flee or attack and may roll up only as a last resort.
Hedgehogs are commonly confused with echidnas, porcupines, and armadillos. Echidnas are egg-laying monotremes, porcupines have detachable quills, and only certain armadillos—notably three-banded species—can form a complete defensive ball.