The hedgehog is a small, insectivorous mammal distinguished by its unique outer covering of sharp spines, which serves as its primary defense against predators. This specialized armor, coupled with a specific behavioral response, allows the hedgehog to transform itself into a nearly impenetrable, prickly sphere. The protective strategy relies on a combination of highly modified hair structures, a specialized muscular system, and a peculiar chemical behavior known as anointing.
The Quill Mechanism
The hedgehog’s coat consists of a dense array of specialized hairs, commonly called quills or spines, which number between 3,000 and 5,000 on the back and flanks of an adult. These quills are not barbed like those of a porcupine and cannot be shot out, instead serving as passive body armor. Each spine is composed of keratin, the same durable protein found in human hair and nails, providing strength and rigidity.
The quill is firmly rooted in the skin by a basal bulb, making it difficult to detach without breaking. A narrowed, flexible portion just above the skin surface allows the spine to be held flat when the animal is relaxed or quickly erected into a defensive position. Erection is achieved by a system of muscles, including the minute arrector pili muscles attached to each follicle, which contract to raise the spines. When raised, the quills crisscross, creating a dense, sharp barrier that points in multiple directions.
The Defensive Ball Formation
The physical defense of the quills is maximized by a specific behavioral maneuver that enables the hedgehog to roll into a tight ball. A threatened hedgehog will first raise its quills and crouch, but if the threat persists, it will engage its musculature. The entire spiny skin, known as the mantle, is loose and controlled by a sheet of muscle called the panniculus carnosus.
A thickened, circular band of this muscle forms the orbicularis muscle, which functions like a biological drawstring around the edge of the mantle. When the hedgehog contracts this muscle, it cinches the spiny skin closed. This action pulls the head, limbs, and the soft, vulnerable belly tightly inside the protective sphere, leaving only the sharp points of the quills exposed.
Anointing Behavior
Hedgehogs also exhibit a non-physical defense behavior known as self-anointing. When encountering a new or strong-smelling substance—such as toxic toads, plants, or chemicals—the hedgehog chews the material to generate a large amount of frothy, sticky saliva. It then contorts its body, using its long tongue to lick the foam mixture and apply it directly onto its quills.
The function of this behavior remains a subject of scientific debate, but it serves a defensive purpose. One theory suggests that coating the quills with the scent of the chewed substance creates chemical camouflage, masking the hedgehog’s odor from predators that rely on smell. Another hypothesis proposes that if the chewed substance is toxic or unpalatable, applying it to the quills may make them mildly poisonous or unpleasant to a predator attempting to bite the hedgehog.