Do porcupine quills grow back?

Yes, they do, in case you were wondering whether shedding their quills leaves these spiny rodents with noticeable bald patches.

Quills are modified hairs composed of keratin, a fibrous structural protein, and consist of an inner spongy core surrounded by a tough, hollow, shaftlike, pointed shell. They grow in clumps in porcupines found in Asia, Africa, and Europe, collectively known as Old World porcupines. In contrast, New World porcupines—those found in North and South America—have individual quills interspersed with sensory guard hairs, bristles, and underfur. The quills of these New World porcupines possess tiny backward-pointing barbs that function like fishhooks when they penetrate the skin, effectively anchoring the quill in place.

Under normal conditions, the quills lie flat against the body of a porcupine. But, when the animal is alarmed or agitated, the musculature around the base of its quills contracts, causing them to stand erect. If these erect quills prick another animal—such as a predator that the porcupine is trying to deter—they readily lodge in the animal’s skin and easily detach from the porcupine at the base, often penetrating deeper as the surrounding muscles contract. Depending on where the quill, or its broken fragments, lodges in the body, the animal may experience injuries that range from minor to fatal.

The porcupine is not injured when a quill detaches from its body because structures at the quill’s base prevent it from springing back into the body and allow it to come off easily. And just as hair that is naturally shed grows back over time, new quills begin to grow within a few days to replace the lost ones. One estimate suggests that a quill grows at a rate of 1 mm (0.04 inch) every two days until it is fully formed. A North American porcupine (Erethizon dorsatum) can have quills measuring 3.5–6 cm (1.38–2.36 inches) in length, whereas the quills of the North African crested porcupine (Hystrix cristata) can reach up to 45 cm (17.72 inches), although they more commonly measure 12–30 cm (4.72–11.81 inches). Depending on the species, porcupine quills can grow back within months of being shed. The time taken to replace a quill may depend on the available diet, and during winter—when food is likely to be scarce—porcupines may not replace lost quills.