Rhino horns are made of tightly woven keratin, which is the same protein that makes up human hair and fingernails. Rhino horn grows in layers from specialized skin cells that are keratinized, becoming hard and inert, with all cellular function ceasing. At the center of the horn, there are layers of calcium and melanin, two materials that help to keep it even tougher. The calcium has a strengthening function, while the melanin protects the core of the horn from UV exposure. Unlike most animal horns, which are comprised of a bony core coated by a relatively thin layer of keratin, rhino horns are solely made of keratin. Rhino horns are solid, and they grow back if lost. Three of the five rhino species have two horns, while the others have only one horn.