The name is a misunderstanding, not a description of colour. "White" is thought to come from the Afrikaans wyd, meaning "wide," a reference to the animal's broad, square upper lip — the feature that separates it from the pointed, prehensile lip of the browsing black rhino. That square lip works like a lawnmower, cropping large mouthfuls of short grass. The two horns are equally distinctive: unlike the bony horns of antelope, a rhino's horn has no bony core and is made entirely of tightly packed keratin, the same protein as hair and fingernails, and the front horn grows throughout life. The species splits into two subspecies — the recovered southern white rhino and the northern white rhino, now functionally extinct after the last males died.