african black-rhinoceros collagen creases critically-endangered Diceros bicornis eating endangered folds food gray grazer grey heavy herbivore herbivorous herbivorous-browser hide hook-lipped-rhinoceros horns keratin large lattice-structured mammal megafauna odd-toed-ungulate offspring perissodactyl placid powerful rhino rhinoceros skin snout southern-white-rhinoceros square-lipped rhinoceros strong surface texture textures thick threatened tough wrinkled wrinkles zoo