african broad-mouthed collagen creases eating endangered feeding flat-mouthed folds food gray grazer grey heavy herbivore herbivorous hide horns keratin large mammal megafauna mother Mud mud-covered muddy odd-toed-ungulate offspring perissodactyl placid powerful rhino rhinoceros skin snout southern-white-rhinoceros square-lipped rhinoceros strong thick threatened tough white-rhinoceros wide-lipped rhinoceros wrinkled wrinkles