african alert biological-interaction bird black broad-mouthed collagen creases curious eating endangered flat-mouthed folds food gray grazer grey heavy herbivore herbivorous hide horns inquisitive keratin large mammal Myna myna-birds mynah Noisy placid plumage powerful precocious rhino rhinoceros skin snout square-lipped-rhinoceros strong symbiosis symbiotic threatened tough watchful white-rhinoceros wide-lipped-rhinoceros wrinkled wrinkles