Such behaviour was originally thought to be an example of mutualism, but recent evidence suggests that oxpeckers may be parasites instead, feeding on rhino blood.
BLACK RHINOCEROS SKIN
Their skin harbors external parasites, such as mites and ticks, which may be eaten by oxpeckers and egrets. Their thick-layered skin helps to protect the rhino from thorns and sharp grasses. This key differentiation is further illustrated by the shape of the two species mouths (lips): the "square" lip of the white rhinoceros is an adaptation for grazing, and the "hooked" lip of the black rhinoceros is an adaptation to help browsing. The black rhinoceros can also be distinguished from the white rhinoceros by its size, smaller skull, and ears and by the position of the head, which is held higher than the white rhinoceros, since the black rhinoceros is a browser and not a grazer. It has a pointed and prehensile upper lip, which it uses to grasp leaves and twigs when feeding. The white rhinoceros has square lips used for eating grass. The black rhino is smaller than the white rhino, and is close in size to the Javan Rhino of Indonesia. The longest known black rhinoceros horn measured nearly 1.5 m (4.9 ft) in length. Sometimes, a third, smaller horn may develop. These horns are used for defense, intimidation, and digging up roots and breaking branches during feeding. Two horns on the skull are made of keratin with the larger front horn typically 50 cm (20 in) long, exceptionally up to 140 cm (55 in).
An adult black rhinoceros stands 140–180 cm (55–71 in) high at the shoulder and is 3–3.75 m (9.8–12.3 ft) in length. An adult typically weighs from 800 to 1,400 kg (1,800 to 3,100 lb), however unusually large male specimens have been reported at up to 2,199–2,896 kg (4,848–6,385 lb). The females are smaller than the males.
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