The Common Hippopotamus (Hippopotamus amphibius) is the third largest land mammal. Hippos are semi-aquatic, and spend most of the day in lakes, waterholes and rivers in Africa south of the Sahara and along the Nile River in Egypt. Hippos leave the water at night, or in the late afternoon to feed, grazing mainly on short grasses, before returning to spend the day in water. Hippos are mostly found in groups in water shallow enough for them to stand in, and each group usually has a dominant territorial bull. Sometimes Hippos are found alone, as with this bull in Kruger National Park in South Africa. He lay mostly submerged in a shallow dam, with his back looking like a rock. He suddenly became more active, yawning, bellowing and rolling over completely, his back covered in grey mud after each roll. This rolling behaviour is most unusual – this is my first observation of it after many hours observing Hippos. The Hippo bull then defaecated a couple of time in the water, then walked to the edge of the dam where he marked his territory, flicking his dung by wagging his tail from side-to-side.