Ostriches normally spend the winter months in pairs or alone. Only 16 percent of ostrich sightings were of more than two birds. During breeding season and sometimes during extreme rainless periods ostriches live in nomadic groups of five to 100 birds that often travel together with other grazing animals, such as zebras or antelopes. Ostriches are diurnal, but may be active on moonlit nights. They are most active early and late in the day. The male ostrich territory is between 2 and 20 km2.With their acute eyesight and hearing, ostriches can sense predators such as lions from far away. When being pursued by a predator, they have been known to reach speeds in excess of 70 km/h, and can maintain a steady speed of 50 km/h, which makes the ostrich the world's fastest two-legged animal. When lying down and hiding from predators, the birds lay their heads and necks flat on the ground, making them appear like a mound of earth from a distance, aided by the heat haze in their hot, dry habitat. When threatened, ostriches run away, but they can cause serious injury and death with kicks from their powerful legs. Their legs can only kick forward. Contrary to popular belief, ostriches do not bury their heads in sand to avoid danger. This myth likely began with Pliny the Elder, who wrote that ostriches "imagine, when they have thrust their head and neck into a bush, that the whole of their body is concealed." This may have been a misunderstanding of their sticking their heads in the sand to swallow sand and pebbles, or, as National Geographic suggests, of the defensive behavior of lying low, so that they may appear from a distance to have their head buried."