Zebras may travel or migrate to better watered areas. Plains zebras have been recorded travelling between Namibia and Botswana, the longest land migration of mammals in Africa. When migrating, they appear to rely on some memory of the locations where foraging conditions were best and may predict conditions months after their arrival. Plains zebras are more water-dependent and live in more mesic environments than other species. They seldom wander from a water source. Grévy's zebras can survive almost a week without water but will drink daily when it is plentiful and conserve water well. Mountain zebras can be found at elevations of up to . Zebras may spend seven hours a day sleeping. During the day, they sleep standing up, while at night they lie down. They regularly rub against trees, rocks, and other objects and roll around in dust for protection against flies and irritation. Except for the mountain zebra, zebras can roll over completely.