Deserts cover about one fifth of the Earth's surface. Most occur at low altitudes, but there are some that are considered cold deserts, occuring at higher altitudes. Vegetation is generally sparse and the soil sandy, rocky and coarse. Temperatures in these regions can be extreme -- either very hot or very cold. There is little rainfall. The animals found in deserts are generally nonmammalian vertebrates such as reptiles. They are small and nocturnal (active at night). The dominant animals are burrowers and kangaroo rats. There are also insects, arachnids, reptiles and birds. During the heat of the day, the animals stay inactive in protected hideaways. They come out to forage at dusk, dawn or at night, when the desert is cooler.