Deserts -- regions that receive less than 10 inches of rain a year -- cover approximately a quarter of the Earth's land surface, mostly in Africa, Asia, Australia and North America. Most desert species are small mammals and reptiles, some of which dig underground burrows to escape from the scorching desert heat. Few large animals can survive in the desert; those that do have special adaptations that allow them to withstand their inhospitable environment.
Large mammals adapted to desert life include hoofed animals like the addax antelope and the Bactrian camel of Africa's Sahara Desert and Asia's Gobi Desert, respectively. Both have wide, flat hooves that allow them to walk on sand without sinking. Bactrian camels, which have two humps, can close their nostrils to keep sand out. Small mammals are more common in deserts. The Sahara alone is home to around 40 species of rodents, including the jerboa. Other mammals include the Mohave ground squirrel, found in the desert of the same name in California, and the elusive marsupial mole of the Central Australian deserts.
The desert-dwelling reptile population includes desert tortoises and desert iguanas, found in the Mohave and Sonora deserts. Both species burrow, though the desert iguana is more resistant to heat and is active during the warmest hours. Desert tortoises spend most of their time underground and hibernate in the winter to reduce water loss. Their bodies can also draw water stored in their bladders. The Mohave and Sonora deserts also harbor the Gila monster, a venomous lizard known to burrow. Gila monsters are nocturnal during the summer and can live off fat stored in their tails during cold winters. Ten species of horned lizards also occur in the Sonora Desert, as do desert grassland whiptail lizards. All of the latter are female; offspring are clones of the mother. Some snakes also live in deserts, including the rattlesnakes of North America and the horned viper of the Sahara.
Several species of owl live in deserts, including the elf owl of the Sonora Desert, which nests in cavities carved into saguaro cacti by another bird, the Gila woodpecker. The aptly named burrowing owl, found in the deserts of North and South America, occupies burrows dug out by squirrels and other small mammals. One of the most iconic desert birds is the roadrunner, an omnivorous bird found in the Sonora Desert. It prefers running to flying and can outrun a person. The deserts of Africa are home to the ostrich, the world's largest bird. Ostriches are also speedy omnivores, but unlike the roadrunner, they cannot fly.
Amphibians begin their lives as aquatic larva. The number of amphibians that can survive in the desert is therefore limited to a few highly adapted species, such as the desert spadefoot, the casque-headed tree frog and the Sonora desert toad of the American Southwest, which spend most of the year in burrows. As its name suggests, the desert spadefoot has hardened areas on its hind legs that enable it to dig. These species lay their eggs in pools of water created by sporadic summer showers.