The deserts of the southern edge of the Palaearctic ecozone are among the harshest environments on Earth. Vast expanses of sand, clay, rock and rubble are baked by a broiling sun during the day and frozen by cold at night. There are no barriers to the chilling winds that blow southwards from the northern icecap, or downwards from the mountain peaks in the south. Rainfall is infrequent and confined to spring and autumn. The vegetation is sparse and only appears when rain has fallen. Animals must be quite hardy to exist in such an environment, and most of the plant-eating creatures found here are cold-blooded. The slow metabolism of such herbivorous animals as tortoises and plant-eating lizards enables them to gorge themselves when the plants are available, and to sleep away the harsher time