Desert regions are not necessarily characterised by great heat, nor do they always consist of vast expanses of shifting sand dunes. The one characteristic common to them all is their aridity throughout most or all of the year. When the air is humid, not only does less solar heat penetrate to the ground during the day, but less is lost from the earth by radiation at night. Consequently, humid climates tend to show daily or seasonal stability, while deserts are characterised by extremes of temperature and humidity. The most adverse conditions for life consist of a combination of aridity and high temperature, and it is the effects of these two factors that have been studied most extensively. The combination presents an unusually acute thermoregulatory challenge to homeothermic animals, because it poses the singularly intractable problem of losing heat to a hot environment while simultaneously keeping water loss at a minimum.