All aspects of ecophysiology are interrelated but, in hot deserts, none more closely than thermal and water relations. Because the drying power of the air is so greatly increased at high temperatures, heat and drought are the two most important elements of the climate of arid environments. The problem of dehydration in desert animals reflects both the high saturation deficiency of the atmosphere, which is usually more difficult to avoid than high temperature, and the use of water for evaporative cooling. Water is lost by evaporation through the integument - the cuticle of arthropods and the skin of reptiles - in respiration and thermoregulation, with the faeces, in nitrogenous excretion, and in the secretion of toxic repellents. It is obtained through drinking, by ingestion of moist food or soil, absorption from the air and as a product of metabolism (Wharton 1987).