Deserts, whether hot or cold, are considered to be one of the most difficult environments for living systems, lacking the essential free water which ac counts for approximately 60-70% of their body mass and more than 98% of their constituent atoms {Macfarlane 1978}. Amongst vertebrates, reptiles are usually thought of as the animals most adapted or suited to such environments because of their diurnal habit, based on a need for external heat, and their ability to survive far from obvious sources of water. This impression is rein forced when one examines the composition of vertebrate faunae characteristic of deserts and arid zones: reptiles predominate and they are often the only vertebrates to be found in hyper-arid areas, such as some parts of the Sahara {Monod 1973}. I recently had occasion to examine this assumption carefully, however, and was led inexorably to the conclusion that reptiles represent a particularly successful desert group, not because of their evolution of superior adaptations, but because of their possession of a basic suite of behavioural and physiologi cal characteristics that suit them uniquely to this very resource-limited environment {Bradshaw 1986a}. These fundamental reptilian characteristics are: 1. their low rates of metabolism, compared with birds and mammals, which result in extremely low rates of resource utilisation and lead to considerable economy in the handling of water 2.

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1 The Osmotic Anatomy of the Reptiles.- 1.1 Evidence for Homeostasis.- 1.2 A Comparative Account.- 1.3 Water and Electrolytes in Reptiles as a Group.- 1.4 Conclusions.- 2 Water and Electrolyte Homeostasis.- 2.1 Basic Concepts.- 2.2 The Maintenance of Homeostasis.- 2.2.1 Regulatory Responses.- 2.2.2 The Concept of Stress.- 2.3 Effector Systems.- 2.3.1 Kidney Morphology.- 2.3.2 Derivation of Renal Parameters.- 2.3.3 The Cloacal-Colonic Complex in Reptiles.- 2.3.4 The Cephalic Salt-Secreting Glands.- 2.4 Osmoregulation in Crocodiles, Alligators and Chelonians.- 2.4.1 Extent of Environmental Exchange.- 2.4.2 Kidney and Cloaca.- 2.4.3 Salt-Excreting Glands.- 2.4.4 Desert Tortoises.- 2.5 Osmoregulation in Lizards.- 2.5.1 Hypernatraemia and Hyperkalemia in Lizards.- 2.5.2 Studies of the Agamid Genus Amphibolurus (Ctenophorus and Pogona) in Australia.- 2.5.3 The Desert Iguana (Dipsosaurus dorsalis) in North America.- 2.5.4 The Chuckwallas (Sauromalus obesus and S. hispidus).- 2.5.5 Two Saharan Lizards: Le Fouette-Queue (Uromastix acanthinurus) and the Varanid Varanus griseus.- 2.5.6 The Mountain or Thorny Devil (Moloch horridus).- 2.6 Osmoregulation in Snakes.- 2.7 Conclusions.- 3 Activity and Hormonal Control of Excretory Organs.- 3.1 Avant Propos.- 3.2 The Kidney.- 3.2.1 Renal Clearances and the Handling of Water and Solutes.- 3.2.2 Neurohypophysial Hormones.- 3.2.3 Adrenocortical Hormones.- 3.2.4 Localisation of Action of Arginine Vasotocin in Kidney Tubules.- 3.3 The Cloacal-Colonic Complex.- 3.4 Cephalic Salt-Secreting Glands.- 3.5 Conclusion.- 4 Thermal Homeostasis.- 4.1 Generalities.- 4.2 The Interpretation of Field Studies.- 4.3 Physiological Mechanisms.- 4.3.1 Metabolic Characteristics of Ectotherms.- 4.3.2 Sources of Heat Production in Reptiles.- 4.3.3 Thermal Hysterisis and Changes in Dermal Vascularity.- 4.3.4 Panting and Evaporative Heat Loss.- 4.3.5 The Pineal Eye and Photoperiod.- 4.3.6 Hormonal Influences on Thermoregulation.- 4.4 Behavioural Mechanisms.- 4.4.1 The Neuronal Basis for Thermoregulation.- 4.4.2 Maintenance of Thermal Homeostasis in the Field.- 4.5 Conclusion.- 5 Conclusion.- References.