Chapter 30: Water, Salts, and Excretion: Mammals of Deserts and Dry Savannas

The physiology of large mammals like oryxes, camels, and gazelles is contrasted with that of small desert mammals. Large body size offers advantages like lower surface-area-to-volume ratios and reduced weight-specific metabolic rates, enabling lower evaporative water losses. Notably, oryxes and other species survive without drinking by obtaining preformed water from food and minimizing evaporative loss through adaptive behaviors and physiological mechanisms. Species are divided into drinking-water-dependent (e.g., wildebeests, zebras) and drinking-water-independent (e.g., oryxes, Grant’s gazelles), and this dichotomy influences migration behavior, habitat use, and shade-seeking tendencies. A deep dive into the Serengeti migration reveals how access to water drives large-scale seasonal movements, while coexisting species with different hydration strategies remain stationary. The chapter also explores water conflicts between human demands and wildlife, such as the impact of river diversion and land privatization on migratory routes and indigenous nomadic cultures. Critical adaptations discussed include evaporative cooling, body temperature modulation (heterothermy), reduced metabolic rates, and enhanced kidney function for urine concentration. Desert plants and fog are highlighted as nuanced sources of water and nutrients. Species-specific adaptations are featured in detail: oryxes display extreme water conservation and can allow body temperatures to vary up to 7.7°C daily; Grant’s gazelles endure temperatures up to 46.5°C; and sand gazelles reduce evaporative loss to just 20% of expected values. Dromedary camels, debunking the myth of water storage in humps, instead survive by tolerating dehydration up to 40% of body weight and reducing water loss through heat shielding fur, reduced urine and fecal water, and thermal tolerance. Together, these physiological strategies illuminate how evolution has enabled large mammals to survive in ecosystems where water availability exerts a dominant controlling effect on life.