Osmoregulation in desert-adapted mammals

Hot deserts devoid of drinking water pose significant problems to mammals whose high body temperatures challenge the restriction of evaporative water loss (EWL) in order to maintain water balance, in addition to the obligatory loss of water for excretion. Desert mammals maintain water balance by physiological adaptations that minimise water loss, and by gaining water from food and/or from metabolism that produces oxidation water. Larger desert mammals such as ungulates depend on heterothermy and selective brain cooling to minimise EWL and generally do not excrete highly concentrated urine. Small desert mammals, as exemplified by the rodents, also restrict EWL, but are renowned for their ability to excrete small volumes of very concentrated urine, especially when they do not drink. In addition, the gastrointestinal tract of desert mammals generally has a greater absorptive area for water and nutrient uptake than that of non-desert mammals. To offset water loss, desert mammals gain preformed water from food, and small desert mammals in particular, can produce a significant amount of metabolic water from the oxidative metabolism of food substrates. The latter source of water gives rise to the iconic view of small desert rodents that consume seeds and survive by metabolic water production. This chapter will provide an overview of the accepted physiological paradigms that permit desert mammals, particularly small rodents, to exist in arid environments, and will emphasise how desert mammals are valuable models for comparative research in osmoregulation. In addition, the recent research in transcriptomics and its application to understanding osmoregulation in desert mammals will be discussed.