Small Mammals in Hot Deserts: Some Generalizations Revisited

Abstract

Life originated in aquatic environments and secondarily invaded terrestrial habitats. This invasion entailed coping with radically different physical environments that vary most noticeably from aquatic ones in that they are desiccating, highly variable in temperature, and often suffused with intense shortwave radiation. This contrast reaches its extreme in deserts, the most terrestrial of environments. Deserts can be defined as regions in which precipitation is so low, so infrequent, and so variable that it plays the dominant role in controlling biological productivity (Noy-Meir 1973). Most deserts combine this lack of water with seasonal periods of high temperatures. This combination of heat and aridity produces a profound challenge for animals, including mammals. Mammals, regardless of the habitats they occupy, typically maintain core body temperatures of approximately 37-38 0c. As a result, their primary thermoregulatory problem in most nondesert regions is to maintain body temperature above environmental temperature. For example, mammals may cope with the low temperatures characteristic of cold winters or polar regions by an exaggeration of the primitive mammalian characteristic of a high capacity for metabolic heat generation combined with substantial insulation. In hot deserts, however, high air temperatures, combined with intense solar radiation, produce conditions in which environmental temperature substantially exceeds body temperature. Effective thermoregulation must therefore rely on a mechanism that will move heat out of the body against a temperature gradient. The only such mechanism known in organisms is evaporation of water, which is driven not by temperature gradients but by differences between the water vapor pressure at the animal's surface and in the surrounding atmosphere. To thermoregulate, therefore, desert mammals must use the single resource whose paucity defines these habitats. This requirement provides the fundamental difficulty for mammals occupying hot deserts. The combination of heat and aridity reaches its extremes in the global belts of subtropical deserts that are located near latitudes 30° Nand 30° S. These deserts include the Sonoran Desert, the Sahara Desert, and the Australian deserts. Summers in these regions can be extraordinarily hot. The typical human experience during a summer day in the core of a hot subtropical desert is a combination of ovenlike air temperatures, intense radiaDESERT RODENTS MAY EXPERIENCE