Terrestrial environments, and particularly deserts, arguably impose the greatest challenges to life with respect to extremes in temperature and potential for dehydration. Small rodents are the dominant mammals within many deserts and, because physiological adaptations are prominent in environmental extremes, these animals represent excellent models for studying the physiology of terrestrial mammals. Geographic variation in physiological parameters related to desiccation has been largely ignored in mammals, yet is important to understand limits on distribution and the evolutionary origin of adaptations.;Accordingly, this research focused on the water economy, thermal biology, and intraspecific variation in physiology in one widely distributed subspecies of nocturnal desert rodent, Merriam's kangaroo rat (Dipodomys merriami merriami). The complex association between these animals and their environments that permits them to reside in the Sonoran Desert, as well as the covariation of physiology and geography, was examined by focusing studies on animals from xeric, intermediate, and mesic sites within this subspecies' distribution. Data show that animals from the xeric site not only are smaller, but have much lower rates of evaporative water loss than those from the intermediate and mesic sites. In fact, kangaroo rats from the intermediate and mesic sites, respectively, exhibited 33% and 64% greater average rates of evaporation than those from the xeric site. Microclimate data show that the xeric site is >5°C hotter than the mesic site throughout the year, both above the surface and to depths of 2 m in the soil, and, presumably, is a driving force behind the intraspecific variation in physiology.;The roles acclimation and developmental plasticity may have in the intraspecific variation in resistance to desiccation in D. m. merriami were investigated. This variation in water loss arises primarily from genetic/maternal effects specific to lineages from areas of contrasting temperature and humidity. However, the effects of the two labile modes (acclimation and developmental plasticity) can equal apparent genetic differences in desiccation resistance among these lineages.;Because there are contradictions in the data surrounding the physiology and ecology of these archetypical organisms, traditional views were re-evaluated with field and laboratory analyses. Thermal and hydric environments were quantified and the physiological and behavioral effects of heat and aridity were examined in intraspecific comparisons. Data demonstrate that most water loss is through the skin, not ventilatory (as has been asserted), and that the latter contributes to total evaporation, at most, only 44%. Also, these animals are much more thermally tolerant than appreciated and remain active for ≥30 min at an ambient temperature (Ta) of 40°C and for 18 min at Ta = 42.5°C.;Finally, although nocturnally active above the surface and diurnally fossorial, these animals are much more tolerant of high temperatures and exposed to substantially greater potential desiccation and thermal stress than believed. In fact, they remain in shallow (1m) burrows at high Ta's (>35°C) throughout summer days. Stomach content and nitrogen isotope analyses show that (rather than being strictly granivorous, as is accepted) they are omnivorous and facultatively consume significant amounts of leafy vegetation and insects to maintain water balance during harsh desert summers. This importantly changes our perception of their ability to survive in desert environments.
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