Water Regulation in Peromyscus

New-World mice of the genus Peromyscus are nearly ubiquitous, and their omnivorous food habits are accompanied by intermediate efficiency in water regulation when compared to dry-food or moist-food specialists among rodents. In virtually all aspects of water economy thus far studied (weight and urinary responses to water deprivation and saline drinking solutions, rates of evaporative water loss), the performances of Peromyscus are modest, with the single exception of P. crinitus; this is a small desert species that is nearly independent of dietary preformed water. Among Peromyscus there is considerable interspecific variability in efficiency of water regulation, and nearly as great intraspecihc variability within west coast races of P. maniculatus. This variability appears to be related to food habits and/or habitat climate, but the specific roles of these factors have yet to be elucidated. The most striking adaptation to drought among Peromyscus is the use of torpor and summer dormancy by P. eremicus, enabling prolonged escape from demanding surface conditions of water stress. These thermoregulatory adjustments may be common among arid-inhabiting Peromyscus and contribute substantially to their powers of water regulation.