Intraspecific variation in water loss in a desert rodent, Dipodomys merriami

A species' thermal and hydric environments may vary significantly throughout its range. However, geographic variation in physiological parameters related to water-saving ability has been largely ignored in mammals, even though there are dramatic differences among the environments in which many species exist. Heteromyid rodents long have been the focus of investigations concerning mammalian adaptations to severely desiccating environments. Representatives of one widely distributed subspecies of Merriam's kangaroo rat, Dipodomys merriami merriami, were collected from three locations that differ in environmental temperatures and aridity. Mass, evaporative water loss, urine osmolality, rate of fractional body-mass loss, fecal water content, and metabolic rate were measured under desiccating conditions. Individuals from the most arid locality were smaller, possessed lower total and mass-specific evaporative water loss (EWL), and lost proportionately less mass under dehydrating conditions than those from less arid locations. These results indicate that EWL, but not kidney function or fecal water loss, is a trait associated with conserving water in this subspecies.