SEASONAL WATER AND ENERGY-METABOLISM OF THE DESERT-DWELLING KANGAROO RAT (DIPODOMYS-MERRIAMI)

Free-living Merriam's kangaroo rats in the Mojave Desert did not drink, even when abundant rainwater was available for short periods in winter and summer, but they still obtained much more water than expected from a diet of dry seeds alone. Year-round measurements of water influx rates and field metabolic rates (via doubly labeled water), diet composition (stomach content analysis), and body mass changes were used to evaluate water and energy balance during each month of the year. From late spring (May) through midwinter (December), kangaroo rats ate primarily the seeds of creosote bush (Larrea tridentata) and were able to maintain water and energy balance on this diet even during the summer drought. Dry seeds collected at the surface apparently were cached in burrows where hygroscopic uptake of water from humid air increased the succulence of seeds before ingestion. In late winter and early spring, kangaroo rats obtained abundant water and energy from their diet of green vegetation (90% of diet dry mass) and arthropods (10%). Only in late April and early May did these animals lose body mass, but that was clue primarily to reproductive effort (lactation) by females. Thus at no time of year were kangaroo rats distressed by their comparatively dry, hot, and barren desert habitat. Field metabolic rates were highest in winter, which suggests that these animals did not use torpor or hibernation. On an annual basis, an individual free-living kangaroo rat (mean body mass, 33.9 g) consumed nearly 1 kg (dry, matter) of food, and the population (density, 17.5 animals per bectare) consumed over 17 kg ba(-1). This is a small fraction of net primary productivity but can be a large proportion of seed production.