Energy Utilization and Temperature Regulation by Jackrabbits (Lepus californicus) in the Mojave Desert

Energy expenditure for black-tailed jackrabbits in the Mojave Desert is highest in winter (172 kcal kg⁻¹ day⁻¹) when ambient temperatures were consistently below the zone of thermoneutrality. During late spring, summer, and early fall energy expenditure is 142 kcal kg⁻¹ day⁻¹ despite appreciable seasonal and diurnal temperature shifts. The annual energy expenditure for maintenance is estimated to be 55,200 kcal kg⁻¹ yr⁻¹. This exceeds standard metabolic rate by a factor of 2.3. Jackrabbits were able to metabolize 65% of the energy contained in the spring diet of succulent annual plants. Metabolic efficiency fell to 45% for the summer diet containing green annuals (mostly Salsola) and some shrub browse, and to only 18% in winter when the animals ate shrub browse and dry annuals. Jackrabbits eat most of the plant material they cut, usually wasting no more than 15% of the amount consumed. By increasing body temperature to circa 41 C on hot summer days, jackrabbits save water that would otherwise be spent for evaporative cooling, but this leaves little margin for heat storage during exercise.