Water, Electrolyte, and Nitrogen Budgets of Jackrabbits (Lepus californicus) in the Mojave Desert

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Abstract
Seasonal changes in water flux rates (HTO) and diet (stomach content analysis) were determined in jackrabbits (Lepus californicus) kept in 1-acre (0.4-hectare) field enclosures that contained natural Mojave Desert vegetation. The water, electrolyte, and nitrogen contents of food plants were measured at different seasons. Itemized water, electrolyte, and nitrogen budgets in jackrabbits were obtained by measuring food utilization in caged animals fed natural diets. Field jackrabbits maintained body mass and obtained excess water while eating succulent annual plants in spring, but in summer and winter animals could not maintain mass or water balance while eating the drier vegetation present during those seasons. Diet utilization studies indicated that (1) jackrabbits could not maintain nitrogen balance on the winter diet, (2) ion regulation was no problem in spring or summer, and (3) jackrabbits can produce fairly concentrated urine (2.5-3.6 osM), very dry feces (38% H₂O), and can reduce evaporative water loss about 70% when water is restricted. Despite these mechanisms for conserving water, jackrabbits in the field have relatively high water requirements. We estimate that in summer a jackrabbit would require about 120 ml H₂O kg⁻¹ day⁻¹ to maintain water balance, and that its diet must contain no less than 68% H₂O in order to meet this requirement. As long as food plants contain sufficient water, their digest ibility, nitrogen, and electrolyte content will probably also be suitable.