SEASONAL AND ANNUAL ENERGY BUDGETS OF FEMALE DESERT TORTOISES (GOPHERUS-AGASSIZII)

do female desert tortoises (Gopherus agassizii) reproduce every year d espite variability in winter rainfall and food availability? To answer this question, I measured energy budgets of individual female desert tortoises from July 1987 to July 1989. Females produced eggs in years with low levels of winter annual plants by relaxing their control of e nergy and water homeostasis. They tolerated large deficits and surplus es in their body dry-matter composition (both nonlipid and lipid) on a seasonal, annual, and longer time scale. They could increase body ene rgy content (lipid and nonlipid energy) before winter and use this res erve (especially nonlipid energy) the following spring to produce eggs . Females used high-protein foods and rainwater, when available, to ac hieve energy surpluses that helped them survive periods of low resourc e availability (e.g., during hibernation and droughts). Adjusting seas onal and annual field metabolic rates (FMR) and food requirements to l evels of food availability, they still managed to produce eggs, even i n a drought year. Egg production in 1988 (mean +/- 1 so: 3.56 +/- 1.94 eggs, N = 9) did not differ from that in 1989 (3.00 +/- 2.69 eggs, N = 9); both were lower than during 1983-1987 (6.75 +/- 3.05 eggs, N = 1 00). Energy per se did not limit egg production in 1988 and 1989, but the availability of nonlipid energy (probably protein) limited egg pro duction in 1988 and was limiting in spring 1989. Yet, water was the pr imary resource limiting egg production in 1989. Females forgoing egg p roduction in 1989 accumulated body nonlipid energy and lost less total body water than did females producing eggs. Females that produced egg s in 1989 forfeited body nonlipid energy. In 1988 and 1989, the paucit y of new annual plants in spring contributed to a lower egg production . The amount of annuals that germinate in summer can affect egg produc tion, because females stored nonlipid energy during summer 1988 when e ating these annuals, and allocated this energy to eggs in the followin g spring (1989). Tortoises stored lipids during summer when consuming dry annuals. These lipids are critical for surviving the winter, but f emales forfeited body water and nonlipid dry matter to digest the dry annuals. Reproductive effort (RE) was higher during the drought year ( July 1988-July 1989: RE = 26.1%)than during the wetter year (July 1987 -July 1988: RE = 13.0%) because tortoises reduced FMR by 70-90% in 198 9. Compared to two other chelonians, RE of desert tortoises was consis tent with the K-selected trend of lower RE for larger, long-lived, and late-maturing species. Forfeiting body condition to produce only a fe w eggs, even under a great environmental stress, was consistent with a life history strategy called bet hedging.