Seasonal and Annual Energy Budgets of Female Desert Tortoises (Gopherus Agassizii)

How do female desert tortoises (Gopherus agassizii) reproduce every year despite 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 energy and water homeostasis. They tolerated large deficits and surpluses in their body dry-matter composition (both nonlipid and lipid) on a seasonal, annual, and longer time scale. They could increase body energy content (lipid and nonlipid energy) before winter and use this reserve (especially nonlipid energy) the following spring to produce eggs. Females used high-protein foods and rainwater, when available, to achieve energy surpluses that helped them survive periods of low resource availability (e.g., during hibernation and droughts). Adjusting seasonal and annual field metabolic rates (FMR) and food requirements to levels of food availability, they still managed to produce eggs, even in a drought year. Egg production in 1988 (mean ± 1 sd: 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 = 100). Energy per se did not limit egg production in 1988 and 1989, but the availability of nonlipid energy (probably protein) limited egg production in 1988 and was limiting in spring 1989. Yet, water was the primary resource limiting egg production in 1989. Females forgoing egg production in 1989 accumulated body nonlipid energy and lost less total body water than did females producing eggs. Females that produced eggs in 1989 forfeited body nonlipid energy. In 1988 and 1989, the paucity of new annual plants in spring contributed to a lower egg production. The amount of annuals that germinate in summer can affect egg production, because females stored nonlipid energy during summer 1988 when eating these annuals, and allocated this energy to eggs in the following spring (1989). Tortoises stored lipids during summer when consuming dry annuals. These lipids are critical for surviving the winter, but females 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 1989. Compared to two other chelonians, RE of desert tortoises was consistent with the K-selected trend of lower RE for larger, long-lived, and late-maturing species. Forfeiting body condition to produce only a few eggs, even under a great environmental stress, was consistent with a life history strategy called bet hedging.