HENEN, B.T.: Reproductive effort and reproductive nutrition of female desert tortoises: essential field methods

The reproductive output of female desert tortoises (Gopherus agassizii) may appear independent of rainfall and the springtime abundance of their preferred food, annual plants. To evaluate whether body reserves (energy, water and other nutrients) influence reproductive success and its apparent independence from such rainfall and food, three innovative, essential field methods (i.e., cyclopropane, doubly-labeled water and radiographic methods) were used to measure individual energy and water budgets of wild, female desert tortoises for two reproductive cycles (years). Body energy and water reserves, field metabolic rates, and water flux rates changed significantly amongst seasons and years, reflecting the females’ opportunistic and copious storage of energy and water for subsequent support of metabolism and egg production when food and rainwater were scarce. Through a 90% reduction of metabolic rates, females conserved enough body reserves to produce eggs during extreme drought conditions, an effective bet-hedging reproductive pattern for animals inhabiting an extreme and unpredictable environment. Ultrasonographic analyses on wild and captive female tortoises are revealing how reproductive cycles enable, and sometimes limit, egg production. Finally, studies of reproductive nutrition are revealing the importance of nitrogen, and perhaps other nutrients, to female reproductive success. New technology and field methods are furthering basic science and conservation efforts.