25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Resource Acquisition and Allocation by Desert Tortoises (Gopherus agassizii): Transducing Unpredictable Resources into Population Parameters

Brian T. Henen^1,2 and Harold W. Avery^3
1Smithsonian Institution, National Zoological Park, Department of Zoological Research, 3001 Connecticut Avenue, Washington, D.C. 20008
²Desert Tortoise Conservation Center, Las Vegas, NV
³U. S. Geological Survey, Biological Resources Division, Department of Biology -
Canyon Crest Field Station, University of California, Riverside, CA 92521

Deserts are defined by their paucity of rainfall and characterized by low primary productivity. For long-lived desert animals, the availability of resources like water and food varies profoundly. Desert tortoises use opportunistic and conservative patterns in their physiology and behavior to prevail in these extreme environmental conditions. When rain falls, desert tortoises seek and imbibe pooled water to eliminate wastes and to replenish their body water reserves. Similarly, when new annual plants become available, desert tortoises selectively consume them, helping to replenish their body nutrient reserves (e.g., water, protein, and lipids). These reserves facilitate egg production, help tortoises overwinter and help tortoises endure seasonal, annual, and multi-annual droughts when water and quality forage are scarce. Under drought conditions, desert tortoises become conservative, reducing losses of body nutrient reserves by reducing allocations of water, energy and other nutrients to reproduction and other functions. The opportunistic acquisition and storage of resources, and conservative allocation of resources, helps explain how population level parameters (e.g., fecundity and survivorship) of desert tortoises are affected by extreme variations in rainfall and food availability. Female body reserves, which are developed when resources are available, enable female tortoises to produce eggs and temper fluctuations in fecundity relative to the fluctuations in environmental resource availability. Similarly, fluctuations in survivorship may be moderated by the conservative use of body reserves and high tolerances to dehydration and starvation.