27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

STUDENT PAPER

Dry Matter, Energy, and Nitrogen Digestibility in Natural Foods Eaten by Desert Tortoises, Gopherus agassizii

Danielle R. Shemanski, Lisa C. Hazard, Kenneth A. Nagy
Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, California 90095-1606

The composition of the diets of desert tortoises has changed in the last fifty years as exotic plants have invaded and flourished in the Mojave Desert. It is possible that these exotic plants do not provide adequate nutrition for tortoises, which might compromise the health and survivorship of this species. We measured the apparent digestibility of dry matter, energy, fiber, and nitrogen in captive juvenile tortoises eating four different diets: Achnatherum hymenoides (a native grass), Schismus barbatus (an exotic grass), Malacothrix glabrata (a native forb), and Erodium cicutarium (an exotic forb). We offered known amounts of dry grass for ~130 consecutive days, and forbs for ~90 consecutive days. Feces and uneaten food (orts) were analyzed for dry matter, energy, fiber, and nitrogen content. Apparent digestibility was calculated as: 100 x (nutrient ingested - nutrient excreted)/ nutrient ingested, and nutrient availability in the foods eaten was calculated as: mg or kJ retained/ g dry food ingested. Bioavailability of dry matter, energy, and nitrogen was greater in forbs than in grass, and fiber content was variable between diets. Thus, nutritional quality of foods available to tortoises varied, but the differences were among food types (i.e. grass vs. forbs) instead of among geographic origin (native vs. exotic).