Nutritional Quality of Natural Foods of Juvenile and Adult Desert Tortoises (Gopherus agassizii): Calcium, Phosphorus, and Magnesium Digestibility

Exotic plants can make up a major component of the diet for some Desert Tortoises (Gopherus agassizii) in the Mojave Desert. If introduced plants differ nutritionally from native plants, they may influence the growth and welfare of young tortoises. Minerals available from a native grass (Achnatherum hymenoides), an introduced grass (Schismus barbatus), a native forb (Malacothrix glabrata), and an introduced forb (Erodium cicutarium) were measured for juvenile Desert Tortoises voluntarily eating single-species diets. We offered tortoises weighed amounts of chopped foods daily for similar to 130 days (dry grasses; summer diet) or similar to 90 days (green forbs; spring diet). Orts and feces were collected daily and dried to constant mass, and calcium, phosphorus, and magnesium contents of food and feces were measured and used to calculate nutrient digestibilities. Overall, tortoises gained more minerals from forbs than from grasses. Tortoises lost small but significant amounts of phosphorus when eating grasses, which may have contributed to observed decreases in tortoise mass and shell volume on grass diets. There were few nutritional differences between native and exotic forbs or between native and exotic grasses. Comparisons of nutrient availability to estimated requirements for growth by juveniles and for egg production by adult females suggest that phosphorus is more limiting than calcium or magnesium and that calcium may pose a significant osmotic challenge for excretion in this desert species. Management practices that promote availability of forbs could increase growth rates and shell ossification, which would enhance predator resistance of juvenile tortoises.