Nutrient balances and maintenance requirements for nitrogen and energy in desert tortoises (Xerobates agassizii) consuming forages

The North American tortoise Xerobates agassizii (synonym: Gopherus agassizii) consumes herbage and grasses produced from low and variable desert rains. Utilization of Sphaeralcea ambigua foliage and the grass Schismus barbatus were studied in 8 adult tortoises (2.3 kg body mass). The herbage contained more N but less fiber than the grass. Tortoises ingested and digested more dry matter and energy from herbage than grass. Intakes and retention of N were greater from the herbage than the grass, but amino acid proportions and true N digestibilities were similar between diets. Herbage contained more Ca than grass, especially in relation to P (Ca:P 14.5 vs. 1.9), but this was ameliorated by lower absorption of Ca from the herbage. K intakes were greater from the herbage than the grass and associated with the digestive loss of Mg from herbage. Low Na content of both forages resulted in net losses of Na. Maintenance requirements for dietary N (14.4 mg • kg • d) and for digestible energy (19.9 kj • kg • d) were lower than estimates for other herbivorous reptiles and consistent with the utilization of desert pastures where quality and abundance may be low and erratic. Modest requirements for these and other nutrients may enable desert tortoises to tolerate temporal deficiencies, but these must ultimately be corrected by a wider dietary selection. Consequently, both diversity and abundance of pasture growth may be critical to long-term nutrient balances of this threatened species.