Digesta passage and functional anatomy of the digestive tract in the desert tortoise (Xerobates agassizii).

The herbivorous tortoise Xerobates agassizii contends with large fluctuations in the quality and abundance of desert pastures. Responses to grass (Schismus barbatus), herbage (Sphaeralcea ambigua) and pelleted diets were studied in captive animals. Digestive anatomy was investigated in wild tortoises. Cornified esophageal epithelia and numerous mucus glands along the digestive tract indicated a resistance to abrasive diets. Gastric contents were acidic whereas hindgut digesta were near neutral pH. The colon was the primary site of fermentation with short-chain fatty acids mainly comprised of acetate (69-84%), propionate (10-15%) and n-butyrate (1-12%). Fibre digestion was extensive and equivalent to 22-64% of digestible energy intakes. Large particles of grass (25 mm; Cr-mordants) were excreted as a pulse but retained longer than either fluids (Co-EDTA) or fine particles (2 mm; Yb). Patterns of marker excretion suggested irregular mixing of only the fluid and fine particulate digesta in the stomach and the colon. Mean retention times of Cr-mordants were 14.2-14.8 days on the grass and high-fibre pellets. Intakes of grass were low and accompanied by smaller estimates of digesta fill than for the high-fibre pellets. Digestive capacity was large and estimated at 11-21% of body mass on these diets. The capacious but simple digestive anatomy of the tortoise may provide the greatest flexibility in utilizing a variety of forages in its unreliable habitat.