Food Intake Rate and Body Mass Influence Transit Time and Digestibility in the Desert Tortoise (Xerobates agassizii)

Studies of digesta transit in the desert tortoise (Xerobates agassizii, formerly Gopherus agassizii) voluntarily eating a succulent diet (green, growing Erodium cicutarium, red-stemmed filaree) or a dry diet (dead Schismus barbatus, split grass), indicated that digesta flow was determined primarily by a gut-fill effect. Total food consumed between the times of marker ingestion and marker excretion remained constant for a given diet, regardless of feeding rate, and was related allometrically to body mass (exponent = 0.707). Higher feeding rates and their correspondingly shorter transit times occasioned lower digestibilities of dry matter, gross energy, and cell-wall components. The gastrointestinal tracts of tortoises eating green E. cicutarium contained nearly three times as much dry matter as when the tortoises were eating dry S. barbatus. This difference in gut fill may be related to the different physical (structural) properties of green and dry plantparts. Gravid females had a reduced gut fill, presumably because eggs in the oviduct reduce the potential gut volume. Unfortunately, insufficient data were obtained for gravid females (n = 3) to allow formulation of a predictive equation for food intake and transit time in females carrying eggs.