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 (gr een, growing Erodium cicutarium, red-stemmed filaree) or a dry diet (d ead Schismus barbatus, split grass), indicated that digesta flow was d etermined primarily by a gut-fill effect Total food consumed between t he times of marker ingestion and marker excretion remained constant fo r a given diet, regardless of feeding rate, and was related allometric ally to body mass (exponent = 0.707). Higher feeding rates and their c orrespondingly shorter transit times occasioned lower digestibilities of dry matter, gross energy, and cell-wall components. The gastrointes tinal 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 differen t physical (structural) properties of green and dry plant parts. Gravi d females had a reduced gut fill, presumably because eggs in the ovidu ct reduce the potential gut volume. Unfortunately, insufficient data w ere obtained for gravid females (n = 3) to allow formulation of a pred ictive equation for food intake and transit time in females carrying e ggs.