Digestive efficiency and gut morphology of omnivorous and herbivorous African tortoises

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Abstract
Most of the 42 living species of tortoise (Testudinidae) are herbivorous, feeding largely on vascular plants, with the exception of the genus Kinixys, which is omnivorous. A comparison of Kinixys spekii and Geochelone pardalis showed the following significant differences. The large intestine was shorter in K. spekii (86% of plastron length and 1.8% of body mass) than in G. pardalis (246 and 2.5%) and held a smaller proportion of the total gut contents (44 and 62%, respectively). The apparent digestibility coefficient (ADC) of organic matter was lower in K. spekii (77%) than in G. pardalis (82%) fed kale; the ADC of holocellulose was higher and did not differ between the two species. Kinixys spekii did not readily consume grass, which is more difficult to digest than kale; the ADC of organic matter was 64% in G. pardalis. The time to first excretion (t0) was lower in K. spekii (2.2 days) than in G. pardalis (3.8 days). Other measures of retention time (t50, tmax, and [Formula: see text]) did not differ between the species, or between the two diets in G. pardalis. Analysis of published data on chelonians fed foliage shows that the ratio of large intestine to small intestine lengths is significantly related to gut retention time (r = 0.88) but not to the ADC (r = −0.28). Thus, although the short large intestine of K. spekii slightly reduces the gut retention time and digestibility of dicotyledonous leaves, the most important effect is a low ability to process grass and other coarse vegetation.