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The utilization of a diet rich in plant cell walls was studied in a large, desert-dwelling, herbivorous lizard, Uromastyx aegyptius (Agamidae). The diet eaten by U. aegyptius in spring in the ’Arava Valley, Israel, consisted almost entirely of leaves and fruits of short-lived annual plant species. The leaves contained only moderate levels of fibre compared with grasses and tree leaves, but those fruits eaten were markedly higher in fibre and lignin. All items had notably high contents of ash.

Following oral doses of [C-14] cellulose, (CO2)-C-14 was detected in respired air from U. aegyptius, demonstrating that the cellulose was digested and that the lizards gained oxidative energy from cellulose degradation. The hind gut was the principal site of microbial activity and the apparent digestibility of the cell-wall fraction was 69%. Similarly, the caecum/proximal colon had the highest concentrations of SCFA (76-120 mM).

The mean rate of SCFA production at 40-degrees-C in vitro was 31 mmol/l h-1. Assuming that this is representative of daily production rate, 69 kJ/kg d-1 would be made available to the animal. This is 47% of the mean digestible energy intake estimated in free-living animals. Microbial fermentation contributes an important part of the energy budget of U. aegyptius but the effects of variation in body temperature on digestion and fermentation need further consideration.