A. HAILEY, R. L. CHIDAVAENZI and J. P. LOVERIDGE

1. Mixed diets are common in vertebrates, and may be explained by several hypotheses including the availability and nutrient composition of different types of food, the presence of toxins, associative effects in digestion or non-nutritional factors such as foraging efficiency or predation. This study investigated diet mixing in the omnivorous tortoise Kinixys spekii, which consumes fungi, vascular plants and invertebrates in the field. Tortoises feeding ad libitum on these three diets had a higher intake of digestible energy from fungi than from leaves or millipedes (69, 42 and 31 kJ kg^-1 day^-1 respectively).

2. Tortoises offered pairwise combinations of foods chose mostly that giving the highest rate of digestible energy intake; 73:27 fungi:leaves, 92:8 fungi:millipedes, and 91:9 leaves:millipedes, by wet mass. Nevertheless, some of the food giving a lower energy intake was eaten.

3. Tortoises feeding ad libitum on a mixed diet (90:10 leaves: millipedes by wet mass) had similar food intake rates and gut retention times to those feeding on leaves. The digestibility of energy was lower for the mixed diet than for diets of leaves or millipedes; a significant negative associative effect.

4. The rate of intake of dry mass did not differ between the three pure diets, the selected diets, or the mixed diet, though this was about 50% higher at 30 ^oC than outdoors, and represents a limitation due to digestion or appetite. Intake of other foods decreased the intake of dry mass of the preferred food, fungi, by an equal amount.

5. The three foods had similar levels of protein as a proportion of dry mass, partly because of the high ash content of millipedes. The digestibility of protein was higher in millipedes, but the rate of intake of total and of digestible protein was in the same rank order as that of energy (fungi > leaves > millipedes). Although the quantity of protein provided no nutritional explanation for inclusion of lower energy foods in the diet, the quality of protein may differ between food types.

6. Herbivorous tortoises are known to face limitations from calcium and sodium balance. Fungi had a lower level of calcium and a lower ratio of sodium:potassiurn than leaves; millipedes had values higher than leaves. Foods that are non-optimal in terms of energy are probably included in the diet for micronutrients such as minerals or essential amino acids.

Key-words: Diet selection, digestive efficiency, energetics, mineral nutrition, protein

Functional Ecology (1998) 12, 373-385