Digestion-Constrained Optimal Foraging in Generalist Mammalian Herbivores

Optimal diet selection of generalist mammalian herbivores under the influence of a digestion constraint is analyzed using an algebraic model. The energy assimilation rate and the digesta mass are first formulated based on an assumed intake process from multiple food types differing in digestible energy content and digestive turnover time. The optimal diet is then analyzed under the assumption that animals maximize energy assimilation under the constraint of digestive capacity. The analysis suggests that diet preference is changeable according to the intensity of the digestion constraint. With a better intake condition (longer foraging time or more abundant food), the digestion constraint is intensified and diet selection is more oriented towards the digestive process: diet is composed of a smaller number of food types of higher digestive quality (digestible energy content per gross intake per digestive turnover time). In contrast, with a loose digestion constraint, diet selection is more oriented towards the intake process: diet is composed of a larger number of food types of higher handling rate (digestible energy content per handling time). Because diet preference is thus changeable, a priori ranking of food types is not justified in digestion constrained herbivore foraging.