Coupling of fermentation and foraging strategies of herbivorous mammals

The advantage of fermentation is studied theoretically in relation to body size and the quality and availability of food plants in habitat. We concentrate specifically on how the adaptation of digestive mode might be coupled with that of diet choice. The retention time of food and the diet composition are assumed to be chosen optimally under the constraints of minimum daily energy gain, the maximum volume of gut contents and the maximum foraging time. The volume of digestive tracts, the basal metabolic rate and the foraging ability are assumed to increase with the body weight. For energy maximizers, fermentation with a long retention time is more advantageous for large sized animals. Low quality and low availability of food and low foraging ability promote the advantage of fermentation. Small mammals are more constrained by the volume of their digestive tracts than by foraging time, and more specialized to food plants with high direct enzymatic digestion rate. In contrast, for large and middle sized animals, both gut volume and foraging time are simultaneously limiting. Fermentation is more likely to be advantageous for gut-content-volume minimizers but not for foraging-time-minimizers.