Mammalian herbivores cannot break down cellulose except by fermentation, and may have Termentation chambers at either end of the gut: ruminants have their principal fermentation chamber in the stomach but horses ferment only in the hindgut. A mathematical model (Alexander, 1991) predicted that foregut fermenters should do better than hindgut fermenters on poor foods, and the reverse on richer. less fibrous foods. Further, the optimum gut for poor foods would have the hindgut fermentation chamber only a little smaller than the foregut chamber. However. it has been claimed that horses do better than ruminants on poor food, and the hindgut of ruminants is much smaller than the rumen.
In this paper, the basic model is modified in ways designed to make it more realistic and the effects are investigated. None of the modifications alters the conclusion that the optimum gut for poor food has a large foregut fermentation chamber. However, the optimum proportions of fore-to hindgut, for poor diets, become more like those of real ruminants when account is taken of the diminishing volume of the food passing through the gut, and of incomplete mixing in the rumen.