Adaptations of digestive systems in non-ruminant herbivores.

Early in the evolution of mammals some lineages veered away from small body size and insectivory, which characterized the earliest mammals, to become larger and herbivorous (e.g., Romer, 1966). Since plant matter is much more difficult to digest than animal matter, because of the high content of structural carbohydrates, such animals should be larger so as to be able to accommodate the more elaborate gastrointestinal tract required for the digestion (initially by bacterial fermentation) of such food. Such adaptations are worthwhile because of the great abundance of potential plant foods, especially the vegetative parts of plants (leaves of varying developmental stages and associated parts, such as stems). While some mammalian orders have maintained the insectivorous habit, such as the Insectivora, Pholidota and Edentata, and others have become more specialized for faunivory, such as the Carnivora (feloids and cynoids), Pinnipedia and Cetacea, retaining small body size or simple gastrointestinal tract, or both, many mammalian orders have evolved to large body size and folivory, such as the Perissodactyla (hippomorphs and ceratomorphs), Artiodactyla (suines, tylopods and pecorans), Proboscidea and Sirenia. This has involved the dramatic enlargement of either the caecum and beginning of the colon or, more recently, as the quality of foliage improved, of the stomach, usually with some caeco-colic enlargement (e.g., Moir, 1968; Janis, 1976; Chivers & Hladik, 1980; Langer, 1987). Some folivores, such as the Rodentia, Lagomorpha and Hyracoidea, have mostly retained small body size with accompanying problems for digestion, such as the need for coprophagy (caecotrophy). Much rarer has been occupancy of the ecological ‘middle ground’, in terms of abundance and digestibility, offrugivory. Among mammals, only the primates and some bats have competed with the successful avian consumers of plant reproductive parts, flowers and fruit. The success of primates seems to lie in their relative lack of specialization, and, while the majority are frugivorous, the smaller ones are faunivores and the larger ones are folivores (with either stomach or caeco-colic enlargement). None are exclusive frugivores, since certain amino acids do not occur in fruit; the smaller species supplement fruit with animal matter, the larger ones with foliage, in all cases showing convergence anatomically on the specialists in other orders. Since no primate, or other mammal (to our current knowledge) consumes significant quantities of animal matter and foliage (because of anatomical and physiological incompatibility), and since mammals with such a variable diet based on fruit do not form