Digestion and assimilation of plant material

The large amounts of fixed carbon in plant materials mean that they are potentially rich sources of energy. It is other components of the diet (e.g. nitrogen) that are more likely to be limiting. Yet most of that energy is only directly available to consumers if they have enzymes capable of mobilizing cellulose and lignins, whereas the overwhelming majority of species in both the plant and animal kingdoms lack these enzymes. Of all the many constraints that put limits on what living organisms can do, the failure of so many to have evolved cellulolytic enzymes is a particular evolutionary puzzle. It may be that gut-inhabiting, cellulolytic prokaryotes have so readily formed intimate, 'symbiotic' relationships with herbivores (see Chapter 13) that there has been little selection pressure to evolve cellulases of their own (Martin, 1991). It is now recognized that a number of insects do indeed produce their own cellulases but the vast majority nevertheless depend on symbionts.

Because most animals lack cellulases, the cell wall material of plants hinders the access of digestive enzymes to the contents of plant cells. The acts of chewing by the grazing mammal, cooking by humans and grinding in the gizzard of birds allow digestive enzymes to reach cell contents more easily. The carnivore, by contrast, can more safely gulp its food.

When plant parts are decomposed, material with a high carbon content is converted to microbial bodies with a relatively low carbon content - the limitations on microbial growth and multiplication are resources other than carbon. Thus, when microbes multiply on a decaying plant part, they withdraw nitrogen and other mineral resources from their surroundings and build them into their own microbial bodies. For this reason, and because micro-bial tissue is more readily digested and assimilated, plant detritus that has been richly colonized by microorganisms is generally preferred by detritivorous animals.

In herbivorous vertebrates the rate of energy gain from different dietary resources is determined by the structure of the gut - in particular, the balance between a well-stirred anterior chamber in which microbial fermentation occurs (AF), a connecting tube in which there is digestion but no fermentation (D), and a posterior fermentation chamber, the colon and cecum (PF). Models of such three-part digestive systems (Alexander, 1991) suggest that large AF, small D and small PF (e.g. the ruminant) would give near-optimal gains cellulases, which most animals lack the gut structures of herbivorous vertebrates from poor-quality food, and that large PF, as in horses, is more appropriate for food with less cell wall material and more cell contents. For very high-quality food (a very high proportion of cell contents and little cell wall material) the optimum gut has long D and no AF or PF.

Elephants, lagomorphs and some rodents eat their own feces and so double the distance traveled by the food resource through the digestive system. This allows further fermentation and digestion but may also allow time for dietary deficiencies (e.g. of vitamins) to be made good by microbial synthesis. These issues are picked up again in Section 13.5.

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