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18 - Foraging and digestion in herbivores
- Edited by D. J. Chivers, University of Cambridge, P. Langer, Justus-Liebig-Universität Giessen, Germany
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- Book:
- The Digestive System in Mammals
- Published online:
- 18 March 2010
- Print publication:
- 21 July 1994, pp 313-314
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Summary
The foraging decisions of animals determine which food items enter their digestive tracts, but it is the digestive tract that determines net energy and nutrient gain from ingested food. The details of digestive function depend upon the physiological and morphological characteristics of an organism; these vary with the food habits, body size and phylogeny for each species. As a result, the morphology of vertebrate digestive tracts and the processes of digestion and absorption of nutrients are complex (Johnson, 1987; Stevens, 1988). Our purpose here is not to summarize the voluminous literature on these subjects for mammals but rather to address some recent developments in the field.
This section focuses on herbivores because the diets of these mammals often contain low concentrations of digestible energy or other nutrients (usually associated with high fibre content) that has led to great variety in the form and function of their gastro-intestinal tracts. The nutritional constituents of a herbivore's diet vary with the species and part of plant eaten, the habitat in which the plants are growing, and the time of year that the plants are harvested. Because most herbivorous mammals eat a variety of plant species and parts, and most do so year around, they must deal with the problem of variable diet quality. Of course, a few specialists in aseasonal environments may use a limited range of higher quality resources (such as nectar or fruit), but when they do so they lose the advantage of having more available food (stems and leaves).
20 - The integrated processing response in herbivorous small mammals
- Edited by D. J. Chivers, University of Cambridge, P. Langer, Justus-Liebig-Universität Giessen, Germany
-
- Book:
- The Digestive System in Mammals
- Published online:
- 18 March 2010
- Print publication:
- 21 July 1994, pp 324-336
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- Chapter
- Export citation
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Summary
The amount of energy and nutrients that herbivorous mammals extract from a diet depends upon the amount of food eaten (intake) and its digestibility, but intake and digestibility do not vary independently. Apparent digestibility for any particular diet depends upon its chemical constituents, its retention time in the gastro-intestinal (GI) tract and the absorptive capacity of the GI tract. The last is a function of epithelial surface area and nutrient transport rates across the epithelium (Karasov and Diamond, 1988). Within limits, forage intake increases as the digestibility of the food decreases; retention time of food within the GI tract decreases as intake increases; and digestibility decreases as retention time decreases (Sibley, 1981; van Soest, 1982; Robbins, 1983). Retention time also decreases as the volume of the GI tract decreases. All of these variables (intake, retention time, GI size and digestibility) can change in response to changes in energy demand or fibre content of the diet (Hammond and Wunder, 1991). We call the relationships among these changes, and their effect on digestible energy (or nutrient) intake, the integrated processing response (IPR).
A wide variety of birds and mammals show an IPR, as indicated by changes in size, morphology and nutrient absorption rates of their GI tract with changes in diet quality (Gross et al., 1985; Karasov and Diamond, 1988; Brugger, 1991 and references therein). The IPR of small mammals is particularly interesting because a considerable body of theory, primarily based on ruminants, argues that small mammals (adult mass less than 1 kg) should not be able to meet their energy requirements on high fibre diets, such as mature grass (Demment and van Soest, 1985).