A Nutritional Explanation for Body-Size Patterns of Ruminant and Nonruminant Herbivores

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Abstract
The gut capacity of mammalian herbivores increases linearly with body weight. This relationship, coupled with the change in basal metabolism with weight, produces an MR/GC ratio (metabolic requirement/gut capacity) that decreases with increasing body size. Since the retention of a food particle within the gut is proportional to this ratio, the extent to which food particles are digested will be related to body size. A positive relationship probably exists between the fiber content of plant parts and their biomass in the environment. This relationship is used to describe a resource axis on which digestion rate is the scaling variable. In response to this resource axis and metabolic requirements, the fiber content of the diet of herbivores increases with body size. Small herbivores are constrained to rapid passage of ingesta by their high MR/GC ratio. They have evolved hindgut fermentation and feed selectivity on rapidly digestible foods. The reduced MR/GC ratio of medium-sized herbivores allows the evolution of gut structures that selectively delay the passage of ingesta. This process produces more efficient fiber digestion in ruminants than that in nonruminants of similar size. Rumination is advantageous over only a limited range body size. The lower limits of ruminant body size are set by maximal fermentation rates. Ruminants must rely almost entirely on the production of microbial volatile fatty acids (VFA) for energy and postruminal digestion of microbes for other nutrients. With decreasing body size, the increasing rate at which energy must be produced per unit volume of the rumen cannot be matched by a concomitant increase in the fermentation rate of forages. Nonruminants are favored by the more efficient energy transfer of enzymatic digestion in the foregut of low-fiber foods often required by small animals. The upper limits may be imposed by 2 factors. First, rumination rates (grams cell wall ruminated/unit time) increase with body size more slowly than does the cell-wall content of the diet. Second, with increased body size the very low MR/GC ratio allows very long retention times. Because of the small body size of early ruminants, the evolution of the rumen was probably initiated by selection for the detoxification or synthetic capabilities of foregut fermentation. The foregut then was preadapted for development as a structure of the selective delay of forages when the grasslands expanded.

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