1. Two contrasting explanations of niche separation in ruminants emphasize the importance of body mass-related trends in energy requirements and food-processing ability (the diet-quality assumption) and of physiological adaptations to diet type (grass or browse; the diet-type assumption).\ud
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2. The quality of the diet consumed in the dry season by 21 species of African ruminants was estimated from observed fermentation rates in the rumens of culled animals using a simulation model of digesta kinetics. The model predicts the effect of food composition (including tannins) on a number of variables on which previous arguments about physiological adaptations have been based: cellulolytic activity, digestion kinetics and volatile fatty acid proportions.\ud
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3. The predicted diet quality (potential digestibility) varied from 0.90 in small animals to 0.75 in large ones, and was negatively related to body mass with a shallow allometric exponent(- 0.035 +/- 0.0109).\ud
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4. The allometric exponent scaling body mass to predicted energy assimilation rate was 0.852+/-0.0474. The abundance and quality of food available were apparently adequate to allow greater net energy intake, relative to requirements, by large animals than small ones.\ud
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5. No difference existed in the predicted potential digestibility or net energy yield from the diet of browsing and grazing species after controlling for body mass. Therefore, although the diets differed in their botanical and chemical composition, the end result had little effect on the nutritional ecology of the animals with different diet types.\ud
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6. The model predicted that the level of cellulolytic activity of microbes in the rumen can affect the energy obtained from the diet. Microbial adaptation would cause browsers to suffer a penalty on switching from browse to grass, thereby reinforcing diet choice.\ud
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7. The presence of tannins in the diets of browsers was predicted to decrease rumen fermentation rate and increase the digestibility of the diet required to match observed fermentation rates
