We tested the hypotheses that small-bodied ruminant browsers have higher weight-specific dry matter intake rates, shorter retention times, and lower digestive efficiency than larger-bodied grazers, and that retention time is inversely related to dietary browse concentration. We compared digestive functions of mule deer (Odocoileus hemionus), mountain sheep (Ovis canadensis), and elk (Cervus elaphus) consuming grass–browse diets and related our comparisons to previously reported diet choices of these species. Consistent with our hypotheses, mule deer had higher intake rates, shorter particulate retention times, and reduced digestibilities relative to mountain sheep and elk. Contrary to our hypothesis, increased dietary browse levels increased retention times in all species. Lignification appeared to render forage particles more resistant to comminution rather than less resistant, and consequently increased retention time rather than decreasing it. Browse prolonged retention time of grass, which in turn resulted in elevated fiber digestion in the grass component of the diet. In contrast to mountain sheep and elk, deer were able to compensate for increased retention time and reduced digestibility of browse diets by increasing gut fill, which allowed constant intake of digestible energy despite declining dry matter digestibility. Our findings suggest highly dynamic relationships among intake, fill, and passage, relationships influenced by physiological and morphological traits of the animal and by the fiber composition of the diet.
Assessing the Jarman–Bell Principle: Scaling of intake, digestibility, retention time and gut fill with body mass in mammalian herbivores
