Scaling of Digestive Efficiency with Body Mass in Neotoma

Previous authors have suggested that larger animals are more efficient at extracting energy and nutrients from food resources because of their more favourable metabolic rate to gut capacity ratio (MR/GC). For herbivores in particular, the utilization of low quality vegetation becomes progressively more difficult with decreasing size because of the time-dependent nature of the microbial fermentation of plant fibre. 2. The relationship between fibre digestibility, a measure of digestive efficiency important to herbivores, and body size was investigated for three species of Neotoma. Animals ranged in mass from 80 to 400 g. A diagram was constructed which reflected allometric predictions, but also incorporated several mediating factors, such as intake and food sorting behaviour. Because of the high correlations between the variables, path analysis was utilized to evaluate the relative importance of each factor in affecting fibre digestibility. 3. Overall, a low correlation (0.105) was found between fibre digestibility and body mass. Path analysis revealed that this low correlation could be partitioned into a relatively strong direct influence of body mass (0.761) and opposing indirect effects through intake and food sorting (-0.563 and -0.094, respectively). 4. While it is clear from the analysis that body mass is of critical importance in influencing fibre fermentation, actual ecological or evolutionary implications are more difficult to assess. It is plausible, for example, that the apparent propensity towards insular gigantism common among small herbivores is at least partially the result of selection acting on digestive efficiency