Nutritional limits of gigantism. Allometry of digestive anatomy and physiology in herbivores with special reference to methane losses

[摘要] An evolutionary increase in body mass has often been considered to be linked with advantages in several terms. One prominent concept is that of an increasing digestive efficiency in larger herbivores, which has found widespread application in ecology. The so-called Jarman-Bell principle suggests that larger herbivores have digestive advantage due to allometric principles. This concept is based on a discrepancy between the allometric scaling of gut capacity and gut fill rate (food intake rate). Metabolic requirements and hence the daily food intake are generally a function of their body mass raised to the power of 0.75, whereas the gut capacity scales linearly, i. e. to a higher exponent (1.00). Therefore, more gut capacity per unit food intake is available with size increase, which might result in a longer ingesta retention time, with increasing body mass. As fermentation in herbivores is a time-dependent factor, longer retention times have been linked with higher digestive efficiency. The JBP suggests that larger animals can subsist on a diet of lower quality (because a longer MRT allows a more thorough digestion), while smallbodied animals are constrained to feed on higher quality items (low in fibre) due to their relatively high metabolism and lower relative gut capacity. The results of this study indicate that gut capacity, measured as wet contents of the gastrointestinal tract, scales nearly isometrically with body mass (BM1.00) and daily food intake scales about the power of 0.75 in reptilian and mammalian herbivores. These results support the considerations of the Jarman-Bell principle. In contrary to the general assumption, less scaling of ingesta retention and digestive efficiency with body mass was found in herbivorous reptiles and mammals. These results imply no advantage with size increase. Even disadvantages are associated with increase in body mass, such as ingesta particle size and potentially methane production. The results of this study suggest that methane output, in a broadscale comparison, scales linearly with body mass in reptilian and mammalian herbivores across a large range of body mass. This translates into an increase of energy losses due to methane as a proportion of overall energy intake with body mass. In methane production ruminant species reached the highest level, followed by non-ruminant mammalian herbivores, which had similar levels as reptilian herbivores. The scaling of methane production with body mass adds to the assumption that, contrary to previous concepts, an increase in body mass does not necessarily translate into a digestive advantage. Whatever the causes of the increased methane output in ruminants are, its scaling with body mass may be responsible for the different body mass ranges achieved by ruminant and non-ruminant herbivores and thus represent an intriguing example of a physiological constraint on the evolutionary history of a particular animal group.

[发布机构] University of Zurich

[效力级别] Department of Small Animals

[关键词] Institute of Evolutionary Biology and Environmental Studies;Department of Small Animals;570 Life sciences;biology;590 Animals (Zoology)