The maximum attainable body size of herbivorous mammals: morphophysiological constraints on foregut, and adaptations of hindgut fermenters

Abstract

The maximum attainable body size of herbivorous mammals: morphophysiological constraints on foregut, and adaptations of hindgut fermenters AbstractAn oft-cited nutritional advantage of large body size is that larger animals have lower relative energy requirements and that, due to their increased gastrointestinal tract (GIT) capacity, they achieve longer ingesta passage rates, which allows them to use forage of lower quality. However, the fermentation of plant material cannot be optimized endlessly; there is a time when plant fibre is totally fermented, and another when energy losses due to methanogenic bacteria become punitive. Therefore, very large herbivores would need to evolve adaptations for a comparative acceleration of ingesta passage. To our knowledge, this phenomenon has not been emphasized in the literature to date. We propose that, among the extant herbivores, elephants, with their comparatively fast passage rate and low digestibility coefficients, are indicators of a trend that allowed even larger hindgut fermenting mammals to exist. The limited existing anatomical data on large hindgut fermenters suggests that both a relative shortening of the GIT, an increase in GIT diameter, and a reduced caecum might contribute to relatively faster ingesta passage; however, more anatomical data is needed to verify these hypotheses. The digestive physiology of large foregut fermenters presents a unique problem: ruminant-and nonruminant-forestomachs were designed to delay ingesta passage, and they limit food intake as a side effect. Therefore, with increasing body size and increasing absolute energy requirements, their relative capacity has to increase in order to compensate for this intake limitation. It seems that the foregut fermenting ungulates did not evolve species in which the intake-limiting effect of the foregut could be reduced, e.g. by special bypass structures, and hence this digestive model imposed an intrinsic body size limit. This limit will be lower the more the natural diet enhances the ingesta retention and hence the intake-limiting effect. Therefore, due to the mechanical characteristics of grass, grazing ruminants cannot become as big as the largest browsing ruminant. Ruminants are not absent from the very large body size classes because their digestive physiology offers no particular advantage, but because their digestive physiology itself intrinsically imposes a body size limit. We suggest that the decreasing ability for colonic water absorption in large grazing ruminants and the largest extant foregut fermenter, the hippopotamus, are an indication of this limit, and are the outcome of the competition of organs for the available space within the abdominal cavity. Our hypotheses are supported by the fossil record on extinct ruminant/tylopod species which did not, with the possible exception of the Sivatheriinae, surpass extant species in maximum body size. In contrast to foregut fermentation, the GIT design of hindgut fermenters allows adaptations for relative passage acceleration, which explai...

The digestive morphophysiology of wild, free-living, giraffes

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“…Overall the conclusion must be drawn that Clauss et al's (2003A; estimates (13.7*Mb^.18; range 1.4-2.1 days) are the best available for retention time of ingesta in a giraffe. If so, rumen retention time could be as short as 33 hours in a young giraffe and 50 hours in a mature adult male.…”

“…Demment and van Soest (1985), van Soest (1996), Clauss et al (2003A;2009), and Muller et al (2013) have suggested that extraction of energy does not only depend on the size of the gastrointestinal tract. Efficient energy extraction also depends on digestibility and fermentation rate of browse, on reduction in particle size by rumination, a longer retention time of ingesta, and selection for diets of high quality, or optimal combinations of these.…”

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The comparative gastrointestinal morphology of five species of muroid rodents found in Saudi Arabia

“…Certain predictions can be made about gut morphology and measurements relating to diet (Schieck and Millar, 1985) although differences in feeding habits alone do not always translate into differences in GIT morphology as life history, phylogeny and body size also play a role (Langer, 2002;Clauss et al, 2003;Stevens & Hume, 1995). Of the animals studied here, M. rex, M. libycus and D. dasyurus have been reported to be herbivorous (Harrison and Bates, 1991;Nowak and Paradiso, 1983).…”