The relationship of food intake and ingesta passage predicts feeding ecology in two different megaherbivore groups

Abstract

Digestion, especially of plant material, is a time-dependent process. In herbivores, an increase in food intake is usually correlated to an acceleration of ingesta passage through the gut, and could hence depress digestive efficiency. Therefore, the nature of the relationship between food intake and ingesta passage (i.e. whether the increase in ingesta passage due to the increase in food intake is mild or drastic) should determine the flexibility of the feeding strategy of herbivore and omnivore species. Using two megaherbivore groups, the elephants and the hippopotamuses, as examples from opposing ends of the range of potential adaptations to this problem, we demonstrate that the species-specific relationship of food intake and ingesta passage can precisely predict feeding ecology and activity budgets. In hippos, the distinct acceleration in ingesta passage due to increased intake limits the additional energy gained from eating more forage, and explains the comparatively low food intake and short feeding times generally observed in these animals. In elephants, increased food intake only leads to a very moderate increase of ingesta passage, thus theoretically allowing to optimize energy gain by eating more, which is in accord with the high food intake and long feeding times observed in these animals. We suggest that the characterization of the intake-passage relationship in herbi-and omnivorous species is of much higher ecological relevance than the determination of a supposedly species-specific ''passage time/mean retention time''.

Is there an influence of body mass on digesta mean retention time in herbivores? A comparative study on ungulates

“…Such a significant 392 negative correlation was only found for ruminants, but not for hindgut fermenters. In insensitivity of MRT to an increase in intake has been considered as a major trait in digestive 398 strategies of herbivores (Clauss et al, 2007b), as appears evident in the group of equids and 399 elephants in this study (Fig. 5).…”

No evidence for directional evolution of body mass in herbivorous theropod dinosaurs

“…For example, holding metabolic rates steady, the decreased surface areato-volume ratio of large body size permits a higher body temperature with lower energy expenditure (gigantothermy) [10]. Likewise, increased gut volume (or, by proxy, BM) maximizes digestibility of fibrous plant material through elongation of the gastrointestinal tract and longer gut retention times in living herbivores [1,3,11,12], despite potential reductions in gut surface area-to-volume ratio [8] and potential intake limits [13]. Given this relationship, one would expect increasing body size to pose a selective advantage during the evolution of herbivorous tetrapods, and such a pattern has indeed been speculated for a myriad of taxa, including Palaeozoic amniotes [14], extant lizards [4,15], Palaeogene and modern mammals [16,17], and non-avian dinosaurs [6,18,19].…”

Decreasing methane yield with increasing food intake keeps daily methane emissions constant in two foregut fermenting marsupials, the western grey kangaroo and red kangaroo

“…A number of studies have demonstrated the negative correlation between DMI and MRT in ratites (Frei et al, 2015c), sloths (Vendl et al, 2015), ruminants (Clauss et al, 2007;Hammond et al, 2014) and macropods (Munn et al, 2008). In essence, a higher food intake leads to a faster passage of digesta through the digestive tract, mostly because of the limited capacity of the gut to expand.…”