Herbivorous reptiles and body mass: Effects on food intake, digesta retention, digestibility and gut capacity, and a comparison with mammals

Abstract

Differences in the allometric scaling between gut capacity (with body mass, BM¹·⁰⁰) and food intake (with BM⁰·⁷⁵) should theoretically result in a scaling of digesta retention time with BM⁰·²⁵ and therefore a higher digestive efficiency in larger herbivores. This concept is an important part of the so-called 'Jarman-Bell principle' (JBP) that explains niche differentiation along a body size gradient in terms of digestive physiology. Empirical data in herbivorous mammals, however, do not confirm the scaling of retention time, or of digestive efficiency, with body mass. Here, we test these concepts in herbivorous reptiles, adding data of an experiment that measured food intake, digesta retention, digestibility and gut capacity in 23 tortoises (Testudo graeca, T. hermanni , Geochelone nigra, G. sulcata, Dipsochelys dussumieri) across a large BM range (0.5-180 kg) to a literature data collection. While dry matter gut fill scaled to BM¹·⁰⁷ and dry matter intake to BM⁰·⁷⁶, digesta mean retention time (MRT) scaled to BM⁰·¹⁷; the scaling exponent was not significantly different from zero for species > 1 kg. Food intake level was a major determinant of MRT across reptiles and mammals. In contrast to dietary fibre level, BM was not a significant contributor to dry matter digestibility in a General Linear Model. Digestibility coefficients in reptiles depended on diet nutrient composition in a similar way as described in mammals. Although food intake is generally lower and digesta retention longer in reptiles than in mammals, digestive functions scale in a similar way in both clades, indicating universal principles in herbivore digestive physiology. The reasons why the theoretically derived JBP has little empirical support remain to be investigated. Until then, the JBP should not be evoked to explain niche differentiation along a body size axis in terms of digestive physiology.

Methane output of rabbits (Oryctolagus cuniculus) and guinea pigs (Cavia porcellus) fed a hay-only diet: Implications for the scaling of methane production with body mass in non-ruminant mammalian herbivores

“…The scaling of methane production with body mass adds to the assumption of Clauss and Hummel (2005) that, contrary to previous concepts, an increase in body mass does not necessarily translate into a net digestive advantage (Franz et al 2010c). Nevertheless, given the comparatively low level of methane production in non-ruminant herbivores, it is questionable whether methane output would ever reach a relevant proportion of overall energy intake.…”

Allometry of animal–microbe interactions and global census of animal-associated microbes

“…The scaling of microbial abundance is consistent with previous scaling of animal gut size. Total gut volume has been estimated to scale with animal body mass with exponents of 1.0 to 1.08, depending on the animals selected [5,[39][40][41]. However, counts of microbes per unit volume or mass of gut contents vary over several orders of magnitude, and the proportion of the gut devoted to intensive microbial activities also varies extensively [16,17].…”

Exceptional avian herbivores: multiple transitions toward herbivory in the bird order Anseriformes and its correlation with body mass

“…2008), reptiles (Franz et al. 2011), ungulates (Steuer et al. 2011), and herbivorous mammals more generally (Clauss et al.…”