Little differences in digestive efficiency for protein and fat in mammals of different trophic guilds and digestive strategies: data constraints or fundamental functional similarity?

Carnivores do not vary markedly in their digestive efficiency for protein and fat, but whether they resemble other trophic guilds (omnivores and herbivores) in this respect has not been evaluated. We collated data on apparent crude protein (CP) and crude fat (ether extracts, EE) digestibility in 157 mammal species, applying the Lucas principle of regressing digestible nutrient content against nutrient content, where the slope of the regression equation represents the true digestibility and the intercept the metabolic losses per unit dry matter intake. The data collection is marked by the evident uneven distribution of dietary nutrient contents across trophic guilds and differences in the nutrient range by which different species have been evaluated, making statistical interpretation difficult. Results indicate a lower true digestibility of CP in herbivores compared to carnivores, most likely due to a lower digestibility of fibre-bound protein in herbivore diets. Metabolic CP losses did not appear to differ between trophic guilds, but herbivores had higher metabolic EE losses, compatible with the hypothesis that a higher proportion of metabolic CP losses were bound in microbes that also contain lipids in herbivores. Among herbivores, no clear pattern was evident that would indicate a difference in metabolic losses associated with microbes between digestive strategies (coprophagy, foregut/hindgut fermentation). Foregut fermenters had a lower true EE digestibility, possibly linked to the hydrogenation of lipids in their forestomach prior to digestion. The results do not demonstrate clear differences in digestive efficiency and metabolic losses for protein and fat between mammalian trophic guilds and digestive strategies, leading to the hypothesis that the process of CP and EE digestion is not physiologically challenging and hence does not lead to a noticeable differentiation between species or species groups.