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

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Carnivorous and herbivorous mammals typically achieve very different nutrient digestibilities, and their diets differ distinctively in nutrient content. For example, a typical beef‐based diet fed to cheetah (Acinonyx jubatus) in captivity contains 45% crude protein (CP; all values per dry matter DM) and 37% ether extracts (EE; ‘crude fat’), and the animals achieved apparent digestibilities (aD) for CP and EE of 93 and 94% respectively (Vester et al., 2008). In contrast, a typical diet of lucerne hay and concentrates fed to black rhinoceros (Diceros bicornis) in captivity has CP and EE contents of 15 and 2% DM, respectively, and black rhinos achieve aD CP and aD EE of 66 and 45% (Clauss et al., 2006). However, whether these differences in apparent digestive efficiency reflect differences in the digestive physiology of carnivores and herbivores, or whether they are simply caused by the difference in dietary nutrient content, has not been investigated so far.
Carnivores and herbivores differ in their digestive anatomy and – presumably – digestive physiology. This is most evident with respect to the extent by which they harbour, and depend on, microbes for digestion (Stevens and Hume, 1998; Karasov and Martínez del Rio, 2007; Barboza et al., 2009). From carnivores to omnivores to herbivores, the contribution of microbes to digestive efficiency increases, in particular with respect to plant fibre digestion. At the same time, the morphological structures of the gastrointestinal tract that offer a habitat to these microbes – the so‐called fermentation chambers – become more prominent (Stevens and Hume, 1998; Karasov and Martínez del Rio, 2007; Barboza et al., 2009). In the literature on comparative digestive physiology, these differences are often emphasized. Herbivorous mammals are further differentiated by the location of the fermentation chamber into foregut or hindgut fermenters, and by their digestive physiology into non‐ruminant or ruminant foregut fermenters, and non‐coprophageous and coprophageous hindgut fermenters (e.g. Müller et al., 2013). Comparative analyses of digestive efficiency have traditionally been performed on a single trophic guild, such as within carnivores (Vester et al., 2008; Clauss et al., 2010) but especially with a large number of studies within herbivores (e.g. Foose, 1982; Udén and Van Soest, 1982; Clauss et al., 2008; Müller et al., 2013; Steuer et al., 2014; Hagen et al., 2015b). In particular, differences in fibre digestion and differences in the reduction in overall digestibility due to increasing dietary fibre levels (Demment and Van Soest, 1985; Hagen et al., 2015a) were investigated between herbivore species. Comparisons across the trophic guilds, in contrast, are rare. The most comprehensive data compilation is that of Robbins (1993; p. 296–297), who showed that true protein digestion (see below for a definition) hardly varies between species of different guilds.
In contrast, such comparisons have been made with respect to nutrient requirements, in particular with respect to protein. They indicated metabolic adaptations to dietary niches, such as the particularly high protein requirement of domestic cats and other obligate carnivores (Morris, 2002) or the particularly low protein requirements of nectarivores (Smith and Green, 1987). Correspondingly, differences in digestive efficiency appear intuitive. They could be explained theoretically by ultimate causes, such as an increased efficiency compensating for a low dietary nutrient content (as protein in herbivore diets), or by proximate causes of digestive physiology, such as a decreased efficiency due to high metabolic losses of a nutrient (for example via the excretion of microbial matter in animals relying on microbial fermentation of plant fibre).
The apparent digestibility (aD) measures the difference of the amount of a nutrient that is ingested and the amount that is excreted via faeces (Schneider and Flatt, 1975; Robbins, 1993).
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