Aspects of food comminution in ungulates and their consequences for energy budget

While green plant material represents the most commonly available food resource in terms of quantity, its nutritional quality poses considerable challenges. For chemical digestion, the lack of endogenous enzymes to digest the fibrous parts of plants (basically cell wall) is overcome in all major vertebrate herbivore lines by the development of large fermentation chambers within the digestive tract hosting symbiotic microbes (Stevens & Hume 1995). However, physical resistance of the material ingested as grass or browse represents another, equally relevant challenge herbivores are confronted with (Welch 1982). Reduction of particle size and disintegration of plant material in terrestrial herbivores is largely depending on the chewing process (Poppi et al. 1980, Murphy & Nicoletti 1984), which has been described to be responsible for about 80% of particle size reduction in forage like ryegrass or alfalfa in cattle (McLeod & Minson 1988). Comminution can be considered to be important because smaller ingesta particles will allow a denser packing of the digesta and therefore a higher intake. Even more important, it is of crucial significance for the speed of fermentation (Bjorndal et al. 1990); in contrast to cells without a cell wall, access to nutrients (cell solubles and cell wall itself) for microbes and consequently the host is depending on mechanical disruption of the fibrous cell wall as the major barrier (Fig. 6.1). Because smaller particles are fermented faster, animals that achieve a higher particle size reduction can afford shorter digesta retention times. This is evident in a comparison of reptiles and mammals (Fritz et al. 2010), but also within mammalian herbivores (Clauss et al. 2009b). Degree of comminution can be regarded as an indicator for the elaborateness of the food processing system and also the level of metabolism of herbivores. Again, this is most obvious in herbivores with a low level of energy metabolism (brachymetabolic) like tortoises and herbivorous lizards, which show little to no particle size reduction beyond bite size (Fritz et al. 2010). At least some herbivorous dinosaurs, for which level of metabolism is a matter of discussion, were capable of some food comminution, e. g. ceratopsids and some ornithopods (Norman & Weishampel 1985, Weishampel & Jianu 2000). In birds, the gizzard is capable of comprehensive particle disintegration (Moore 1999, Fritz et al. 2011), while among extant mammals, a more or less comprehensive comminution by elaborated teeth is present in all lineages specialized on herbivory. Mammalian dentitions and chewing apparatus must surely be considered key innovations, contributing significantly to mammalian success in the large herbivore niche observed today (Ungar 2010, Berkovitz & Shellis 2018). While in general all mammalian herbivores have developed an elaborate food comminution system, relevant differences become obvious when looking at it in more detail; important variables like the species-specific level of intake, retention time of food in the gut, digestion and metabolism of mammals influence each other, and food comminution is a crucial variable in this concert (Clauss et al. 2013). Differences between taxonomic groups are obvious and have triggered comparative research on their performance (Udén & Van Soest 1982, Fujikura et al. 1989, Fritz et al. 2009). Besides phylogenetic relationships, feeding preference (browse vs. grass) also takes considerable influence on the process of food comminution (Clauss et al. 2008, Codron et al. 2019). In this contribution the focus will be on the process of food comminution in its effect on energy digestion and energy budget. While overall efficiency of the food comminution process of mammalian herbivores must be considered to be influenced by a concert of different factors, including, for example, the number of erupted teeth in juvenile animals (Grandl et al. 2018), effectiveness of Aspects of food comminution in ungulates and their consequences for energy budget