Comparative selective retention of particle size classes in the gastrointestinal tract of ponies and goats

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Mechanisms for the selective retention or expulsion of certain particle size fractions are an important feature of the digestive physiology of many herbivores. Sieve analyses of gut contents typically reveal selective retention mechanisms for large particles in the forestomach of functional ruminants (i.e., taxonomic ruminants and camelids) (Clauss et al., 2017; Lechner‐Doll & von Engelhardt, 1989), and selective retention mechanisms for small particles in the caecum of small hindgut fermenters (Björnhag, 1972; Lanyon & Sanson, 1986; Vispo & Hume, 1995). The relevance of such mechanisms lies in the fact that, due to surface–volume relationships, larger particles are digested at a slower rate than smaller particles (Bjorndal, Bolten, & Moore, 1990). Therefore, the retention of large particles in the ruminant forestomach is coupled with the mechanism of particle size reduction via rumination to enhance the digestibility of these particles, whereas the selective expulsion of larger particles in hindgut fermenters has been interpreted as a strategy to rid the digestive tract of difficult‐to‐digest material in order to enhance the animal's potential for higher food intake (Hume & Sakaguchi, 1991).
In horses, a similar observation on selective retention of small particles in the dorsal large colon was published based on sieve analysis of gut contents, which the authors interpreted as an adaptation to maintain large populations of bacteria in the fermentation chamber (Björnhag, Sperber, & Holtenius, 1984; Sperber, Björnhag, & Holtenius, 1992). The equine large intestine has a distinct anatomy with two prominent “narrow points” that suggest a functional interpretation in the sense of retention mechanisms (Figure 1a). Other observations, based on passage experiments with polyethylene particle markers of different sizes rather than sieve analysis, led to the conclusion that large particles are selectively retained in the caecum and colon as compared to small particles (Argenzio, Lowe, Pickard, & Stevens, 1974), as summarized in Figure 1b, and descriptions of colonic motility in horses appeared to match this pattern (Sellers et al., 1982). To our knowledge, the conceptual problem of explaining the value of a selective retention of both, small and large particles (even though at different sites of the digestive tract), has not been solved, and the adaptive value of such a double mechanism remains obscure. Whether these putative mechanisms lead to a selective net retention of any particle size category has not been investigated.
The differential retention of different‐sized particles can be investigated using passage markers of different sizes, either applied via a tube or fistula or offered via food for regular ingestion. In particular, in ruminants and camelids, differences in the retention of different‐sized markers are evident even when animals ingest these markers via food (Dittmann et al., 2015). An example of a typical excretion pattern for markers ingested via food is displayed in Figure 2a, where it is evident that a cattle‐type ruminant (Bos javanicus) retains larger particles of 10 mm for a longer time in its digestive tract than smaller particles of less than 2 mm (Schwarm, Ortmann, Wolf, Streich, & Clauss, 2008). An implicit prerequisite for the interpretation is that differences in marker particle size are not extinguished due to ingestive mastication. The presence of large particles in the forestomach of ruminants, with an overall mean particle size of 8–10 mm in the dorsal rumen contents (Clauss, Fritz et al., 2009), corroborates this concept. In contrast, when feeding the same marker set to a domestic horse (Figure 2b), no difference in the excretion pattern between the different‐sized markers was evident.
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