Comparative omasum anatomy in ruminants: Relationships with natural diet, digestive physiology, and general considerations on allometric investigations

Abstract

The omasum is the third forestomach compartment of pecoran ruminants. It is assumed that the re-absorption of fluid present in the forestomach digesta (that facilitates particle sorting, digestion, and harvest of microbes) is its main function, so that less diluted digesta is submitted to enzymatic digestion in the lower digestive tract. Here, we evaluate measures of omasum size (representing 84 ruminant species in the largest data set) against body mass and proxies of the natural diet (% grass) or forestomach physiology (fluid throughput), using phylogenetically controlled models. The origin of specimens (free-ranging or captive) did not have an effect in the data set. Models with the best support invariably either included %grass or a physiology proxy in addition to body mass.These effects were not necessarily additive (affecting the intercept of the allometric regression), but often indicated a change in the allometric body mass-exponent with diet or physiology. Only models that allowed an influence on the allometric exponent yielded basic exponents compatible with predictions derived from geometry. Species that include more grass in their natural diet, or that have a "cattle-type" physiology marked by a high forestomach fluid throughput, generally have larger omasa. However, the existence of outliers, as well as the overall data pattern, suggest that this is not an obligatory morphophysiological condition. Circumstantial evidence is presented leading to the hypothesis that the comparatively small and less complex omasa of "moose-type" species do not necessarily represent an "original" state, but may be derived from more complex states by ontogenetic reduction and fusion of omasal laminae.

Mammalian intestinal allometry, phylogeny, trophic level and climate

“…The scatter evident from plots recommend that while macroevolutionary trends can be stated, they should not be considered fixed biological laws, and that results may depend to a large extent on the size and composition of the investigated sample. The typical constraints of large literature compilations apply [52] that are not re-iterated here. Given the state of both the published literature and the information available on dissected specimens, it was not possible to account for the diet actually ingested by the animals either within the last months before measurements, or during their ontogeny.…”

Gross intestinal morphometry and allometry in ruminants

“…The typical constraints of long‐term anatomical data collections, outlined in detail in Ehrlich et al (), apply to the present study and shall not be repeated here. While in general, GLS and PGLS models yielded similar results in the present study, this was not the case for all of the investigated models and datasets, indicating that in spite of the considerable number of species included, the results were sensitive to small changes in sample size (and hence species composition).…”

“…The second proxy is the selectivity factor in the reticulorumen (SF RR), that is, the ratio of small particle to fluid retention times, with higher values indicating a higher fluid throughput relative to particle retention, typical for “cattle‐type” ruminants (Dittmann et al, ). Third, the relative size of the omasum serves as an indicator of the degree of fluid throughput through the reticulorumen and the subsequent necessity for water reabsorption, with higher values typical for “cattle‐type” ruminants with a high fluid throughput through the reticulorumen (Ehrlich et al, ). These proxies were used to test the prediction that “cattle‐type” ruminants and other grazers have comparatively longer small intestines.…”

“…Sources: 1 Dittmann et al (); 2 Meier, Schmuck, Meloro, Clauss, and Hofmann (); 3 Zerbe et al (); 4 Gagnon and Chew (); 5 Bagchi, Goyal, and Sankar (); Jaipal (); 6 Campos‐Arceiz, Takatsuki, and Lhagvasuren (); 7 Tomas and Salis (); Orrell (); 8 Serbent, Periago, and Leynaud (); 9 Jones et al (); 10 Clauss, Lechner‐Doll, and Streich (); 11 Clauss, Hofmann, et al (); Clauss, Reese, and Eulenberger (); 12 Ehrlich, Codron, Hofmann, Hummel, and Clauss ().…”

“…Only the percentage of grass and climate variables of the natural habitat were available for all species, reducing the datasets when other variables were added to the models. Following the detailed considerations in Ehrlich et al (), we also assessed models that included the respective interaction terms. In addition to generalised least squares (GLS) regressions, we also performed phylogenetic generalised least squares (PGLS) analyses using a mammalian supertree (Fritz, Bininda‐Emonds, & Purvis, ) pruned to include the relevant taxa in our dataset, to account for any phylogenetic influence on these allometries.…”