Gross intestinal morphometry and allometry in ruminants

Abstract

While some descriptions of ruminants' dietary adaptations suggest that the length of the intestinal tract reflects the proportion of grass or browse in the diet, this assumption has been questioned. We collated data on body mass (BM), as well as small intestine, caecum, colon/rectum, large and total intestine length in 68 ruminant species, and, while accounting for the phylogenetic structure of the dataset, evaluated both allometric scaling and the potential influence of diet, digestive physiology or climate proxies on measures of intestine length. Intestinal length generally scaled to BM at an exponent higher than the 0.33 expected due to geometry. Diet or digestive physiology proxies did not have an influence on any intestinal length measures, though some proxies indicating more arid natural habitats were positively correlated with measures of the large intestine. The relative size of a forestomach compartment, the omasum, was negatively correlated with intestine length. The results indicate that intestine length measures provide little indication of feeding type or digestive physiology, but rather indicate adaptations to aridity. Higher-than-geometry scaling of intestinal length may be related to the necessity of maintaining geometric (or metabolic) scaling of intestinal surface area while keeping gut diameter, and hence the diffusion distances, small. The way in which space trade-offs determine the macroanatomy of different organs in the abdominal cavity, such as the omasum and the intestine, deserves further investigation. K E Y W O R D S digestion, feeding type, fluid throughput, omasum, ruminant

Gross intestinal morphometry and allometry in primates

“…Based on the general geometric relationship between a length and a volume measure, one would expect intestinal lengths to scale approximately with body mass to the power of 0.33. However, three previous data compilations -on mammals in general (Lavin et al, 2008), on mammalian carnivorans (McGrosky, Navarrete, Isler, Langer and Clauss, 2016) and on ruminants (McGrosky et al, 2019) -unexpectedly yielded higher exponents. The explanation for this phenomenon was, to our knowledge, first proposed by Woodall & Skinner (1993), who suggested that animals should evolve so that their intestinal surface retains a geometric or metabolic scaling (i.e., at an exponent between 0.67 and 0.75), but that the diameter of the intestine should scale less-than-geometrically to maintain short diffusion distances, and that hence, to compensate, intestinal length should scale more-than-geometrically.…”

“…For more confidence in the phylogenetic signal of intestinal length measurements, a larger species sample would be required. This would also help to clarify if primates are really different from carnivores, ruminants or mammals in general (Lavin et al, 2008, McGrosky et al, 2016, 2019, which all show a more-than-geometrically scaling of intestine length, possibly to achieve geometrical constancy of absorptive intestinal surface while keeping diffusion distances in the instestine small by increasing the intestinal diameter at less-than-geometrically scaling (Woodall and Skinner, 1993). The large 95% confidence intervals for the scaling exponents in the present study do not allow a clear answer to this question.…”

Intraspecific macroscopic digestive anatomy of ring-tailed lemurs (Lemur catta), including a comparison of frozen and formalin-stored specimens

“…In contrast to the expected geometric scaling of length measurements, several studies found a higher scaling exponent (positive allometry) for interspecific scaling relationships of various intestinal section lengths with body mass in mammals (Woodall and Skinner 1993;Lavin et al 2008;McGrosky et al 2016McGrosky et al , 2019a. The common explanation for this observation, developed to our knowledge by Woodall and Skinner (1993), is that on the one hand, both intestinal volume and surface area (calculated from length and circumference measurements in their study) do indeed scale geometrically with body mass, but that intestinal diameter scales to a lower exponent in order to maintain short diffusion distances from the lumen to the secretory and absorptive surfaces.…”

Mammalian intestinal allometry, phylogeny, trophic level and climate

“…A large number of original studies that investigated digestive tract anatomy came to supportive conclusions, albeit always necessarily on very small datasets [11][12][13][14][15][16], and generally also without accounting for phylogeny. By contrast, large-scale studies that accounted for phylogeny did not confirm an association between diet and intestinal length [17][18][19][20][21] or GIT complexity [22]. Other factors than diet thought to influence GIT anatomy include special adaptations to a volant [17,23,24] or a marine [25][26][27] lifestyle, or the aridity of the habitat [18,28,29].…”