Morphometrics of the Avian Small Intestine Compared with That of Nonflying Mammals: A Phylogenetic Approach

Abstract

Flying animals may experience a selective constraint on gut volume because the energetic cost of flight increases and maneuverability decreases with greater digesta load. The small intestine is the primary site of absorption of most nutrients (e.g., carbohydrates, proteins, fat) in both birds and mammals. Therefore, we used a phylogenetically informed approach to compare small intestine morphometric measurements of birds with those of nonflying mammals and to test for effects of diet within each clade. We also compared the fit of nonphylogenetic and phylogenetic models to test for phylogenetic signal after accounting for effects of body mass, clade, and/or diet. We provide a new MATLAB program (Regressionv2.m) that facilitates a flexible model-fitting approach in comparative studies. As compared with nonflying mammals, birds had 51% less nominal small intestine surface area (area of a smooth bore tube) and 32% less volume. For animals <365 g in body mass, birds also had significantly shorter small intestines (20%-33% shorter, depending on body mass). Diet was also a significant factor explaining variation in small intestine nominal surface area of both birds and nonflying mammals, small intestine mass of mammals, and small intestine volume of both birds and nonflying mammals. On the basis of the phylogenetic trees used in our analyses, small intestine length and nominal surface area exhibited statistically significant phylogenetic signal in birds but not in mammals. Thus, for birds, related species tended to be similar in small intestine length and nominal surface area, even after accounting for relations with body mass and diet. A reduced small intestine in birds may decrease the capacity for breakdown and active absorption of nutrients. Birds do not seem to compensate for reduced digestive and absorptive capacity via a longer gut retention time of food, but we found some evidence that birds have an increased mucosal surface area via a greater villus area, although not enough to compensate for reduced nominal surface area. We predict that without increased rate of enzyme hydrolysis and/or mediated transport and without increased passive absorption of water-soluble nutrients, birds may operate with a reduced digestive capacity, compared with that of nonflying mammals, to meet an increase in metabolic needs (i.e., a reduced spare capacity).

Comparative digesta retention patterns in ratites

“…This scenario matches the intermediate position of Rheas in terms of gastrointestinal anatomy, digesta retention, and gut volume. Additionally, this scenario combines all ratites with a comparatively simpler gastrointestinal anatomy in one group with a common ancestor, which is in better line with the finding of Lavin et al (2008) that closely related avian taxa have a similar intestinal morphology.…”

Comparative transcriptomics across 14 Drosophila species reveals signatures of longevity

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The effect of body size on the wing movements of pteropodid bats, with insights into thrust and lift production

“…We used a consensus tree of several recent pteropodid phylogenies (Giannini and Simmons, 2005;Jones et al, 2002;O'Brien et al, 2009), with branch lengths scaled using the method of Pagel (Pagel, 1992). GLM analyses with phylogeny were carried out using REGRESSIONv2 (Lavin et al, 2008) in Matlab. The slope of each regression was compared with that expected under isometry using two-tailed t-tests with four degrees of freedom.…”