Hidden diversity: comparative functional morphology of humans and other species

Abstract

Gastrointestinal (GI) morphology plays an important role in nutrition, health, and epidemiology; yet limited data on GI variation have been collected since 1885. Here we demonstrate that students can collect reliable data sets on gut morphology; when they do, they reveal greater morphological variation for some structures in the GI tract than has been documented in the published literature. We discuss trait variability both within and among species, and the implications of that variability for evolution and epidemiology. Our results show that morphological variation in the GI tract is associated with each organ's role in food processing. For example, the length of many structures was found to vary significantly with feeding strategy. Within species, the variability illustrated by the coefficients of variation suggests that selective constraints may vary with function. Within humans, we detected significant Pearson correlations between the volume of the liver and the length of the appendix (t-value = 2.5278, df = 28, p = 0.0174, corr = 0.4311) and colon (t-value = 2.0991, df = 19, p = 0.0494, corr = 0.4339), as well as between the lengths of the small intestine and colon (t-value = 2.1699, df = 17, p = 0.0445, corr = 0.4657), which are arguably the most vital organs in the gut for nutrient absorption. Notably, intraspecific variation in the small intestine can be associated with life history traits. In humans, females demonstrated consistently and significantly longer small intestines than males (t-value₁₅ = 2.245, p = 0.0403). This finding supports the female canalization hypothesis, specifically, increased female investment in the digestion and absorption of lipids.

Keywords: Anatomy; Comparative variation; Dissection; Gastrointestinal morphology; Human variation.

Figure 1. Representative gut diagrams for the fetal pigs (Sus scrofa), frogs (Lithobates catesbeianus), and rats (Rattus norvegicus).

Illustration by Chris Hedstrom.

Figure 2. Mean length (in cm) of small intestine, cecum, and colon in (A) rats (Rattus norvegicus), (B) frogs (Lithobates catesbeianus), (C) fetal pigs (Sus scrofa), and (D) humans (Homo sapiens).

Empty shapes indicate small intestine measurements; filled shapes indicate colon measurements. Previously published data reported from Permezel & Webling (1971), Fisher & Parsons (1950), Toloza & Diamond (1990), Wang et al. (2005), Miller & Ullrey (1987), Treves (1885), Dreike (1894), Bloch (1904), Bryant (1924), Blankenhorn, Hirsch & Ahrens (1955), Underhill (1955), Standring, Borley & Gray (2008), Netter (2014), and Tubbs, Shoja & Loukas (2016). Asterisks indicate medical texts.

Figure 3. The amount of variation detected for (A) small intestine and (B) colon length increases with sample size for all species except fetal pig.

Empty shapes denote previously published studies (Permezel & Webling, 1971; Fisher & Parsons, 1950; Toloza & Diamond, 1990; Wang et al., 2005; Blankenhorn, Hirsch & Ahrens, 1955). Filled shaped denote the present study.

Figure 4. Boxplots by sex for small intestine length, in centimeters.

Model significance is based on α =0.05.