Gastrointestinal (GI) morphology plays an important role in nutrition, health, and epidemiology, yet limited data on gastrointestinal 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 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 correlations between the various lengths of the liver and appendix (p = 0.0174) and with the colon (p = 0.0494), as well as between the small intestine and colon (p = 0.0445), 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 (p = 0.0403). This finding supports the female canalization hypothesis, specifically, increased female investment in the digestion and absorption of lipids.

We measured the length of internal organs to the nearest 0.01 cm using sliding calipers, spreading calipers, ruler tape, rulers, and string. We also measured the body length of each specimen and calculated the volume of the liver using the formula for a cone. 

All data were analyzed in the R environment.