Phenotypic plasticity in laboratory mice and rats: a meta-analysis of current ideas on gut size flexibility

Abstract

Background: Animals’ digestive tract represents the functional link between foraging and the energy available for survival, growth, and reproduction. In addition, gut tissue is one of the most expensive tissues to maintain in terms of energy and protein metabolism. Thus, gut size flexibility has been suggested to be one of the most important physiological adjustments to cope with changes in environmental conditions. Hypotheses: (1) Animals adjust their digestive features to cope with changing environmental conditions, and increase gut size in parallel to increased food consumption. (2) Changes in different energy-demanding factors, such as lactation, pregnancy, and temperature, determine different magnitudes of gut size flexibility. (3) Changes in the amount of undigestible material in the diet mainly affect the size of the fermentative chambers, while changes in energy demands mainly affect the size of the small intestine. (4) The qualities of a demand, such as its relative intensity or duration, affect the amount of digestive flexibility. Data incorporated: We focus on digestive organ length and dry mass of laboratory mice (Mus musculus) and white rats (Rattus norvegicus). We found and reviewed 25 studies that reported a total of 55 comparisons for small intestine length, 67 for small intestine dry mass, 25 for hindgut length, and 26 for hindgut dry mass. Method of analysis: We conducted a meta-analysis in which we calculated the Hedges difference for each comparison, and using mixed-effects models plus heterogeneity tests to evaluate differences among factors. Results: Although gut size flexibility was noted for all the experimental factors evaluated, there was variation in the amount of digestive flexibility determined by each factor. Changes in large intestine length were greater for changes in diet quality than for changes in energy-demanding factors, but this result was not observed for large intestine mass. A clear correlation between gut size flexibility and the number of pups reared during lactation was recorded. Conclusions: The present analysis confirms previous hypotheses on gut size flexibility, providing strong quantitative support for them.