Intraspecific Trait Variation in the Mammalian Gastrointestinal Tract: A Digestive Dive and Systematic Literature Review.

The gastrointestinal tract (GIT) represents the functional link between food and energy for organisms. It also harbors the gut microbiome, protects against toxins, and eliminates waste, all of which mediate individual health and fitness. The form and function of the GIT can be dynamic, allowing organisms to respond to-and buffer against-rapid changes in their environment and energetic demands. However, knowledge of intraspecific trait variation (ITV) in the GIT is sparse among different taxa and traits (e.g., microstructural traits and physiology). Additionally, the actual mechanics of how these changes occur (e.g., cell proliferation, cellular redistribution) remain largely unknown. This systematic review summarizes the current state of knowledge on mammalian GIT ITV at multiple levels (macroscopic, microstructural, and physiological), speculates as to how global change drivers will affect ITV, and suggests new directions for future work. A comprehensive list of mammal species heretofore examined for GIT morphology and ITV from a total of 260 journal articles or book chapters identified 824 mammal species with quantitative GIT traits available (12% of all mammal species), but only 79 species (1.1%) investigated for GIT ITV. This highlights the increased need to preserve and collect trait data in standardized ways for mammalian GITs, a series of organs typically discarded or unused during specimen preparation. Understanding how wild species utilize GIT ITV to cope with energetic costs will be crucial to predicting how species may fare under rapidly changing environments.