For example, food types can be ranked in terms of increasing amount of material that is refractory to rapid digestion with endogenous enzymes (i.e., localized to the digestive tract), such as plant cell-wall or arthropod cuticle/chitin ( Fig. Variation in food chemistry drives diversification of digestive systemsįeatures of food chemistry ultimately drive diversification of digestive system morphology, physiology, and biochemistry, and account for a lot of the variation among animals in efficiency of digestion (proportion retained/consumed). Two sections focus on enzymatic and transport changes within animals during development and when they switch diets, and the final section is on interactions with natural toxins in foods. Subsequent sections cover mechanisms and patterns of variation across taxa in chemical digestion by animals and their microbiota, and absorption of breakdown products. This overview also introduces the economy of nature as an evolutionary organizing principle that can be used to predict and explain many patterns. We begin with an overview of the architecture of animals’ guts, including a description of simple integrative models that have advanced understanding of how gut size, digesta flow, and biochemical capacity are matched to food intake to achieve efficient nutrient extraction. We include a new analysis of interactions between digestive physiology and naturally occurring toxins because these biochemicals are nearly ubiquitous in foods consumed by wild animals and many of their effects are mediated through interactions with the gut. Where sufficient information is available, phylogenetically informed analyses are included to provide better evidence of evolutionary trajectories and stronger inferences about the adaptive nature of certain traits. Our review complements and updates many earlier reviews ( 248, 249) to provide broader taxonomic coverage, and incorporates increased molecular information to characterize further the mechanistic bases of patterns of change within and across species.
The primary functions considered in this article are the extraction of nutrients and toxins from diverse foods consumed by vertebrates and invertebrates. The gastrointestinal (GI) tract of animals can serve multiple functions including digestion, osmoregulation, and protection (e.g., by detoxification or immune function). Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption Paracellular absorption is important in many birds. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K + rather than Na + as a counter ion). The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. Many species respond to higher food intake by flexibly increasing digestive compartment size. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose).