Modulation of digestive enzyme activities in the avian digestive tract in relation to diet composition and quality
In nature, birds are faced with variable food sources that may differ in composition (protein vs. carbohydrates) and quality (highly digestible material vs. indigestible fiber). Studies in passerine birds and some commercial poultry demonstrate that the gastrointestinal tract can respond to varying diet composition and quality by changing morphology and/or activities of digestive enzymes. However, studies in additional avian species are warranted to understand generalities of these trends. We first fed juvenile mallards (Anas platyrhynchos), chickens (Gallus gallus), and quails (Coturnix coturnix) on either high-carbohydrate or high-protein diets. For the most part, birds fed the high-carbohydrate diet had higher small intestinal and cecal disaccharidase activities (maltase and sucrase). However, only mallards exhibited higher small intestinal aminopeptidase-N (APN) activities when fed the high-protein diet. These results differ from passerine birds, which largely modulate small intestinal proteases, but not disaccharidases. In another trial, we fed Canada geese (Branta canadensis) diets that varied in both their protein and fiber concentrations for approximately 3.5 months. Birds fed the high-fiber diets had significantly longer small intestines and caeca compared to those fed low-fiber diets. Additionally, geese fed the high-fiber diets exhibited lower mass-specific activities of small intestinal sucrase, and higher activities of APN when summed across the small intestine and ceca. Similar to the avian species above, geese fed the high-protein diets did not exhibit flexibility in their small intestinal APN activities. Overall, these experiments demonstrate that responsiveness of the avian digestive tract to diet composition may have phylogenetic or ecological constraints. Studies on other avian taxa are needed to understand these patterns.