Pancreatic and Intestinal Carbohydrases Are Matched to Dietary Starch Level in Wild Passerine Birds

Evolutionary shifts in diet composition are presumably accompanied by simultaneous changes in digestive physiology. The adaptive modulation hypothesis predicts that activities of digestive enzymes should match the relative levels of their substrates in an animal's diet so that available membrane space and synthetic energy are not wasted on enzymes in excess of need. However, previous studies on captive passerine birds showed high intraspecific phenotypic flexibility only in proteases but not in carbohydrases in response to varying diet composition. In this study, we measured the activities of pancreatic, intestinal, and hepatic enzymes in six wild-caught passerine species. We predicted that if the adaptive modulation hypothesis holds during evolutionary shifts in diet composition in birds, then mass-specific activities of digestive enzymes should be correlated positively with the content of their relevant substrates in species' diets. Whereas mass-specific activities of proteases (aminopeptidase-N, trypsin, chymotrypsin, alanine aminotransferase) were not correlated with estimated dietary protein content, mass-specific activities of all studied carbohydrases (amylase, maltase, sucrase) were positively correlated with estimated dietary starch content. We conclude that activities of carbohydrases but not proteases are evolutionarily matched to diet composition in passerine birds. We hypothesize that the need for nitrogen and essential amino acids can prevent the evolution of a low activity of proteases, even in species feeding on a low-protein diet.