DIETARY FLEXIBILITY AND INTESTINAL PLASTICITY IN BIRDS - A FIELD AND LABORATORY STUDY

The adaptive modulation hypothesis posits that the expression of diges tive proteins should be modulated in response to intake of their respe ctive substrates. A corollary of this hypothesis suggests that dietary flexibility and digestive plasticity should be correlated. We examine d these two hypotheses in two granivorous Chilean birds (Zonotrichia c apensis and Diuca diuca) that differ in dietary breadth. D. diuca is a strict granivore, whereas Z. capensis also eats insects. In field-cau ght birds, the activity of the intestinal dipeptidase aminopeptidase-N was positively correlated with intake of insects in Z. capensis but n ot in D. diuca. This is the first field documentation of maturation of intestinal enzymes by diet in birds. Intestinal maltase and sucrase a ctivities were not correlated with seed (vs. insect) intake in either species. In the laboratory, captive birds of both species exhibited si milar modulation of membrane-bound intestinal hydrolases when fed on s ynthetic diets of contrasting carbohydrate and protein composition. Ma ltase, sucrase, and aminopeptidase-N activities were significantly hig her in birds fed on the carbohydrate-free than those on the carbohydra te-containing diet. Activities of the three enzymes were positively co rrelated. Therefore, this increase probably resulted from nonspecific increases of all enzymes resulting from intake of the carbohydrate-fre e diet. Principal components analysis separating the effect of diet on specific and on nonspecific modulation revealed that diet had a stron g effect on nonspecific activity of intestinal enzymes in both Z. cape nsis and D. diuca. Diet also significantly affected aminopeptidase-N a ctivities when the effect of diet on nonspecific modulation was remove d. Birds fed on the carbohydrate-free, high-protein diet had significa ntly higher specific aminopeptidase-N activities than those fed on the carbohydrate-containing diet. Our results cast doubts on the notion t hat dietary flexibility and the plasticity of the gut's enzymes are ne cessarily correlated and on the general validity of the adaptive modul ation hypothesis.