DIGESTIVE-SYSTEM TRADE-OFFS AND ADAPTATIONS OF FRUGIVOROUS PASSERINE BIRDS

Researchers have historically assumed that short food retention time is a typical trait of frugivorous birds, and our data support this. Because absorptive efficiency is directly related to retention time and absorption rate, we predicted that either rates of nutrient absorption would be higher in frugivores than other birds to compensate for short retention or that absorptive efficiencies would be lower. In the four passerine species that we studied, the more frugivorous species had higher intestinal glucose (but not proline) transport activity than the more insectivorous and carnivorous species and higher intestinal sucrase activity. But compared with those of chickens and hummingbirds, small intestines of frugivorous passerines (four species) did not have high rates of glucose and proline uptake in vitro. In accordance with our prediction, in vivo digestive efficiency of radiolabeled glucose was 92% in frugivorous cedar waxwings (88% for fructose) and 73% in fruit-eating American robins, less than the expected value near 100%. Digestive efficiency for sucrose was even less (62% in cedar waxwings, 0% in American robins). Thus, it appears that the anatomy and physiology of fruit eaters result in less than complete digestion and absorption of sugars. Presumably there is some compensating advantage to short digesta retention that perhaps increases net rate of energy intake. It may lie in the ability of frugivores to process large amounts of fruit per unit time in spite of the constraint gut volume might place on fruit intake.