Digestion Strategies in Nectar- and Fruit-Eating Birds and the Sugar Composition of Plant Rewards

We present a model for the digestion of sugars in avian frugivores and nectarivores (nectar feeders) and use it to analyze the digestive strategies and preferences of birds. The main feature of the model is the assumption that the intestine of birds works as a steady-state chemical reactor (a storage organ coupled to a tubular digestor) where hydrolysis of sucrose and absorption of monosaccharides occurs. We examine two variants of this basic model. In the "hummingbird model," all the uptake of sugars results from carrier-mediated processes. Under such assumptions, the model predicts that energy-maximizing birds feeding on energy-concentrated foods should exhibit longer food-retention times than birds feeding on energy-dilute foods and that birds should exhibit digestibilities of sugars close to 100%. The optimal throughput times and sugar digestibilities predicted by the hummingbird model are independent of the cost of feeding. In the "frugivore model," the uptake of sugars had a substantial passive-diffusion component. This model predicts that when birds feed on hexoses, optimal retention times and digestibilities of sugar decrease as functions of the increased concentration of sugars in food. In the frugivore model, increasing the cost of feeding increases the time food should be retained in the intestine and, hence, the digestibility of sugars. Both models predict that birds should prefer hexoses to sucrose if the rate-limiting step in the absorption of sugars is sucrose hydrolysis. The ratio of maximal hydrolysis of sucrose to maximal uptake of hexoses required for hydrolysis not to be limiting is much higher in the frugivore model than in the hummingbird model. We suggest that the digestive physiology of birds will be a necessary component in an evolutionary explanation of the patterns of sugar distribution in nectars and fruit pulps.