Test of a reactor-based digestion optimization model for nectar-eating rainbow lorikeets

We tested predictions of a chemical reactor model of digestion of glucose solutions by rainbow lorikeets (Trichoglossus haematodus, 125 g). The optimization criterion was the maximization of net energy gain. The predictions were that residence time in the small intestine would be inversely related to sugar concentration, and extraction efficiency would be inversely related to sugar concentration and generally below 90%. Residence time in the small intestine was indexed by the transit time (TT) of the nonabsorbed marker polyethylene glycol (molecular weight = 4,000), and extraction efficiency was measured by the inert indicator ratio technique. Birds were tested when ingesting 0.4 or 1.2 M D-glucose. All predictions were rejected: TT teas independent of concentration (average = 37 +/- 4 min, n = 7 birds), and extraction efficiency was uniformly very high (98.0% +/- 0.4%) and also not influenced by glucose concentration. At low and high sugar concentration the rates of glucose absorption were similar (average = 4 mmol/h). Extraction efficiency of sucrose solution at one concentration, 1.2 M, was 90.5% +/- 1.0% (n = 6). Stomach-emptying rate appeared slower at high glucose concentration, perhaps owing to negative feedback arising from intestinal receptors. Total (mouth-to-anus) mean retention time was independent of glucose concentration and averaged 88 +/- 12 min. Because the birds were not growing, storing fat, or reproducing, perhaps the assumption about energy maximization was inappropriate. The birds' responses were consistent with the goal of minimizing feeding time.