A TEST FOR PASSIVE ABSORPTION OF GLUCOSE IN YELLOW-RUMPED WARBLERS AND ITS ECOLOGICAL IMPLICATIONS

In an earlier study, we found that yellow-rumped warblers had in vitro active uptake rates of D-glucose that were only a few percent of the glucose absorption rate achieved at the whole-animal level. Here we us ed a pharmacokinetic technique to test whether a substantial amount of sugar can be absorbed passively. We used yellow-rumped warblers (Dend roica coronata), known for their seasonal frugivory, freely feeding on a synthetic mash formulated with naturally occurring concentrations o f D-glucose. Birds absorbed 89.8% +/- 1.0% (SE) of the D-glucose in th e mash. When fed the same mash with trace-labeled H-3 L-glucose, the s tereoisomer that does not interact with the intestinal Na+-glucose cot ransporter, H-3 appeared in plasma, an indication that this stereoisom er of glucose was absorbed. We used H-3 levels in plasma and excreta i n a pharmacokinetic model to calculate L-glucose extraction efficiency (i.e., the percent absorbed). Calculated mean extraction efficiency f or the passively absorbed L-glucose averaged 91% +/- 23%. Our finding of considerable passive absorption reconciles the in vitro and in vivo results for D-glucose absorption and is in concert with results from five other avian species. The passive pathway appears to provide birds with an absorptive process that call respond quickly to changing lumi nal concentration and that is energetically inexpensive to maintain an d modulate in real time but that may bear a cost. Less discriminate pa ssive absorption might increase vulnerability to toxins and thus const rain foraging behavior and limit the breadth of the dietary niche.