MCWHORTER, T.J.*; HARTMAN BAKKEN, B.; KARASOV, W.H.; MARTINEZ DEL RIO, C.; University of Wisconsin-Madison; University of Wyoming: Hummingbirds fuel high metabolism with both paracellular and carrier-mediated intestinal glucose absorption

Twenty years ago, the highest active glucose transport rate and lowest passive glucose permeability in vertebrates were reported in Rufous and Anna�s hummingbirds (Selasphorus rufus, Calypte anna). These first measurements of intestinal nutrient absorption in nectarivores provided an unprecedented physiological foundation for understanding the foraging ecology of these exceptional creatures. They showed that physiological processes are determinants of feeding behavior. The conclusion that active, mediated transport accounts for essentially all glucose absorption in hummingbirds influenced two decades of subsequent research on the digestive physiology and nutritional ecology of nectarivores. We report new findings demonstrating that the passive permeability of hummingbird intestines to glucose is much higher than previously reported. The fractional absorption of L-glucose (not actively transported in birds) measured using standard pharmacokinetic techniques (59 � 5%), an in vivo test of passive permeability, demonstrates that not all sugar uptake is carrier-mediated in hummingbirds. Even while possessing the highest active glucose transport rates measured in vertebrates, hummingbirds must rely partially on passive non-mediated intestinal nutrient absorption to meet their high mass-specific metabolic demands. Supported by NSF grants IBN-0216709 to WHK and IBN-0110416 to CMR.