Meeting Abstract
The capacity of animals to assimilate energy depends on both mechanisms of nutrient absorption and gastrointestinal morphology. Flying vertebrates must fuel high metabolic demands while minimizing mass of digesta and gut size. Small birds and bats have shorter small intestines, less small intestine nominal surface area, shorter digesta retention times, and rely to a greater extent on non-mediated paracellular nutrient absorption than similarly sized nonflying mammals. In this talk, I will focus on functional studies of water-soluble nutrient absorption in volant endotherms at the extreme: hummingbirds and migratory passerines. Hummingbirds have exceptionally high mass-specific metabolic rates, and more than 35 years ago Bill Karasov and Jared Diamond showed that they have the highest mediated glucose transport rate amongst vertebrates. In vitro techniques also indicated that passive permeability to glucose was very low. In later in vivo studies, we found that the passive permeability of hummingbird intestines to glucose is much higher than previously reported, suggesting that they must rely partly on passive, non-mediated nutrient absorption to meet their high metabolic demands. Passerine birds migrating long distances arrive at stopover sites to refuel having lost as much as 50% of their initial body mass, including losses to the gastrointestinal tract that may serve as a reservoir of protein catabolised for fuel during flight, and show delays of 2-3 days in regaining mass. We found that a small passerine newly arrived at a migratory stopover had increased paracellular nutrient absorption, which when combined with extended digesta retention time may thus allow these birds to maintain higher digestive efficiency during initial stages of refuelling while digestive organs are rebuilt.
