THE INTEGRATION OF OSMOREGULATION AND ENERGY-BALANCE IN HUMMINGBIRDS

Hummingbirds subsist almost entirely on a liquid diet composed of floral nectar, and, when energy demands are high, they can consume more than three times their body mass in fluid per day. At the same time, however, the hummingbird's high metabolism requires efficient extraction of energy and nutrients from a dilute food source that is passing rapidly through the gastrointestinal (GI) tract. The ability of the hummingbird to efficiently process and excrete such large volumes of water must surely entail structural or functional specializations of the kidney and GI tract. The rate of water flux and nutrient extraction efficiency are also influenced, however, by the animal's feeding behavior. Because meal size affects the passage rate of food through the digestive tract (and, therefore, assimilation efficiency), feeding frequency and the amount of nectar consumed per feeding bout will affect the efficiency of nutrient absorption. Ultimately, the water and nutrient content of the nectar produced by the plants should reflect the ability of the hummingbird pollinator to balance its required intake of energy and electrolytes with its ability to excrete the accompanying water load. The simultaneous regulation of water and energy balance in hummingbirds consequently involves the complex integration of renal and intestinal functions and of these physiological processes with behavior and ecology. The hummingbird is a unique animal: its kidney appears to be structurally similar to that of a reptile, but its rate of water flux is more typical of an amphibian. Nonetheless, it sustains a metabolic level as high as that of any endotherm. The inextricable links among energetics, nutrition, and osmotic regulation in hummingbirds provide a fascinating example of the functional integration of vertebrate organ systems operating at the extreme.