9514351 Beuchat Hummingbirds have the highest metabolic rate of any vertebrate, and they fuel this tremendous metabolism almost entirely with the sugar found in nectar. Supporting a high metabolic rate on liquid diet presents these birds with some special physiological problems. Most obviously, hummingbirds must consume tremendous quantities of nectar, up to five times the bird's own body mass per day! Then, the gastrointestinal tract must be able to extract, with great efficiency, the sugar and other nutrients in the nectar as it passes rapidly through the bird. The kidney must also be able to filter and excrete excess water to avoid dilution of body fluids. Finally, because concentrations of salts and minerals in nectar are typically low but are needed by the bird, the amounts excreted in the urine must be minimal. This research investigates the ways in which hummingbirds deal simultaneously with these extreme physiological challenges of fluid, electrolyte, and energy balance. The study focuses not only on understanding fundamental aspects of the physiology of the kidney and intestinal tract in hummingbirds, but also on how these organ systems play coordinated roles in the physiological maintenance of energy and osmotic homeostasis. The work is further extended by considering the integration of renal and intestinal function with hummingbird biology at the organismal level in a series of questions that address the interface of physiology with behavior, morphology, and ecology, and evolution. Much of what comparative physiology has contributed to our understanding of osmoregulation in terrestrial birds and mammals has been based upon studies of the ultimate concentrators, the desert rodents, and only a few species of birds. We know relatively little, by contrast, about the special osmoregulatory problems faced by birds and mammals for which the challenge is an exc ess of water rather than desiccation. This study is unusual in the breadth of its perspective and its simultaneous consideration of processes from the cellular level to that of the animal in the field. It is the first study to closely examine the osmoregulatory physiology of a bird or mammal that is a model organism for the study of diuresis in vertebrates.
