Coping with a Watery Diet: Integration of Metabolic, Digestive, and Osmoregulatory Processes

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The maintenance of water and electrolyte balance can pose a serious challenge for terrestrial animals. Consequently, the strategies used by terrestrial animals to conserve water have been well studied. This proposal emphasizes another, much less studied, aspect of osmoregulation in land animals. Unlike many animals living in dry environments, nectar feeding animals must cope with excessive water ingestion. Nectar-feeding birds respond to variation in the sugar concentration of floral nectar by modulating how much they eat. They decrease ingestion as sugar concentration increases. The intake response of nectar-feeding birds to sugar concentration has the consequence that at low to moderate sugar concentrations the birds ingest phenomenal amounts of water. The drinking rates exhibited by nectar-feeding birds would lead to pathological consequences in other terrestrial vertebrates. This project investigates how nectar-feeding birds cope with this apparent polydipsia (water over-ingestion). Hummingbirds can cope with natural voluntary polydipsia through two non-exclusive mechanisms: 1) They can avoid absorbing water in the intestine, and 2) they can get rid of ingested and absorbed water by producing copious urine. A primary objective of this project is to determine the relative importance of these two mechanisms. Because birds mix urine and feces in the cloaca, achieving this objective is not easy. A minimally invasive method that relies on a pharmacokinetic mathematical model was designed to measure the fraction of ingested water that is absorbed in the intestines of unrestrained birds. Preliminary data suggests that in hummingbirds most of the water ingested is absorbed in the intestine. Consequently, in these birds the task of disposing of ingested water seems to be assigned to the kidneys. The rate at which the kidney filters plasma is a good example of a trait whose magnitude is determined by many potentially competing demands. The need to conserve water and the need to eliminate metabolic by-products can be in discord. In this proposal another potential conflict is identified: The need to dispose of absorbed water is better served by high glomerular filtration rates, but these can be accompanied by the loss of valuable, albeit easily filterable, metabolites like glucose. Hummingbirds exhibit extremely high plasma glucose concentrations (several times higher than those of a diabetic human), and presumably show high glomerular filtration rates to get rid of absorbed dietary water. However, the concentration of glucose in the excreta of these birds is extremely low. A goal of this project is to elucidate how hummingbirds manage to avoid excreting glucose in the face of high plasma glucose levels and high levels of filtration by the kidney.The project is significant because it will allow development of an integrated quantitative description of kidney function under the range of water loads and hydration conditions that birds experience. Also, understanding the limits to water processing will provide general insights into how animals are designed, on how aspects of design constrain ecological performance of animals, and into how aspects of design in one physiological system can impose limits on other systems. The osmoregulatory processes of nectar feeding birds highlight the relevance of understanding the impact that events taking place in the gut can have for feeding behavior, and renal and metabolic function. Comparative physiologists have emphasized low water availability as a constraint on energy and microhabitat use. Our research will emphasize a novel aspect of the water-energy interaction and fill a gap in our understanding of osmoregulation in land animals.