The integration of digestive, metabolic and osmoregulatory processes in nectar-eating birds

Abstract

Nectarivorous birds are represented by three major radiations: hummingbirds, honeyeaters, and sunbirds. These lineages share a number of convergent features in ecology, morphology, physiology, and behavior, and have served as important models in the study of foraging strategies and energetics. Because their diet is rich in water and sugar but poor in nitrogen and electrolytes, nectarivores provide a striking opportunity for evaluation of physiological constraints. My research emphasizes a novel aspect of the water-energy interaction: water overingestion in nectar-eating birds. The dual purpose of my dissertation research was to investigate the physiological mechanisms that allow nectar-eating birds to cope with exceedingly high ingestion of water and to elucidate the consequences of ingesting and processing large quantities of water for energy intake and for the maintenance of balance of important metabolites such as glucose. In nectar-eating birds, water overabundance in food has the potential effect of constraining energy procurement by overwhelming osmoregulatory processes and limiting digestive function. My research has allowed the development of an integrated quantitative description of gut and kidney function under the broad range of water loads and hydration conditions that birds can experience in the wild. Understanding limits to water processing will provide general insights into how animals are designed, on how aspects of design constrain their ecological performance, and into how aspects of design in one physiological system can impose limits on other systems. The osmoregulatory processes of nectar-eating 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. Adopting a broadly comparative approach to understanding the interaction between feeding behavior, digestion, and osmoregulation is pertinent because is unclear whether the many extreme physiological characteristics of hummingbirds that have traditionally been assumed to be associated with a nectar-feeding habit are shared by other nectar-eating birds. In my dissertation research I have begun to examine the similarities, and have found some important differences, in the responses of two major radiations of nectar-eating birds to their sugary and watery diets.