| dc.description.abstract | Hummingbirds are confronted with an osmoregulatory dilemma: when feeding, their high
rates of water intake require them to eliminate ingested water quickly; yet, during fasts, their capacity to
eliminate water makes them susceptible to dehydration. Indeed, although their food is mostly water, hummingbirds
may even dehydrate when they are feeding on moderately concentrated nectars. What mechanisms
then do hummingbirds use to maintain water balance? To avoid overhydration, hummingbirds rely
on two renal processes, the most important of which is to reduce water reabsorption; however, glomerular
filtration rate (GFR) appears to be responsive to water loading under some circumstances. To mitigate
dehydration during fasts, hummingbirds reduce, even cease, GFR. Although this strategy conserves body
water by reducing losses in urine, rates of evaporative water loss suggest that hummingbirds still dehydrate
during natural, overnight fasts. At present, however, it is unclear if hummingbirds are able to regulate evaporative
water losses in response to hydration status. Of the eight avian lineages that independently evolved
specialized nectar-feeding, the mechanisms of water balance are best understood in hummingbirds.
Although more work in other lineages is needed to understand the evolutionary patterns of water balance
strategies, studies to date have revealed both convergence and diversity in the physiological processes that nectar-feeding birds use to meet the osmoregulatory challenges resulting from their diet. | |
