Hummingbirds arrest their kidneys at night:: diel variation in glomerular filtration rate in Selasphorus platycercus

Small nectarivorous vertebrates face a quandary. When feeding, they must eliminate prodigious quantities of water; however, when they are not feeding, they are susceptible to dehydration. We examined the role of the kidney in the resolution of this osmoregulatory dilemma. Broad-tailed hummingbirds (Selasphorus platycercus) displayed diurnal variation in glomerular filtration rate (GFR). During the morning, midday and evening, GFRs were 0.9 +/- 0.6, 1.8 +/- 0.4 and 2.3 +/- 0.5 ml h(-1), respectively. At midday, GFR increased linearly with increased water intake. During the evening, hummingbirds decreased renal fractional water reabsorption linearly with increased water intake. Broad-tailed hummingbirds appeared to cease GFR at night (4.1 +/- 0.2 ml h(-1)) and decreased GFR in response to short-term (similar to1-5 h) water GFR seems to be very responsive to water deprivation. deprivation in hummingbirds. Although hummingbirds and other nectarivorous birds can consume astounding amounts of water, a phylogenetically explicit allometric analysis revealed that their diurnal GFRs are not different from the expectation based on body mass.