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

SUMMARY 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 (–0.1±0.2 ml h–1) and decreased GFR in response to short-term (∼1.5 h) water deprivation. GFR seems to be very responsive to water 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.