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First published online November 19, 2004
Journal of Experimental Biology 207, 4383-4391 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01238

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

Bradley Hartman Bakken^1,*, Todd J. McWhorter², Ella Tsahar³ and Carlos Martínez del Rio¹

¹ Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA
² Department of Wildlife Ecology, University of Wisconsin, Madison, WI 53706, USA
³ Department of Biology, Technion – Israel Institute of Technology, Haifa 32000, Israel

^* Author for correspondence (e-mail: bradley{at}uwyo.edu)

Accepted 11 August 2004

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.

Key words: hummingbird, Selasphorus platycercus, glomerular filtration rate, renal fractional water reabsorption, diurnal variation, phylogenetically independent contrast, nectarivory, glomerular intermittency

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