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First published online August 23, 2004
Journal of Experimental Biology 207, 3391-3398 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01169

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Renal function in Palestine sunbirds: elimination of excess water does not constrain energy intake

Todd J. McWhorter^1,*, Carlos Martínez del Rio², Berry Pinshow³ and Lizanne Roxburgh³

¹ Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA
² Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA
³ Mitrani Department of Desert Ecology, Jacob Blaustein Institute for Desert Research, and Department of Life Sciences, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990 Midreshet Ben-Gurion, Israel

^* Author for correspondence at present address: Department of Wildlife Ecology, 226 Russell Labs, 1630 Linden Drive, University of Wisconsin-Madison, Madison, WI 53706, USA (e-mail: tjmcwhorter{at}wisc.edu)

Accepted 30 June 2004

Although the renal responses of birds to dehydration have received significant attention, the consequences of ingesting and processing large quantities of water have been less studied. Nectar-feeding birds must often deal with exceptionally high water intake rates in order to meet their high mass-specific energy demands. Birds that ingest large volumes of water may either eliminate excess water in the kidney or regulate the volume of water absorbed in the gastrointestinal tract. Because water absorption in the gastrointestinal tract of Palestine sunbirds (Nectarinia osea) decreases with increasing water ingestion rate, we predicted that glomerular filtration rate (GFR) in these birds would not be unusually high in spite of large ingested water loads. When feeding on dilute sucrose solutions, sunbirds ingested between 4 and 6 times their body mass in nectar per day, yet they were able to compensate for varying nectar energy density and increased thermoregulatory energy demands with no apparent difficulty. GFR was lower than predicted (1976.22±91.95 µl h^-1), and was not exceptionally sensitive to water loading. Plasma glucose concentrations were high, and varied 1.8-fold between fasted (16.08± 0.75 mmol l^-1) and fed (28.18±0.68 mmol l^-1) sunbirds, but because GFR was low, glucose filtered load also remained relatively low. Essentially the entire glucose filtered load (98%) was recovered by the kidney. Renal fractional water reabsorption (FWR) decreased from 0.98 to 0.64 with increasing water intake. The ability of Palestine sunbirds to reduce the absorption of ingested water in the gastrointestinal tract may resolve the potential conflict between filtering a large excess of absorbed water in the kidney and simultaneously retaining filtered metabolites.

Key words: Palestine sunbird, Nectarinia osea, glomerular filtration rate, nectar, glucose, osmoregulation, water balance, kidney, renal function

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