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WATER AND ELECTROLYTE HOMEOSTASIS

Gastrointestinal and renal responses to water intake in the green-backed firecrown (Sephanoides sephanoides), a South American hummingbird

¹Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming; ²Facultad de Ciencias, Departamento de Ciencias Ecológicas, Universidad de Chile, Casilla, Santiago, and; ³Facultad de Ciencias Biológicas, Center for Advanced Studies in Ecology and Biodiversity, Pontificia Universidad Católica de Chile, Santiago, Chile

Submitted 27 February 2006 ; accepted in final form 8 April 2006

To maintain water balance, nectar-feeding vertebrates oscillate between meeting the challenges of avoiding overhydration and preventing dehydration. To understand how green-backed firecrowns (Sephanoides sephanoides) accomplish this, we examined the response of water-handling processes in the gastrointestinal tract (GIT) and kidney to different rates of water intake during the evening, night, and morning. Fractional water absorption in the GIT was independent of water intake rate (evening: 0.91 ± 0.08; morning: 0.88 ± 0.04). Consistent with this nonregulated water absorption, we found linear increases in water flux, fractional turnover of body water, and the rate of renal water loading as water intake rate increased during both the evening and morning. Despite these relationships, glomerular filtration rate (GFR) was insensitive to water loading (evening: 2.08 ± 0.56 ml/h; morning: 1.84 ± 0.68 ml/h) and less than the allometric expectation (2.92 ml/h). During the evening, fractional renal water reabsorption decreased linearly as the rate of water intake increased. At night, a period of natural fasting for hummingbirds, mean GFR was not different from zero (0.00 ± 0.05 ml/h). These findings indicate that green-backed firecrowns eliminate excess ingested water by decreasing water reabsorption in the kidney; to conserve water, it appears that hummingbirds arrest whole kidney GFR, effectively preventing urinary water losses. After discounting evaporative water losses, our results show that hummingbirds rely principally on their renal system to resolve the osmoregulatory quandary posed by nectarivory.

glomerular filtration rate; glomerular intermittency; osmoregulation; renal water reabsorption; water turnover

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				S. W. Nicolson Water management in nectar-feeding birds Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2006; 291(3): R828 - R829. [Full Text] [PDF]