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Journal of Experimental Biology, Vol 202, Issue 20 2851-2858, Copyright © 1999 by Company of Biologists
JOURNAL ARTICLES |
TJ McWhorter and C Martinez del Rio
Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA. mcwhorte@u.arizona.edu.
Hummingbirds are specialized nectarivores that feed on dilute solutions of sugars with trace amounts of amino acids and electrolytes. Their diets contain excess water that, if absorbed, must be eliminated. It has been hypothesized that in hummingbirds only a small fraction of this dietary water may be absorbed in the intestine. Here, we report the results of experiments designed to examine the relationship between nectar intake and water turnover in hummingbirds. Our results also allow the estimation of water absorption across the intestine and therefore test the hypothesis that ingested water in hummingbirds passes largely unabsorbed through the gastrointestinal tract. We found that fractional and total water turnover increased linearly with water ingestion. At low sucrose concentrations, food intake rates between four and five times body mass per 12 h were not unusual. A simple mass-balance model suggested that 78 % of ingested water was absorbed in the gastrointestinal tract and hence must be processed by the kidneys. However, fractional water absorption was variable and did not appear to be correlated with food or water intake parameters. Our results do not lend support to the hypothesis that the bulk of dietary water passes through the intestine unabsorbed. Although hummingbird kidneys appear well suited to excrete large volumes of dilute urine, rates of energy assimilation in hummingbirds may be constrained by excess water elimination when these birds are feeding on nectars with a low sugar concentration.
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B. H. Bakken, T. J. McWhorter, E. Tsahar, and C. M. del Rio Hummingbirds arrest their kidneys at night: diel variation in glomerular filtration rate in Selasphorus platycercus J. Exp. Biol., December 1, 2004; 207(25): 4383 - 4391. [Abstract] [Full Text] [PDF]
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T. J. McWhorter, C. M. del Rio, B. Pinshow, and L. Roxburgh Renal function in Palestine sunbirds: elimination of excess water does not constrain energy intake J. Exp. Biol., September 1, 2004; 207(19): 3391 - 3398. [Abstract] [Full Text] [PDF]
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P. A. Fleming and S. W. Nicolson Osmoregulation in an avian nectarivore, the whitebellied sunbird Nectarinia talatala: response to extremes of diet concentration J. Exp. Biol., June 1, 2003; 206(11): 1845 - 1854. [Abstract] [Full Text] [PDF]
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T. J. McWhorter, C. M. del Rio, and B. Pinshow Modulation of ingested water absorption by Palestine sunbirds: evidence for adaptive regulation J. Exp. Biol., February 15, 2003; 206(4): 659 - 666. [Abstract] [Full Text] [PDF]
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L. Roxburgh and B. Pinshow Ammonotely in a passerine nectarivore: the influence of renal and post-renal modification on nitrogenous waste product excretion J. Exp. Biol., June 15, 2002; 205(12): 1735 - 1745. [Abstract] [Full Text] [PDF]
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 C. Martinez del Rio, J. E. Schondube, T. J. McWhorter, and L. G. Herrera Intake Responses in Nectar Feeding Birds: Digestive and Metabolic Causes, Osmoregulatory Consequences, and Coevolutionary Effects Integr. Comp. Biol., August 1, 2001; 41(4): 902 - 915. [Abstract] [Full Text] [PDF]
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