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
Renal function in Palestine sunbirds: elimination of excess water does not constrain energy intake
Todd J. McWhorter1,*,
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
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Fig. 1. Palestine sunbirds reduced their nectar intake rates in response to
increased sucrose concentration in nectar. Energy intake therefore remained
relatively constant at a level that appeared to be dictated by ambient
temperature, and hence by thermoregulatory demands. (A) Sucrose intake by
sunbirds was not significantly correlated with sucrose concentration in
nectar, despite nectar intake rates that varied 7.2-fold (for 30°C; filled
circles) to 9.5-fold (for 15°C; open circles) between the lowest and the
highest sucrose concentrations. Sucrose intake was 1.6 times greater at
15°C than at 30°C, averaging 119.45±5.08 mg h-1
(27.76±1.18 kJ day-1) at 15°C and 75.28±5.17 mg
h-1 (17.49±1.2 kJ day-1) at 30°C. (B)
Sunbirds consumed significantly less nectar as dietary sucrose concentration
increased. Nectar intake rate was significantly higher at 15°C than at
30°C. We described the relationship between nectar intake and sucrose
concentration using a power function for each temperature separately
(15°C, y=313.24x-1.11,
r2=0.95; 30°C,
y=224.63x-0.93, r2=0.87). The
exponents of these relationships were not significantly different from -1.
When feeding on the most dilute sucrose solution (0.146 mol l-1),
sunbirds consumed between 4 and 6 times their body mass in nectar in 14 h of
daylight, depending on ambient temperature. Note that both axes in B and the
x-axis of A are logarithmic scales.
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Fig. 2. The relationships between the concentration of 14C-labeled
inulin in excreta (disints min-1 µl-1) and time were
well described by exponential functions (r2=0.61-0.99,
N=37). The decline in the concentration of 14C-labeled
inulin in excreta with time therefore followed one-compartment, first-order
kinetics. Data are shown here for two individuals and were
semi-loge transformed for clarity. Analysis was performed on
untransformed data (Motulsky and Ransnas,
1987 ).
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Fig. 3. Glomerular filtration rate (GFR) as a function of rate of water intake and
ambient temperature in Palestine sunbirds. GFR ranged from 820.7 to 3597.31
µl h-1 and was correlated with water intake rate at 15°C
(open circles; y=0.37x+1435.8, r2=0.3),
but not at 30°C (filled circles). Mean GFR was significantly higher at the
higher temperature (1792.4±129.78 vs 2192.48±111.65
µl h-1 for 15 and 30°C, respectively).
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Fig. 4. Fractional water reabsorption (FWR) in the kidney ranged from 0.64 to 0.98
(0.82±0.02, N=29) and decreased significantly with water
intake rate as predicted
(y=-1.6x10-4x+0.91,
r2=0.34). There was no significant effect of ambient
temperature (15°C, open circles; 30°C, filled circles) on FWR as a
function of water intake rate.
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Fig. 5. Glucose and osmotic concentrations in excreted fluid and ureteral urine of
Palestine sunbirds varied with rate of water intake. (A) Glucose concentration
declined significantly with increasing water intake rate, and was
significantly higher in ureteral urine (open squares) than in excreted fluid
(filled diamonds). Glucose concentration was not significantly correlated with
water intake rate when ureteral urine data were considered separately,
probably because of small sample size, particularly at higher rates of water
intake. The relationship between glucose concentration in excreted fluid and
rate of water intake was adequately described by a power function
(y=26.18x-0.62, r2=0.4,
N=31). Glucose concentration in ureteral urine ranged from 0.28 to
10.39 mmol l-1 (2.97±1.05, N=11), and that in
excreted fluid ranged from 0.12 to 3.52 mmol l-1 (0.6±0.12,
N=31). (B) Osmotic concentration declined significantly with
increasing water intake rate, and was significantly greater in ureteral urine
than in excreted fluid. We described the relationship between osmotic
concentration and water intake rate using separate power functions for
ureteral urine and excreted fluid
(y=18045.61x-0.82, r2=0.49,
N=13, and y=1101.14x-0.57,
r2=0.65, N=32, respectively). Osmotic
concentration of ureteral urine ranged from 14.96 to 329 mOsm kg-1
(115.5±25.28, N=13), and that of excreted fluid ranged from
12.33 to 95 mOsm kg-1 (30.82±3.82, N=32). Note that
the scales of all axes are logarithmic.
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Fig. 6. Water intake rate, estimated water load (ingested water that is absorbed in
the gastrointestinal tract plus metabolic water) and urine flow rate as
functions of diet sucrose concentration in Palestine sunbirds. Estimated water
load increases much more slowly with decreasing diet sucrose concentration
than does water intake rate, and roughly parallels urine flow rate. The
ability of sunbirds to modulate the absorption of preformed water in nectar
substantially reduces the water load that must subsequently be eliminated by
the kidney. Data for both temperatures were combined; see text for an
explanation of how variables were estimated. No inferential statistics were
performed on estimated water loads.
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© The Company of Biologists Ltd 2004
