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Modulation of ingested water absorption by Palestine sunbirds: evidence for adaptive regulation

Todd J. McWhorter^1,*, Carlos Martínez del Rio² and Berry Pinshow^3,4

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, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel
⁴ Department of Life Sciences, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel

View larger version (18K): [in a new window]   Fig. 1. Behavioral responses of sunbirds to varying sucrose concentration in food. (A) The rate of sugar intake did not increase significantly with sucrose concentration (rs=0.12, P=0.49, N=35). Mean sucrose intake was 77.17±3 mg h-1 (17.94±0.7 kJ day-1). (B) Volumetric food intake rate declined significantly with dietary sucrose concentration (F1,29=107.0, P<0.0001). The relationship was well described by a power function (r2=0.76) with an exponent that was not significantly different from 1.0 (t=-1.52, d.f.=33, P>0.1). Changes in food energy density from 0.292 mol l-1 to 1.168 mol l-1 sucrose led to an approximately 3.5-fold variation in food (and thus water) intake. The right-hand axis shows food intake in multiples of body mass (5.74±0.07 g, N=4) per 14 h. At low sucrose concentrations, sunbirds consumed 0.8-2.2 times their body mass in food in 14 h of daylight.

View larger version (21K): [in a new window]   Fig. 2. Fractional absorption of ingested water (fW) across the gut of Palestine sunbirds ranged from 0.33 to 1.02 (mean ± S.E.M., 0.59±0.04, N=35) and declined significantly and non-linearly with water intake rate (F1,29=40.03, P<0.0001, r2=0.54). Palestine sunbirds may therefore avoid absorbing up to 64% of ingested water when feeding on dilute nectars. In sunbirds feeding on 584 mmol l-1 sucrose solutions, fW was not significantly different between 15°C (filled circles) and 30°C (unfilled circles; 0.44±0.02 vs 0.43±0.02, respectively; F1,5=0.22, P=0.66).

View larger version (19K): [in a new window]   Fig. 3. Water flux in sunbirds (filled circles) estimated from fractional water turnover rate (K3H) and total body water (TBW) measurements ranged from 112.97 µl h-1 to 463.83 µl h-1 and increased linearly with water intake rate (r2=0.89, F1,29=237.29, P<0.0001). The slope of this relationship was significantly less than 1.0 (slope ± S.E.M.=0.42±0.03, t=22.8, d.f.=33, P<0.001), and significantly lower than that of the same relationship in broad-tailed hummingbirds (unfilled diamonds; slope ± S.E.m.=0.88±0.05, ANCOVAslopes F1,35=27.8, P<0.0001; data for hummingbirds from McWhorter and Martínez del Rio, 1999).

View larger version (18K): [in a new window]   Fig. 4. Water absorption as a function of sucrose assimilation and food intake rate in sunbirds. (A) The volume of water absorbed per mass of sucrose assimilated declined significantly with the sucrose concentration of the diet (r2=0.78; F1,29=106.66, P<0.0001), despite constant sucrose intake, suggesting that sunbirds regulate transepithelial water flux independently of sugar absorption. (B) The absorbed water load (fractional absorption of ingested water x water intake rate) was positively correlated with food intake rate (r2=0.84; F1,29=152.53, P<0.0001).