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Osmoregulation in an avian nectarivore, the whitebellied sunbird Nectarinia talatala: response to extremes of diet concentration

P. A. Fleming^* and S. W. Nicolson

Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa

View larger version (15K): [in a new window]   Fig. 1. Daily water gain (A) and cloacal fluid volume (B) were strongly affected by diet concentration (0.07–2.5 mol l–1 sucrose solutions) and the provision of supplementary drinking water (solid bars; open bars indicate no added water). Water gain (ml day–1) was calculated as the sum of metabolic + preformed + free water volumes (for details see text). Horizontal lines indicate mean body mass in g. Values are means ± 1 S.D. (N=7).

View larger version (16K): [in a new window]   Fig. 2. Estimated evaporative water loss (EWL, i.e. the difference between water gain and cloacal fluid output) for sunbirds fed sucrose solutions of varying concentrations (0.07–2.5 mol l–1), with (solid bars), or without (open bars), supplementary drinking water. EWL is expressed as a volume (A) and as a percentage of total water gain (B). Values are means ± 1 S.D. (N=7).

View larger version (18K): [in a new window]   Fig. 3. Cloacal fluid (CF) osmolality (note the logarithmic scale) as a function of water flux. (A) CF osmolality increased with increasing dietary sucrose concentration. Sunbirds were fed sucrose solutions of varying concentrations (0.07 to 2.5 mol l–1), with (solid bars) or without (open bars) supplementary drinking water. Values are means ± 1 S.D. (N=7). (B) CF osmolality was inversely correlated with CF volume (both logarithmic scales). Values are individual data (N=7) for the 14 diet trials with (solid circles), or without (open diamonds), supplementary water; the regression line is fitted to data from all diet trials (osmolality=253.5[CF volume+1]–1.054). The volume of CF excreted per day is given as log(ml+1) since the log of values <1 yields negative values.

View larger version (17K): [in a new window]   Fig. 4. Total osmotic excretion in cloacal fluid CF (osmolality x CF volume) as a function of water flux. (A) Total osmotic excretion compared with diet concentration and the provision of supplementary water. Values are means ± 1 S.D. (N=7). (B) Total osmotic excretion was positively correlated with CF volume (note log scale), being highest on more dilute diets, when birds have higher water fluxes. Values are individual data (N=7) for the 14 diet trials with (solid circles), or without (open diamonds), supplementary water; the regression line is fitted to data from all diet trials (osmotic excretion = 0.045[CF volume+1]0.563). The volume of CF excreted per day is given as log(ml+1) since the log of values <1 yields negative values.

View larger version (18K): [in a new window]   Fig. 5. Electrolyte (Na+ + K+) outputs in the CF (concentration x CF volume) as a function of water flux. (A) Electrolyte outputs were highest for the most dilute diets, and lowest for concentrated diets without supplementary water, when birds voided minimal volumes of CF. Values are means ± 1 S.D. (N=7). (B) Electrolyte output increased significantly with increasing cloacal fluid volume (note log scale). Values are individual data (N=7) for the 14 diet trials with (solid circles) or without (open diamonds) supplementary water; the regression line is fitted to data from all diet trials (electrolyte output=0.0008[CF volume+1] + 0.0047). The volume of CF excreted per day is given as log(ml+1) since the log of values <1 yields negative values.