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First published online June 29, 2006
Journal of Experimental Biology 209, 2622-2627 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02293

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Metabolic substrate use and the turnover of endogenous energy reserves in broad-tailed hummingbirds (Selasphorus platycercus)

Scott A. Carleton^*, Bradley Hartman Bakken and Carlos Martínez del Rio

Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA

View larger version (19K): [in a new window]   Fig. 1. Experiments consisted of three phases. In phase 1, broad-tailed hummingbirds (Selasphorus platycercus) were fed on a C3 diet for roughly 3 months. Phase 2 began on day 0 when the isotopic composition of the diet was changed from C3 to C4. Phase 2 lasted 20 days. Phase 3 began on day 20, when the isotopic composition of the hummingbirds' diet was shifted back to the original C3. Vertical lines denote the days we measured 13C of exhaled CO2 in fasted and fed birds. We collected exhaled air from hummingbirds that were lightly restrained within a centrifuge tube that had been previously flushed with CO2-free air.

View larger version (12K): [in a new window]   Fig. 2. Broad-tailed hummingbirds (Selasphorus platycercus) maintained mass in phase 1, but lost mass in phase 2. Birds regained mass during the first 10 days of phase 3. Regression lines were drafted from the averages of the intercepts and slopes of the relationships between body mass and time for individual birds. The coefficients of correlation of these relationships were significantly negative in phase 2 (r=-0.85, t7=17.8, P<0.001) and significantly positive in the first 10 days of phase 3 (r=0.81, t7=22.2, P<0.001). The slopes of these relationships (the rate of daily mass change) are reported in the text.

View larger version (26K): [in a new window]   Fig. 3. 13C of expired breath in fasted and fed broad-tailed hummingbirds (Selasphorus platycercus). The dotted gray lines parallel to the x- and y-axes denote the 13C of diets and diet shift days, respectively. (A) After the carbon isotopic composition of the diet was changed from C3 to C4 (day 0 to 19), the 13C of exhaled CO2 in fasted birds changed following one-compartment, first order kinetics with a fractional rate of isotopic incorporation equal to 0.47±0.19 day-1. When the isotopic composition of the hummingbirds' diet was changed from C4 to C3 (day 20 to 43), the 13C of exhaled CO2 in fasted birds changed with a faster fractional rate of isotopic incorporation (0.86±0.16 day-1). Note that the asymptotic value of 13C in the fasted hummingbirds' breath was more negative than that of their diet (represented by dotted horizontal lines). We constructed these curves using the parameters in Table 1. (B) Immediately after a diet shift, the 13C of breath in fed birds changed to resemble closely (but not exactly) that of the new diet. The broken curve is the value predicted for fasted birds and is presented to compare the differences between measurements done on fed and fasted birds.