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Oxygen uptake during post dive recovery in a diving bird Aythya fuligula: implications for optimal foraging models

Roland Parkes¹, Lewis G. Halsey^1,*, Anthony J. Woakes¹, Roger L. Holder² and Patrick J. Butler¹

¹ School of Biosciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
² School of Mathematics and Statistics, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

View larger version (9K): [in a new window]   Fig. 1. The optimal breathing model (Kramer, 1988). The abscissa shows time spent travelling to and from the foraging site to the left of the ordinate, and time at the surface to the right. The ordinate shows the amount of oxygen consumed during travel and gained during surface periods. The value ts* denotes the optimal surface time for the diver, according to the model, in terms of maximising the proportion of time at the foraging site.

View larger version (11K): [in a new window]   Fig. 2. Model, as in Fig. 1, incorporating a biphasic oxygen uptake curve (Walton et al., 1998). Due to the initial rapid resaturation of oxygen into the respiratory oxygen stores upon surfacing, a biphasic oxygen resaturation curve means that a range of dive depths (d1-d3), associated with travel time (tT), have identical optimal surface times (ts*).

View larger version (22K): [in a new window]   Fig. 3. Diagram of the experimental apparatus showing a tufted duck in an enclosed section of a large dive tank. For further details, see Materials and methods.

View larger version (23K): [in a new window]   Fig. 4. Charts to show the methodology of biphasic modelling using broken stick analysis. (A) VO2 data for four tufted ducks were smoothed and plotted on a logarithmic axis. The break point was arbitrarily chosen at 6 s. Linear regression was performed up to and including the break point (Linear phase 1) and also on all remaining data points (Linear phase 2). Regression parameters were then used to determine the point of inflection (C) where the two lines intersect. (B) VO2 data for four tufted ducks were regressed onto the two regression lines shown (biphasic and linear). The biphasic regression line was calculated as shown in Fig. 4A. Commonly in this type of analysis the biphasic point of inflection (C) was not identical to the selected break point.

View larger version (23K): [in a new window]   Fig. 5. Bar chart showing mean oxygen uptake for four tufted ducks during the first 14.5 s post surface (values are means + S.E.M.). The line graphs show the cumulative oxygen gain curve for short (<16.0 s; solid line) and long dives (16.0 s; broken line) for the first 14.5 s post dive.

View larger version (15K): [in a new window]   Fig. 6. Comparison of the mean oxygen uptake curve of four tufted ducks at 5 s, 10 s and 15 s surface duration ± S.E.M. after short dives and long dives.

View larger version (18K): [in a new window]   Fig. 7. Histogram showing the calculated residual variances with the break point every 0.25 s for the first 6 s post dive for long dives of four tufted ducks. The break point with the lowest residual variance occurred 3.00 s post dive and was used to define the biphasic regression line. See statistics in Materials and methods for details of the biphasic determination.

View larger version (13K): [in a new window]   Fig. 8. Chart to show biphasic modelling for long duration dives during the first 15 s post surface for four tufted ducks. The smoothed VO2 data (open circles) have been log-transformed. Least-squares regression is then performed using the linear and the biphasic regression lines. Phases 1 and 2 of the biphasic regression line can be seen with the point of inflection at 3.28 s post surface.

View larger version (21K): [in a new window]   Fig. 9. Mean VO2 observed after long dives of four tufted ducks. Data were smoothed by taking a three-point moving average. As well as the general exponential decrease in VO2 over the first 15 s post surface, there is a cyclical aspect to the data. This is highlighted subjectively on the graph (black bars), and is believed to correspond to expiration (peak) and inspiration (trough), occurring at frequencies similar to those recorded by Butler and Woakes (1979) in a diving pochard (Aythya ferina). The arrow indicates the point of inflection for biphasic uptake.

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