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As for the other model parameters, there was no concrete data available for MR as a function of speed for killer whales and therefore uncertainty regarding this value existed. Of all parameters, the O2 model appeared most sensitive to variation in the slope of MRL vs. speed3,

with slope of the correlation over 15 min periods notably changing according to the variation of this value for the lower tested values. In relation to the sensitivity analyses outcomes it could be argued that for females the slope of MRL vs. speed3 was set too high; the relationship

between EO2 and VO2 over the 15 min intervals was not as strong as for a lower slope of MRL vs.

speed3. However, according to measurements made by Kriete (1995) a captive adult male and female killer whale had a MR of 0.595 and 0.492 LO2·s-1, respectively, during a show, implying

mean VO2 and breathing rate and maximum VT. To produce a same MR using Eq. 10, a male and

speed for during a show; Guinet et al. (2007) measured a maximal speed in free-ranging killer whales of 5.6 m·s-1 while chasing tuna.

According to the equations used in the present study to establish the VO2-speed3 relationship,

CD ranged between 0.0138 and 0.0228 for females, and 0.0111 and 0.0215 for males,

according to their minimum and maximum speed measured over 15 min intervals. Fish (1998) found a theoretical CD of 0.0026 for a Re of 3.65·107 for killer whales, which is 3.8 times as low

as by calculating CD using Eq. 7. If the CD found by Fish (1998) was true, then using Eq.7 would

have yielded a VO2 of 3.8 too large, and the overestimation of energetic requirements

estimated through respiration rate would be even more drastic. However, Guinet et al. (2007), using a similar theoretical approach as Fish (1998) to define MR in killer whales, found a VO2 of

0.02 LO2·kg-1·min-1 for a swim speed of 4.7 m·s-1. This would mean that a female killer whale

(2,800 kg) of the present study had a VO2 of 0.8886 LO2·s-1 for the same swim speed. Whereas,

according to the MRL vs. speed3 slope used in the present study a female swimming at a similar

speed would have a VO2 1.6 times as low (0.5481 LO2·s-1). This aspect indicates once more the

uncertainty in predicting actual MR in killer whales.

The relationship between Re and the CD, as found for killer whales by Fish (1998) and used in

the present study to estimate the slope of MRL vs. speed3, was based on the unsteady wing

theory, accounting for drag by actively swimming movements. Doing so, the CD varied

according to swimming speed, which is related to body swimming movements (Fish, 1998). During other previous studies on drag in swimming animals it was shown that the so-called active drag could be 3-7 times higher than the passive drag (Lighthill, 1971; Fish et al., 1988). Understandably, firm values of drag forces caused by propulsions remain difficult to obtain. Sensitivity analysis of the present study indicated that significant increase of the slope of MRL

vs. speed3, would not improve the correlation between VO2 and speed3 over 15 min intervals.

Data available on MR measured for other cetaceans showed great variation, with all of them being relatively considerably higher than the estimates upon which the O2 model is based and

which were derived using the same method as used by Guinet et al. (2007). Highest estimated speed during the present study was 4.1 m·s-1 and 2.8 m·s-1 for males and females, respectively, with corresponding MR of 0.527 and 0.181 LO2·s-1, respectively. When increasing the slope of

tested during sensitivity analyses (found by Williams et al. (1993) for bottlenose dolphins) O2

store would become so low that a whale would take up O2 maximally during every single

breath without being able to replenish its store, which is unrealistic. Despite the fact that smaller cetaceans are expected to have a higher mass-specific MR compared to larger cetaceans (Costa and Williams, 1999; Boyd, 2002; Costa, 2009), the corresponding extrapolated MR for males and females killer whales of 5.58 and 1.29 LO2·s-1, respectively, for

maximal speed is expected to overestimate true MR. In relation to MR measurements made by Otani et al. (2001) on harbour porpoises a male and female killer whale would exhibit a MR of around 15.867 and 3.961 LO2·s-1, respectively, at the highest measured speed, which is as well

high compared to the corresponding MR found during this study. According to measurements for beluga whales by Shaffer et al. (1997) an adult male killer whale would have a MR of around 1.471 LO2·s-1 during maximal speed. The beluga whale is on average about half the size

of a killer whale (O'corry-Crowe, 2009) and thus has a higher mass-specific MR which could explain a higher slope of MRL vs. speed3 per kg. Though beluga whales are known for being

poorly adapted for swimming at high speed (Shaffer et al., 1997) this still does not explain the elevated MR compared to the ones found during this study for killer whales. On the contrary, MR found for average speed of the killer whales in the present study was 0.1507 LO2·s-1 and

0.0980 LO2·s-1 for males and females, respectively, and appeared in proportion with the

average MR for slightly larger body sized minke whales (Perrin and Brownell, 2009) found by Folkow and Blix (1992), which was 0.1817 LO2·s-1.

Thus, though most MR data collected on other cetacean species were higher than the MR estimated through the slope of MRL vs. speed3 as used in the O2 model of the present study,

these did not improve the correlation between VO2 and speed3 over 15 min intervals when

applied to the O2 model in combination with the other used parameters.