Predicting rate of oxygen consumption from heart rate while little penguins work, rest and play

The relationship between heart rate (fH) and rate of oxygen consumption (V˙O2) was investigated under changing conditions of ambient temperature, digestive state and exercise state in the little penguin (Eudyptula minor). Both fH and V˙O2 were recorded simultaneously from 12 little penguins while they each (a) rested and exercised within their reported thermo-neutral zone (TNZ), (b) rested and exercised below their reported TNZ and (c) digested a meal of sardines within their reported TNZ. Contrary to our expectations, we found that minimum V˙O2 did not vary between the two temperatures used. Comparison with values from the literature suggests that both minimum V˙O2 and the extent of the TNZ in this species may vary along a latitudinal gradient. Furthermore, while minimum V˙O2 was unchanged at the lower temperature, minimum fH was significantly higher, suggesting a hitherto undescribed cardiac response to lowered ambient temperature in an avian species. This response was maintained when the penguins exercised within and below their apparent TNZ as fH was significantly greater in cold conditions for a given level of V˙O2. Furthermore, both fH and V˙O2 were slightly but significantly elevated for a given walking speed during exercise at the lower temperature. This suggests that the penguins may have been close to their TNZ and that the measures employed to counteract heat loss while at rest may have been compromised during exercise. There was no significant difference in the relationship between fH and V˙O2 while the penguins were inactive in a post-digestive state or inactive and digesting a meal within their TNZ, though both of these relationships were significantly different from that during exercise. This suggests that while digestion has no effect on the fH/V˙O2 relationship, for little penguins at least, it is of little value in deriving a predictive relationship for application to active free-ranging animals.