The use of data loggers to determine the energetic and physiology of aquatic birds and mammals
By deploying a data logger specifically designed for the purpose, it was possible to record heart rate, fH (beats/min), from free-ranging gentoo penguins, Pygoscelis papua, and Antarctic fur seals, Arctocephalus gazella, at the British Antarctic Survey Base at Bird Island. The heart rate data were then converted into oxygen consumption (VO2, mlO2 min-1 kg-1) and/or energy expenditure (W/kg) using equations that had been derived from calibration experiments. In these experiments the relationships between fH and VO2 were determined in animals at rest and while exercising at different levels on a treadmill or in a static water channel (penguins) and in a variable speed flume (California sea lions, Zalophus californianus, as surrogate fur seals) or in a static water channel (fur seals). The validity of using these relationships was tested by recording simultaneously fH, VO2 by direct respirometry and VCO2 by the doubly labelled water (DLW) technique in six penguins and in six California sea lions during 72 h and 24 h, respectively, at various levels of activity. For the penguins, both indirect methods gave mean algebraic errors within 2% of the measured VO2, whereas for the sea lions, the mean algebraic errors were 36.4% for the DLW method and 2.7% for the fH method. The range of errors was greater for the DLW method, in both species. Field data from 15 penguins indicate that the fH method provides data that are comparable to those obtained using the DLW method, but with the added advantage that they can be broken down into the energy costs for specific types of behaviour. The implanted data loggers also recorded the temperature of the abdominal cavity (T(ab)) and it was evident that this routinely decreased by approximately 2 degrees C during diving bouts and by a maximum of almost 5 degrees C. Such temperature decreases, particularly if representative of similar decreases in other tissues, may at least partly explain why the energy costs of travelling to and from the foraging site and of foraging itself are similar to those for penguins resting in water at 5 degrees C. Field data from 15 female fur seals indicate that, when the animals are ashore, there is good agreement between the values for mean energy expenditure obtained by the fH and DLW methods. However, when the animals are at sea, the values obtained by DLW are substantially greater than those obtained by fH. When the at-sea values are corrected for the apparent overestimations referred to above, there is good agreement between the data obtained using the two methods. The data derived from fH indicate that, as with the penguins, the energy expenditures during travelling to and from the foraging site and during foraging are similar to those of fur seals resting in water at 7 degrees C.