Abstract
Direct measures of energy expenditure are difficult to obtain in marine mammals, and accelerometry may be a useful proxy. Recently its utility has been questioned as some analyses derived their measure of activity level by calculating the sum of accelerometry-based values and then comparing this summation to summed (total) energy expenditure (the so-called “time trap”). To test this hypothesis, we measured oxygen consumption of captive fur seals and sea lions wearing accelerometers during submerged swimming and calculated total and rate of energy expenditure. We compared these values with two potential proxies of energy expenditure derived from accelerometry data: flipper strokes and dynamic body acceleration (DBA). Total number of strokes, total DBA, and submergence time all predicted total oxygen consumption ({boldsymbol{sV}}{{boldsymbol{O}}}_{{boldsymbol{2}}} ml kg−1). However, both total DBA and total number of strokes were correlated with submergence time. Neither stroke rate nor mean DBA could predict the rate of oxygen consumption (smathop{{boldsymbol{V}}}limits^{{boldsymbol{.}}}{{boldsymbol{O}}}_{{boldsymbol{2}}} ml min−1 kg−1). The relationship of total DBA and total strokes with total oxygen consumption is apparently a result of introducing a constant (time) into both sides of the relationship. This experimental evidence supports the conclusion that proxies derived from accelerometers cannot estimate the energy expenditure of marine mammals.
Highlights
ODBA and VeDBA are calculated from body acceleration measured on three axes[14], while stroke rate can be calculated from the peaks in the dynamic acceleration of the x-axis[15,16]
We found that different combinations of thresholds and running means greatly influenced the overall correlation of dynamic body acceleration (DBA) measures with both sV O2 and sVO2 for different groups of animals, but there was very little difference between ODBA or VeDBA
We provide evidence using a range of otariids of different ages, sizes, sexes, and species that support Halsey’s contention that many of the strong relationships observed elsewhere between total energy expenditure and total number of strokes or measures of total DBA are a result of time being incorporated into both sides of the equation
Summary
Tehitehererlsattriookneshraipteonf toortmaleDaBnADaBnAdctooutladl predict the strokes with total oxygen consumption is apparently a result of introducing a constant (time) into both sides of the relationship This experimental evidence supports the conclusion that proxies derived from accelerometers cannot estimate the energy expenditure of marine mammals. ODBA and VeDBA are calculated from body acceleration measured on three axes[14], while stroke rate can be calculated from the peaks in the dynamic acceleration of the x-axis[15,16] While these methods have demonstrated strong predictive relationships to oxygen consumption in terrestrial animals[17], the results in marine mammals and birds have so far been mixed[18,19,20]. Species: AFS – Australian fur seal; ASL – Australian sea lion; NZFS – New Zealand fur seal; SSL – Steller sea lion. *Indicates seals identified as subadults during trials
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