Flipper strokes can predict energy expenditure and locomotion costs in free-ranging northern and Antarctic fur seals
Résumé
Flipper strokes have been proposed as proxies to estimate the energy expended by marine vertebrates
while foraging at sea, but this has never been validated on free-ranging otariids (fur seals and sea lions).
Our goal was to investigate how well flipper strokes correlate with energy expenditure in 33 foraging
northern and Antarctic fur seals equipped with accelerometers, GPS, and time-depth recorders. We
concomitantly measured field metabolic rates with the doubly-labelled water method and derived
activity-specific energy expenditures using fine-scale time-activity budgets for each seal. Flipper strokes
were detected while diving or surface transiting using dynamic acceleration. Despite some inter-species
differences in flipper stroke dynamics or frequencies, both species of fur seals spent 3.79 ± 0.39 J/kg per
stroke and had a cost of transport of ~1.6–1.9 J/kg/m while diving. Also, flipper stroke counts were good
predictors of energy spent while diving (R2 = 0.76) and to a lesser extent while transiting (R2 = 0.63).
However, flipper stroke count was a poor predictor overall of total energy spent during a full foraging
trip (R2 = 0.50). Amplitude of flipper strokes (i.e., acceleration amplitude × number of strokes) predicted
total energy expenditure (R2 = 0.63) better than flipper stroke counts, but was not as accurate as other
acceleration-based proxies, i.e. Overall Dynamic Body Acceleration.