Flipper strokes can predict energy expenditure and locomotion costs in free-ranging northern and Antarctic fur seals

International audienceFlipper strokes have been proposed as proxies to estimate the energy expended by marine vertebrateswhile 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 foragingnorthern and Antarctic fur seals equipped with accelerometers, GPS, and time-depth recorders. Weconcomitantly measured field metabolic rates with the doubly-labelled water method and derivedactivity-specific energy expenditures using fine-scale time-activity budgets for each seal. Flipper strokeswere detected while diving or surface transiting using dynamic acceleration. Despite some inter-speciesdifferences in flipper stroke dynamics or frequencies, both species of fur seals spent 3.79 ± 0.39 J/kg perstroke and had a cost of transport of ~1.6–1.9 J/kg/m while diving. Also, flipper stroke counts were goodpredictors 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 foragingtrip (R2 = 0.50). Amplitude of flipper strokes (i.e., acceleration amplitude × number of strokes) predictedtotal energy expenditure (R2 = 0.63) better than flipper stroke counts, but was not as accurate as otheracceleration-based proxies, i.e. Overall Dynamic Body Acceleration