Season and time of day affect the ability of accelerometry and the doubly labeled water methods to measure energy expenditure in northern fur seals (Callorhinus ursinus)

• The best DLW method estimates overestimated respirometry by 13.1 ± 16.5%.
• The best accelerometry method estimates overestimated respirometry by 5.4 ± 29.3%.
• Both DLW and accelerometry methods require corrections for seasonal effects.
• The accelerometry method also depends on the temporal resolution and time of day.

Estimates of energy expenditure for free-ranging animals are essential to answering a range of fundamental questions in animal biology, but are challenging to obtain and difficult to validate. We simultaneously employed three methods to measure the energy expenditure of 6 captive female northern fur seals (Callorhinus ursinus) during 5-day trials across 4 seasons: respirometry (oxygen consumption), doubly labeled water (DLW), and accelerometry. The DLW method estimated that the fur seals expended 13.1 ± 16.5% more energy than indicated by the more direct measures of oxygen consumption. Accelerometry failed to predict the average mass-specific rate of oxygen consumption (V˙O2DEE) within the individual seasons over entire 5-day trials. However, on a finer time scale (15 or 60 min) and adjusted for time of day, accelerometry estimated energy expenditure within an average difference of 5.4 ± 29.3% (60 min intervals) and 13.8 ± 39.5% (15 min intervals) of respirometry measured values. Our findings suggest that accelerometers have the potential to be more effective than the DLW method for measuring energy expenditure of free-ranging animals. However, rates of oxygen consumption varied with season, independent of overall activity. Seasonal effects (and time of day for accelerometry) must therefore be accounted for when estimating energy expenditure from measures of DLW and acceleration of free-swimming northern fur seals. Such corrections required for estimating energy expenditures in northern fur seals have implications for using accelerometers and DLW to estimate the energy expenditure of other species.