Accelerometers can measure total and activity-specific energy expenditures in free-ranging marine mammals only if linked to time-activity budgets
Abstract
1. Energy expenditure is an important component of foraging ecology, but is extremely difficult
to estimate in free-ranging animals and depends on how animals partition their time
between different activities during foraging. Acceleration data have emerged as a new way to
determine energy expenditure at a fine scale but this needs to be tested and validated in wild
animals.
2. This study investigated whether vectorial dynamic body acceleration (VeDBA) could accurately
predict the energy expended by marine predators during a full foraging trip. We also
aimed to determine whether the accuracy of predictions of energy expenditure derived from
acceleration increased when partitioned by different types of at-sea activities (i.e. diving, transiting,
resting and surface activities).
3. To do so, we equipped 20 lactating northern (Callorhinus ursinus) and 20 lactating Antarctic
fur seals (Arctocephalus gazella) with GPS, time-depth recorders and tri-axial accelerometers
and obtained estimates of field metabolic rates using the doubly labelled water (DLW)
method. VeDBA was derived from tri-axial acceleration, and at-sea activities (diving, transiting,
resting and surface activities) were determined using dive depth, tri-axial acceleration and
travelling speed.
4. We found that VeDBA did not accurately predict the total energy expended by fur seals
during their full foraging trips (R2 = 036). However, the accuracy of VeDBA as a predictor
of total energy expenditure increased significantly when foraging trips were partitioned by
activity and when activity-specific VeDBA was paired with time-activity budgets (R2 = 070).
Activity-specific VeDBA also accurately predicted the energy expenditures of each activity
independent of each other (R2 > 085).
5. Our study confirms that acceleration is a promising way to estimate energy expenditures of
free-ranging marine mammals at a fine scale never attained before. However, it shows that it
needs to be based on the time-activity budgets that make up foraging trips rather than being
derived as a single measure of VeDBA applied to entire foraging trips. Our activity-based
method provides a cost-effective means to accurately calculate energy expenditures of fur seals
using acceleration and time-activity budgets, that can be transfered to studies on other species.