Activity-specific metabolic rates for diving, transiting, and resting at sea can be estimated from time-activity budgets in free-ranging marine mammals
Résumé
Time and energy are the two most important currencies in animal bioenergetics. How
much time animals spend engaged in different activities with specific energetic costs
ultimately defines their likelihood of surviving and successfully reproducing. However,
it is extremely difficult to determine the energetic costs of independent activities for
free-ranging
animals. In this study, we developed a new method to calculate activity-specific
metabolic rates, and applied it to female fur seals. We attached biologgers
(that recorded GPS locations, depth profiles, and triaxial acceleration) to 12 northern
(Callorhinus ursinus) and 13 Antarctic fur seals (Arctocephalus gazella), and used a hierarchical
decision tree algorithm to determine time allocation between diving, transiting,
resting, and performing slow movements at the surface (grooming, etc.). We
concomitantly measured the total energy expenditure using the doubly-labelled water
method. We used a general least-square model to establish the relationship between
time–activity budgets and the total energy spent by each individual during their foraging
trip to predict activity-specific
metabolic rates. Results show that both species allocated
similar time to diving (~29%), transiting to and from their foraging grounds
(~26–30%), and resting (~8–11%). However, Antarctic fur seals spent significantly
more time grooming and moving slowly at the surface than northern fur seals (36% vs.
29%). Diving was the most expensive activity (~30 MJ/day if done non-stop for 24 hr),
followed by transiting at the surface (~21 MJ/day). Interestingly, metabolic rates were
similar between species while on land or while slowly moving at the surface (~13 MJ/
day). Overall, the average field metabolic rate was ~20 MJ/day (for all activities combined).
The method we developed to calculate activity-specific
metabolic rates can be
applied to terrestrial and marine species to determine the energetic costs of daily activities,
as well as to predict the energetic consequences for animals forced to change
their time allocations in response to environmental shifts.