Studying Acceleration and Energetics… in 3-D

A fur seal “fitness tracker” called a Daily Diary Tag records three-dimensional acceleration, depth, water temperature, and many other variables while the fur seals are at sea.

How many calories did you burn today? If you wear a fitness tracker, the answer is easy. You can see how many steps you took, and estimate how much energy you expended. You can even plan your meals to replace those calories.

For scientists studying energetics in wild fur seals, the answers are not so straightforward. Seals move in three dimensions through the water, and they expend different amounts of energy while diving, swimming, grooming, and even sleeping. How much energy does each activity consume, and how to accurately measure this? Can this information ultimately help to conserve fur seal populations?

A new Consortium study, led by Dr. Tiphaine Jeanniard-du-Dot (University of British Columbia), tracked the movements of 40 wild fur seals over extended foraging trips in the northern and southern hemispheres. Using the data from the tracking devices, the authors developed a novel way to estimate energy expenditure in fur seals.

The new study was published in the journal Functional Ecology, and extends from Jeanniard-du-Dot’s previous work measuring flipper strokes in fur seals.

Female northern fur seals nurse their pups for about a day between week-long foraging trips.

Dynamic Body Acceleration

“In the flipper stroke study, I measured forward acceleration,” says Jeanniard-du-Dot. “This new study focuses on Dynamic Body Acceleration [DBA], which measures acceleration in three dimensions—for example, when the animals roll, twist, turn, or swim up or down. Our results give us a more complete estimate of how much energy they expend when hunting for food.”

The study involved temporarily fitting 20 lactating northern fur seals and 20 lactating Antarctic fur seals with tracking devices to measure their position, acceleration, and movements in three dimensions.

The scientists wanted to know whether DBA could accurately predict the energy expended over a full foraging trip that can last up to 15 days. At the same time, they wanted to know whether it was more accurate to measure the energy expended over the entire trip, or by partitioning the trip into periods of different types of activity, such as traveling, diving, grooming and resting.

“Different activities have different energy costs,” says Jeanniard-du-Dot. “Our goal was to see whether 3-D accelerometers could be used to estimate how much energy an animal burns each day while swimming, diving and resting at sea.”

Relationships between dynamic-body acceleration and energy expenditure in lactating northern fur seals (triangles, n=16 seals) and Antarctic fur seals (squares, n=17 seals) while diving, transiting, and grooming and resting on the surface. The graphs show fur seals spend more energy diving compared to transiting and engaging in surface activities.  They also show northern fur seals spend more energy to swim (transiting) compared with Antarctic fur seals.

It might seem logical to expect faster activities to require more energy. But the relationship between acceleration and energy expenditure isn’t linear, she says. For example, fur seals foraging at depth spend more energy overcoming drag, which reduces acceleration but increases energy expenditure. At the surface there is less drag, making porpoising a faster and more energy efficient activity.

From Individuals to Populations

The study concludes that measuring Dynamic Body Acceleration is a promising way to estimate energy expenditures of free-ranging fur seals at a fine scale never attained before. The methodology can be applied to other species by linking time-activity budgets to acceleration, says Jeanniard-du-Dot.

However, she adds that the results also underscore the importance of measuring expenditure based on the individual activities that make up foraging trips, rather than as a single measure applied to entire foraging trips. In all, the study provides valuable baseline data for both species of fur seal.

“As an energetics researcher, I think having estimates of energy expenditures for a wild species is paramount to understanding everything that is happening in their environment,” she says. “These animals can only use whatever energy they can capture in the form of food. If we know how much energy they need to survive, then we know how much they extract from their environment. We can then extrapolate from the individual animal to an entire population.”

“This is particularly important in a changing marine environment,” she notes. “At the individual level, if animals can’t capture enough prey, they will begin to have problems with their health, reproduction, and survival. This can be an indicator of a declining population.”

A female Antarctic fur seal with her pup in the Sub Antarctic.

“Climate change is coming, and more quickly in polar regions,” notes Jeanniard-du-Dot. “Prey will potentially be less available, accessible and abundant. This means that fur seals might have to work harder to get their prey—go further from their colony, dive deeper for longer, and so on. They will probably have to increase the time spent in the two most energetically expensive activities, which are transiting and diving.”

When time-activity budgets of marine mammals change due to fishing or environmental conditions, scientists will be able to use the methods developed by Jeanniard-du-Dot and colleagues to determine what is happening, and anticipate the consequences at the population level.

PublicationsPUBLICATION

Accelerometers can measure total and activity-specific energy expenditure in free-ranging marine mammals only if linked to time-activity budgets.
Jeanniard du Dot, T., C. Guinet, J. P. Y. Arnould, J. R. Speakman and A. W. Trites. 2017.
Functional Ecology 31:377-386.
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 has emerged as a new way to determine energy expenditure at a fine scale but 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) vs calculated activity-specific metabolic rates. 3-To do so, we equipped 20 lactating northern (Callorhinus ursinus) and 20 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 traveling speed. 4-We found that VeDBA did not accurately predict the total energy expended by fur seals during their full foraging trips (R2 = 0.36). However, the accuracy of VeDBA as a predictor of total energy expenditure increased significantly when foraging trips were partitioned by activity and used activity-specific VeDBA paired with time activity budgets (R2 = 0.70). Activity-specific VeDBA also accurately predicted the energy expenditures of each activity independent of each other (R2 > 0.85). 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 budget 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, a stepping stone for numerous other research fields.

keywords     Antarctic fur seal, Arctocephalus gazella, Callorhinus ursinus, diving, energy expenditure, foraging, metabolic rate, northern fur seal, time-activity budget
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