63
Chapter three: Exercise energetics
For marine mammals, the use of doubly labeled water has three major limitations.
First, it is prohibitively expensive for exceptionally large animals, most notably mysticete whales. For example, as of this writing, an average-sized, adult blue whale would
require a 30 L injection of D 2 O 18 , at a minimum cost of over $13 million! Second, because
the DLW method requires blood samples at both the beginning and end of the measurement period, it is of limited utility when the chance of recapture is low, as is the case for
many far- ranging and elusive marine mammals, particularly cetaceans. Finally, calculations of energy expenditure using the DLW method are very sensitive to estimates of the
animal’s mass; this limits the utility of the method to instances where precise body mass
measurements are possible—a rare luxury for biologists studying free-ranging marine
mammals, and impossible for those studying mysticetes. Despite these limitations, DLW
is still considered one of the best methods for estimating field metabolic rate when logistically feasible (Speakman 1997; Sparling et al. 2008).
3.4 Unsolved mysteries and future directions
Estimates of energy expenditure and, by extension, the prey-energy requirements of freeranging marine mammals are fundamental to many questions surrounding the biology,
management, and conservation of populations. For each member of any population, survival
and successful reproduction depend on achieving positive energy balance, whereby individuals acquire enough energy from foraging to fuel critical life processes. In gray whales,
for example, a sexually mature female may not successfully produce a calf if her energy
intake is reduced by just 4% during pregnancy. Her own survival is predicted to be impacted
if energy intake is reduced by 42% during this same time (Villegas-Amtmann et al. 2015).
Similarly, the high energetic demands of raising a pup superimposed on the daily energetic
costs of foraging and activity by female sea otters can push the mother over the energetic
cliff and into a state of starvation. The potential for pup abandonment is escalated under
these energetic circumstances (Thometz et al. 2014). Thus, energy balance in individuals has
consequences for reproductive fitness, survival and, ultimately, demography.
Strong evolutionary pressures are in place for energy minimizing mechanisms and
behaviors that help balance the trade-offs between the costs and benefits of all activities.
Today, cumulative anthropogenic disturbances of marine mammals, especially during
foraging, are expected to challenge energetic balance by instigating avoidance behaviors
that increase movement costs. Consequently, the movement energetics of marine mammals has received growing attention in the scientific community (e.g., New et al. 2013;
Braithwaite et al. 2015).
Overall, exercise and movement are important life-history behaviors with marked
consequences for fecundity and fitness. In general, the temporal and spatial scales over
which marine mammals move and the consequent balance of energy budgets are correlated with body size (Boyd 2002). Movement in these groups can refer to the relatively
short distances associated with daily territorial patrols in male sea otters, to extraordinary annual, ocean basin scale foraging migrations of mysticete whales and phocid seals.
Movement can also refer to the depths to which marine mammals dive. Although motivations and cues vary, one constant unifies the movements of all active animals: it comes at
a high energetic cost. One can only imagine the total energetic costs that supported the
remarkable movements of a female gray whale that migrated 22,511 km (nearly 14,000
miles) roundtrip between Russia and Baja, Mexico to set the record for the longest migratory movement of any mammal (Mate et al. 2015).
Chapter three: Exercise energetics
For marine mammals, the use of doubly labeled water has three major limitations.
First, it is prohibitively expensive for exceptionally large animals, most notably mysticete whales. For example, as of this writing, an average-sized, adult blue whale would
require a 30 L injection of D 2 O 18 , at a minimum cost of over $13 million! Second, because
the DLW method requires blood samples at both the beginning and end of the measurement period, it is of limited utility when the chance of recapture is low, as is the case for
many far- ranging and elusive marine mammals, particularly cetaceans. Finally, calculations of energy expenditure using the DLW method are very sensitive to estimates of the
animal’s mass; this limits the utility of the method to instances where precise body mass
measurements are possible—a rare luxury for biologists studying free-ranging marine
mammals, and impossible for those studying mysticetes. Despite these limitations, DLW
is still considered one of the best methods for estimating field metabolic rate when logistically feasible (Speakman 1997; Sparling et al. 2008).
3.4 Unsolved mysteries and future directions
Estimates of energy expenditure and, by extension, the prey-energy requirements of freeranging marine mammals are fundamental to many questions surrounding the biology,
management, and conservation of populations. For each member of any population, survival
and successful reproduction depend on achieving positive energy balance, whereby individuals acquire enough energy from foraging to fuel critical life processes. In gray whales,
for example, a sexually mature female may not successfully produce a calf if her energy
intake is reduced by just 4% during pregnancy. Her own survival is predicted to be impacted
if energy intake is reduced by 42% during this same time (Villegas-Amtmann et al. 2015).
Similarly, the high energetic demands of raising a pup superimposed on the daily energetic
costs of foraging and activity by female sea otters can push the mother over the energetic
cliff and into a state of starvation. The potential for pup abandonment is escalated under
these energetic circumstances (Thometz et al. 2014). Thus, energy balance in individuals has
consequences for reproductive fitness, survival and, ultimately, demography.
Strong evolutionary pressures are in place for energy minimizing mechanisms and
behaviors that help balance the trade-offs between the costs and benefits of all activities.
Today, cumulative anthropogenic disturbances of marine mammals, especially during
foraging, are expected to challenge energetic balance by instigating avoidance behaviors
that increase movement costs. Consequently, the movement energetics of marine mammals has received growing attention in the scientific community (e.g., New et al. 2013;
Braithwaite et al. 2015).
Overall, exercise and movement are important life-history behaviors with marked
consequences for fecundity and fitness. In general, the temporal and spatial scales over
which marine mammals move and the consequent balance of energy budgets are correlated with body size (Boyd 2002). Movement in these groups can refer to the relatively
short distances associated with daily territorial patrols in male sea otters, to extraordinary annual, ocean basin scale foraging migrations of mysticete whales and phocid seals.
Movement can also refer to the depths to which marine mammals dive. Although motivations and cues vary, one constant unifies the movements of all active animals: it comes at
a high energetic cost. One can only imagine the total energetic costs that supported the
remarkable movements of a female gray whale that migrated 22,511 km (nearly 14,000
miles) roundtrip between Russia and Baja, Mexico to set the record for the longest migratory movement of any mammal (Mate et al. 2015).