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Marine Mammal Physiology: Requisites for Ocean Living
plasma or serum from red blood cells) and stabilized by freezing to limit the degradation
of metabolites. Hence, we know the most about the biochemistry of fasting in elephant
seals, and it is not hard to imagine that this is partly because they fast on accessible
beaches. Collecting and carefully preserving a blood sample from a migrating, fasting
whale in pelagic waters is obviously more daunting.
8.2.4 Turnover of metabolic tracers
Similar to the doubly labeled water method, the dilution and breakdown of tracer chemicals can be used to track substrate use during fasting. This requires an even more complicated field sampling strategy than a single blood draw. An initial blood sample is collected
for baseline information, and subsequent samples are collected to determine the rate of
tracer chemical turnover. Potentially, several samples must be collected to confirm this
rate. Often, the animal’s blood volume (typically measured by dilution of IV-injected
Evans blue dye) is also determined. Depending on the timeframe and the study species,
it is necessary to sedate the animals to collect blood or give injections. The length of these
procedures can complicate sampling strategy. However, this type of experiment provides
very useful, specific data about the metabolism of substances and has been successfully
accomplished in fasting elephant seals for labeled urea, fatty acids, and glucose, to name a
few (Pernia et al. 1980; Castellini et al. 1987; Houser and Costa 2001).
8.3 Lingering mysteries
There are many specific details of fasting physiology that we have only begun to explore.
While our depth of understanding of basic fasting physiology is rapidly increasing, its
breadth is still very narrow. The bulk of our knowledge on hormonal control mechanisms,
for example, comes from a single species, the northern elephant seal. Studies with other
pinnipeds have supported a basic conclusion that marine mammals are well-adapted to
their natural fasts, but the manner in which they accomplish this feat differs significantly
between species and from terrestrial mammals. But these scientific glimpses are exceedingly limited. Perhaps the most important focus for future research is to explore fasting physiology in a wider range of species, with particular emphasis on cetaceans. This
increased knowledge will also allow us to determine what evolutionary processes have
honed species-specific fasting strategies, even among closely related animals.
The question of the fasting capacity of marine mammals is of more than mere academic interest but also has important conservation implications. For example, in 2014–2015,
unprecedented numbers of stranded, starving California sea lions (Zalophus californianus)
began arriving on beaches of the U.S. west coast. Scientists believe that anomalously warm
coastal waters shifted the food base, requiring lactating mothers to prolong their trips
to sea. The result is an extension beyond natural fasting durations that the pups cannot
endure, driving them into the water to begin their own foraging too early.
With concern increasing over the impacts to wild populations of both natural and
anthropogenic disturbance, it is timely that we start addressing how predictable periods of
fasting interact with unpredicted perturbations. How much of an additional physiological
burden are imposed by threats such as disease, pollution, and human harassment? How
might survival and reproductive capacity be impacted by environmental changes resulting in slightly longer or more expensive fasts, slightly smaller energy reserves, or altered
seasonal timing of fasts? Future research will help us understand how changing oceans
could affect marine mammal populations.
Marine Mammal Physiology: Requisites for Ocean Living
plasma or serum from red blood cells) and stabilized by freezing to limit the degradation
of metabolites. Hence, we know the most about the biochemistry of fasting in elephant
seals, and it is not hard to imagine that this is partly because they fast on accessible
beaches. Collecting and carefully preserving a blood sample from a migrating, fasting
whale in pelagic waters is obviously more daunting.
8.2.4 Turnover of metabolic tracers
Similar to the doubly labeled water method, the dilution and breakdown of tracer chemicals can be used to track substrate use during fasting. This requires an even more complicated field sampling strategy than a single blood draw. An initial blood sample is collected
for baseline information, and subsequent samples are collected to determine the rate of
tracer chemical turnover. Potentially, several samples must be collected to confirm this
rate. Often, the animal’s blood volume (typically measured by dilution of IV-injected
Evans blue dye) is also determined. Depending on the timeframe and the study species,
it is necessary to sedate the animals to collect blood or give injections. The length of these
procedures can complicate sampling strategy. However, this type of experiment provides
very useful, specific data about the metabolism of substances and has been successfully
accomplished in fasting elephant seals for labeled urea, fatty acids, and glucose, to name a
few (Pernia et al. 1980; Castellini et al. 1987; Houser and Costa 2001).
8.3 Lingering mysteries
There are many specific details of fasting physiology that we have only begun to explore.
While our depth of understanding of basic fasting physiology is rapidly increasing, its
breadth is still very narrow. The bulk of our knowledge on hormonal control mechanisms,
for example, comes from a single species, the northern elephant seal. Studies with other
pinnipeds have supported a basic conclusion that marine mammals are well-adapted to
their natural fasts, but the manner in which they accomplish this feat differs significantly
between species and from terrestrial mammals. But these scientific glimpses are exceedingly limited. Perhaps the most important focus for future research is to explore fasting physiology in a wider range of species, with particular emphasis on cetaceans. This
increased knowledge will also allow us to determine what evolutionary processes have
honed species-specific fasting strategies, even among closely related animals.
The question of the fasting capacity of marine mammals is of more than mere academic interest but also has important conservation implications. For example, in 2014–2015,
unprecedented numbers of stranded, starving California sea lions (Zalophus californianus)
began arriving on beaches of the U.S. west coast. Scientists believe that anomalously warm
coastal waters shifted the food base, requiring lactating mothers to prolong their trips
to sea. The result is an extension beyond natural fasting durations that the pups cannot
endure, driving them into the water to begin their own foraging too early.
With concern increasing over the impacts to wild populations of both natural and
anthropogenic disturbance, it is timely that we start addressing how predictable periods of
fasting interact with unpredicted perturbations. How much of an additional physiological
burden are imposed by threats such as disease, pollution, and human harassment? How
might survival and reproductive capacity be impacted by environmental changes resulting in slightly longer or more expensive fasts, slightly smaller energy reserves, or altered
seasonal timing of fasts? Future research will help us understand how changing oceans
could affect marine mammal populations.