170
Marine Mammal Physiology: Requisites for Ocean Living
Predictable fasting periods generally occur as part of an animal’s natural life history.
Suites of behavioral and physiological adaptations permit survival across these periods
with minimal long-term consequences.
Although a general model describes fasting physiology in vertebrates, marine mammal studies suggest that this group does not always follow expectations—in fact, even
closely related species seem to cope with fasting in very different ways. Many different
marine mammals withstand extended fasts; however the ways that they do so are constantly surprising scientists. Not only is the physiology of marine mammals specialized
to cope with episodes of fasting, but these mechanisms can be altered over the course of
the fast to adjust to ongoing changes. As is typical in other fasting-adapted vertebrates,
lipid stores provide the primary support for this physiology. However, marine mammals
uniquely tap into lipids stored in their expansive hypodermal blubber layer. While fasting
often occurs during energetically demanding life-history stages (e.g., periods of reproduction and growth), marine mammals extend fasting tolerance by saving energy through
reducing other energetic costs. The physiological and energetic changes that support
and regulate fasting are hormonally controlled. A diversity of responses across species,
and divergence between marine and terrestrial mammals in the suite of key hormones
that regulate fasting provide an important example of the unique physiology of marine
mammals.
8.1.1 What is fasting and when does it occur?
Fasting refers to the physiological state of living with a complete lack of food intake. In the
strictest sense, short-term fasting occurs constantly in most mammals, since individuals
are not continuously consuming prey—or more accurately, processing previously acquired
prey. In this chapter, we will not be discussing such short-term events (what is referred to
as dynamic equilibrium) (Kleiber 1975). Rather, we will address “fasting” as a complete lack
of food intake over longer intervals of days or months.
It might be surprising to learn that scientists have not agreed upon clear definitions
for the terms “fasting state” versus “starvation state.” Both imply a lack of food intake, and
while they are sometimes used interchangeably, they should more appropriately differentiate two different physiological states. More specifically, the terms fasting and starvation
distinguish differences in the severity of the effects of a lack of food intake. Whereas fasting can be sustained over a period of time, a “starvation state” is characterized by lifethreatening or potentially irreversible physiological and anatomical changes.
Fasting and starvation can also be distinguished by cause or motivation (McCue 2010).
A fasting animal is unwilling to eat (or forage) even though food is available, due to a noncompatible requirement, such as reproduction (breeding, nursing), or thermoregulation,
etc. While fasting also occurs during hibernation or estivation, no pinnipeds or cetaceans
exhibit these strategies (although pregnant female polar bears, Ursus maritimus, which are
sometimes discussed with marine mammals, hibernate in the winter). In contrast, starvation would be said to occur when an animal is willing to consume food but some extrinsic
factor (e.g., lack of prey, inclement weather, and predator avoidance) is limiting this ability.
This difference between these states is tied to predictability; fasting is a natural part
of life history and may, therefore, occur very predictably. Since fasting is “natural,” and
timed to life history, an animal will be more physiologically able to withstand this lack of
food intake at certain times of the year or life stage. We, therefore, expect that the impacts
of a natural fasting episode be less detrimental than those incurred during “unpredicted”
episodes of starvation.
Marine Mammal Physiology: Requisites for Ocean Living
Predictable fasting periods generally occur as part of an animal’s natural life history.
Suites of behavioral and physiological adaptations permit survival across these periods
with minimal long-term consequences.
Although a general model describes fasting physiology in vertebrates, marine mammal studies suggest that this group does not always follow expectations—in fact, even
closely related species seem to cope with fasting in very different ways. Many different
marine mammals withstand extended fasts; however the ways that they do so are constantly surprising scientists. Not only is the physiology of marine mammals specialized
to cope with episodes of fasting, but these mechanisms can be altered over the course of
the fast to adjust to ongoing changes. As is typical in other fasting-adapted vertebrates,
lipid stores provide the primary support for this physiology. However, marine mammals
uniquely tap into lipids stored in their expansive hypodermal blubber layer. While fasting
often occurs during energetically demanding life-history stages (e.g., periods of reproduction and growth), marine mammals extend fasting tolerance by saving energy through
reducing other energetic costs. The physiological and energetic changes that support
and regulate fasting are hormonally controlled. A diversity of responses across species,
and divergence between marine and terrestrial mammals in the suite of key hormones
that regulate fasting provide an important example of the unique physiology of marine
mammals.
8.1.1 What is fasting and when does it occur?
Fasting refers to the physiological state of living with a complete lack of food intake. In the
strictest sense, short-term fasting occurs constantly in most mammals, since individuals
are not continuously consuming prey—or more accurately, processing previously acquired
prey. In this chapter, we will not be discussing such short-term events (what is referred to
as dynamic equilibrium) (Kleiber 1975). Rather, we will address “fasting” as a complete lack
of food intake over longer intervals of days or months.
It might be surprising to learn that scientists have not agreed upon clear definitions
for the terms “fasting state” versus “starvation state.” Both imply a lack of food intake, and
while they are sometimes used interchangeably, they should more appropriately differentiate two different physiological states. More specifically, the terms fasting and starvation
distinguish differences in the severity of the effects of a lack of food intake. Whereas fasting can be sustained over a period of time, a “starvation state” is characterized by lifethreatening or potentially irreversible physiological and anatomical changes.
Fasting and starvation can also be distinguished by cause or motivation (McCue 2010).
A fasting animal is unwilling to eat (or forage) even though food is available, due to a noncompatible requirement, such as reproduction (breeding, nursing), or thermoregulation,
etc. While fasting also occurs during hibernation or estivation, no pinnipeds or cetaceans
exhibit these strategies (although pregnant female polar bears, Ursus maritimus, which are
sometimes discussed with marine mammals, hibernate in the winter). In contrast, starvation would be said to occur when an animal is willing to consume food but some extrinsic
factor (e.g., lack of prey, inclement weather, and predator avoidance) is limiting this ability.
This difference between these states is tied to predictability; fasting is a natural part
of life history and may, therefore, occur very predictably. Since fasting is “natural,” and
timed to life history, an animal will be more physiologically able to withstand this lack of
food intake at certain times of the year or life stage. We, therefore, expect that the impacts
of a natural fasting episode be less detrimental than those incurred during “unpredicted”
episodes of starvation.