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Marine Mammal Physiology: Requisites for Ocean Living
The paradox concerning exercise while diving stems from two seemingly conflicting
physiological responses that occur when a marine mammal submerges. The ensuing dive
response is characterized by the cessation of breathing (apnea), a slowing of the heart
(bradycardia), peripheral vasoconstriction, and metabolic downregulation of noncritical
tissues (Chapter 2). Conversely, the exercise response of mammals promotes an increase
in metabolism, heart rate (tachycardia), and respiratory rates. How can both responses
occur when a submerged marine mammal is actively swimming, migrating, or pursing
underwater prey? We might expect that limited on-board oxygen stores would constrain
energetically expensive behaviors in diving mammals. Instead, we find that many species
of marine mammal that chase and consume prey while holding their breath often demonstrate exceptional speeds and complex, energetically costly maneuvers during foraging
(Aguilar de Soto et al. 2008; Goldbogen et al. 2008).
Studies that simultaneously measured swimming performance, stroke frequency,
and heart rate in diving mammals provide clues about the balance between diving and
exercise responses in cetaceans and pinnipeds (Davis and Williams 2012; Noren et al.
2012; Williams et al. 2015b). Rather than a single dominant response, there is an interplay
between exercise and dive responses in which both the depth of the dive and the intensity
of exercise alter the level of bradycardia. This variability impacts the energetic and physiological costs associated with a dive. The deeper the dive by cetaceans and pinnipeds, the
lower the minimum heart rate achieved (Figure 3.1a–c). Instead of maintaining a single
level of bradycardia when submerged, marine mammals show considerable variability in
diving heart rate. Swimming exercise plays an important role in dictating this variability.
Periods of high stroke frequency show a relaxation in bradycardia (higher heart rates)
while low stroke frequency swimming during a dive is associated with the most intense
levels of bradycardia (and lowest heart rates). Such responses have been demonstrated for
a wide range of species including bottlenose dolphins (Tursiops truncatus), Weddell seals
(Leptonychotes weddellii), gray seals (Halichoerus grypus), California sea lions (Zalophus californianus), and human breath-hold divers. Importantly, because the heart rate of marine
mammals, like terrestrial mammals, correlates directly with oxygen consumption (Figure
3.1d), cardiac variability will influence the movement and utilization of oxygen and carbon
dioxide during a dive.
The physical separation of two critical resources required for survival by diving mammals, air at the water surface and prey at depth, should not be considered a simple biological constraint. Instead, the physiological challenge associated with this dichotomy may
have led to a unique selection pressure for locomotor efficiency in pinnipeds, cetaceans,
and other mammalian groups that hunt while submerged (Williams et al. 2015a). In the following section, we examine the energetics of marine living mammals and the costs related
to resting, swimming, and diving activities. These costs represent the maintenance energetic requirements of the animal as well as the additional energy that must be expended
to move though water. It is important to recognize that depending on the species, age, and
reproductive status, the total energetic balance of an individual marine mammal must
also account for the energy required for thermoregulation, growth, reproduction, and the
assimilation of food, which will be addressed in subsequent chapters.
3.2 Energetic costs of marine mammals
How many fish must a marine mammal eat to survive or to successfully reproduce?
Answering such questions requires an understanding of how energetic costs are partitioned across time by an animal. Often, metabolic rate is used as the common metric for
Marine Mammal Physiology: Requisites for Ocean Living
The paradox concerning exercise while diving stems from two seemingly conflicting
physiological responses that occur when a marine mammal submerges. The ensuing dive
response is characterized by the cessation of breathing (apnea), a slowing of the heart
(bradycardia), peripheral vasoconstriction, and metabolic downregulation of noncritical
tissues (Chapter 2). Conversely, the exercise response of mammals promotes an increase
in metabolism, heart rate (tachycardia), and respiratory rates. How can both responses
occur when a submerged marine mammal is actively swimming, migrating, or pursing
underwater prey? We might expect that limited on-board oxygen stores would constrain
energetically expensive behaviors in diving mammals. Instead, we find that many species
of marine mammal that chase and consume prey while holding their breath often demonstrate exceptional speeds and complex, energetically costly maneuvers during foraging
(Aguilar de Soto et al. 2008; Goldbogen et al. 2008).
Studies that simultaneously measured swimming performance, stroke frequency,
and heart rate in diving mammals provide clues about the balance between diving and
exercise responses in cetaceans and pinnipeds (Davis and Williams 2012; Noren et al.
2012; Williams et al. 2015b). Rather than a single dominant response, there is an interplay
between exercise and dive responses in which both the depth of the dive and the intensity
of exercise alter the level of bradycardia. This variability impacts the energetic and physiological costs associated with a dive. The deeper the dive by cetaceans and pinnipeds, the
lower the minimum heart rate achieved (Figure 3.1a–c). Instead of maintaining a single
level of bradycardia when submerged, marine mammals show considerable variability in
diving heart rate. Swimming exercise plays an important role in dictating this variability.
Periods of high stroke frequency show a relaxation in bradycardia (higher heart rates)
while low stroke frequency swimming during a dive is associated with the most intense
levels of bradycardia (and lowest heart rates). Such responses have been demonstrated for
a wide range of species including bottlenose dolphins (Tursiops truncatus), Weddell seals
(Leptonychotes weddellii), gray seals (Halichoerus grypus), California sea lions (Zalophus californianus), and human breath-hold divers. Importantly, because the heart rate of marine
mammals, like terrestrial mammals, correlates directly with oxygen consumption (Figure
3.1d), cardiac variability will influence the movement and utilization of oxygen and carbon
dioxide during a dive.
The physical separation of two critical resources required for survival by diving mammals, air at the water surface and prey at depth, should not be considered a simple biological constraint. Instead, the physiological challenge associated with this dichotomy may
have led to a unique selection pressure for locomotor efficiency in pinnipeds, cetaceans,
and other mammalian groups that hunt while submerged (Williams et al. 2015a). In the following section, we examine the energetics of marine living mammals and the costs related
to resting, swimming, and diving activities. These costs represent the maintenance energetic requirements of the animal as well as the additional energy that must be expended
to move though water. It is important to recognize that depending on the species, age, and
reproductive status, the total energetic balance of an individual marine mammal must
also account for the energy required for thermoregulation, growth, reproduction, and the
assimilation of food, which will be addressed in subsequent chapters.
3.2 Energetic costs of marine mammals
How many fish must a marine mammal eat to survive or to successfully reproduce?
Answering such questions requires an understanding of how energetic costs are partitioned across time by an animal. Often, metabolic rate is used as the common metric for