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Marine Mammal Physiology: Requisites for Ocean Living
to body composition (these equations are often species- and even age-specific) and the
hydration state of the animal (which may be an issue in fasting animals).
For many marine mammals, including most cetaceans, it is impractical to consider
any techniques that require repeated capture and handling, and so new, innovative measures of physiological condition have to be developed. Photogrammetry—using photographs to make morphological measurements—have been investigated as a possible tool
(De Bruyn et al. 2009), but is usually limited to detecting large changes in “body condition” (Pettis et al. 2004). The animal’s diving behavior, which can be more readily monitored via attached dive recorders, may also provide an indirect measure of its relative
body stores. For many marine mammals, a portion of the natural dive sequence is made
up of an unpowered glide. The rate at which an animal ascends or descends through the
water during a glide is dependent upon the hydrodynamic drag (a factor of body shape),
water depth, and the buoyancy of the individual. An individual’s buoyancy is altered by
changes in the relative proportion of lipid mass in their body (as well as factors as the
amount of air they dive with). Hence, rates of ascent/descent while gliding and changes
in stroke rate have been used to detect differences in body condition in a number of species of marine mammals (Watanabe et al. 2006; Aoki et al. 2011; Schick et al. 2013). There
have even been attempts to determine the metabolic status of cetaceans by chemically
analyzing their captured exhalations (Aksenov et al. 2014).
8.2.2 Measuring metabolism and calculating metabolic depression
Metabolic rate is technically the amount of energy liberated or expended in a given unit
of time by an animal. While the earliest studies of metabolism measured the amount of
heat an animal produced (often by measuring the change in temperature of a surrounding water bath or ice mixture), later studies realized that aerobic metabolism consumed
a set amount of oxygen (and produced a set amount of carbon dioxide) depending on the
exact fuel source. Respirometry, the science of measuring the rate of oxygen consumption, has become the standard method of measuring rates of metabolism. The method
is quite simple; usually ambient air is drawn at a known rate through a sealed chamber
containing the organism (or at least within which it must breath). The excurrent airflow is
sampled to determine the concentration of oxygen (and often carbon dioxide). Knowing
the rate of airflow, and the difference in gas concentrations between the sampled and the
ambient air (which is essentially constant) allows one to calculate the rate of oxygen consumption. Often, metabolic rate is presented as a rate of oxygen consumption; while this
can be converted to a rate of energy use, this step also involves several assumptions. One
of these assumptions is the nature of the metabolic substrate, which can be elucidated
by examining the ratio of carbon dioxide produced to oxygen consumed (known as the
respiratory quotient).
Unfortunately, respirometry has limited applications in the field. Scientists have developed a number of proxies to indirectly estimate rates of energy expenditure from wild
marine mammals, including heart rate, flipper strokes, and body acceleration (Iverson et al.
2010). By far, the most common method involves the differential turnover of two isotopically labeled waters, known as the doubly labeled water method (Lifson and McClintock
1966). Still, most of these methods cannot provide estimates of Resting Metabolic Rate, but
only estimate an animal’s average metabolic rate over a period of time, known as its field
metabolic rate (FMR), which may be affected by parallel changes in activity, thermoregulation, and other factors.
Marine Mammal Physiology: Requisites for Ocean Living
to body composition (these equations are often species- and even age-specific) and the
hydration state of the animal (which may be an issue in fasting animals).
For many marine mammals, including most cetaceans, it is impractical to consider
any techniques that require repeated capture and handling, and so new, innovative measures of physiological condition have to be developed. Photogrammetry—using photographs to make morphological measurements—have been investigated as a possible tool
(De Bruyn et al. 2009), but is usually limited to detecting large changes in “body condition” (Pettis et al. 2004). The animal’s diving behavior, which can be more readily monitored via attached dive recorders, may also provide an indirect measure of its relative
body stores. For many marine mammals, a portion of the natural dive sequence is made
up of an unpowered glide. The rate at which an animal ascends or descends through the
water during a glide is dependent upon the hydrodynamic drag (a factor of body shape),
water depth, and the buoyancy of the individual. An individual’s buoyancy is altered by
changes in the relative proportion of lipid mass in their body (as well as factors as the
amount of air they dive with). Hence, rates of ascent/descent while gliding and changes
in stroke rate have been used to detect differences in body condition in a number of species of marine mammals (Watanabe et al. 2006; Aoki et al. 2011; Schick et al. 2013). There
have even been attempts to determine the metabolic status of cetaceans by chemically
analyzing their captured exhalations (Aksenov et al. 2014).
8.2.2 Measuring metabolism and calculating metabolic depression
Metabolic rate is technically the amount of energy liberated or expended in a given unit
of time by an animal. While the earliest studies of metabolism measured the amount of
heat an animal produced (often by measuring the change in temperature of a surrounding water bath or ice mixture), later studies realized that aerobic metabolism consumed
a set amount of oxygen (and produced a set amount of carbon dioxide) depending on the
exact fuel source. Respirometry, the science of measuring the rate of oxygen consumption, has become the standard method of measuring rates of metabolism. The method
is quite simple; usually ambient air is drawn at a known rate through a sealed chamber
containing the organism (or at least within which it must breath). The excurrent airflow is
sampled to determine the concentration of oxygen (and often carbon dioxide). Knowing
the rate of airflow, and the difference in gas concentrations between the sampled and the
ambient air (which is essentially constant) allows one to calculate the rate of oxygen consumption. Often, metabolic rate is presented as a rate of oxygen consumption; while this
can be converted to a rate of energy use, this step also involves several assumptions. One
of these assumptions is the nature of the metabolic substrate, which can be elucidated
by examining the ratio of carbon dioxide produced to oxygen consumed (known as the
respiratory quotient).
Unfortunately, respirometry has limited applications in the field. Scientists have developed a number of proxies to indirectly estimate rates of energy expenditure from wild
marine mammals, including heart rate, flipper strokes, and body acceleration (Iverson et al.
2010). By far, the most common method involves the differential turnover of two isotopically labeled waters, known as the doubly labeled water method (Lifson and McClintock
1966). Still, most of these methods cannot provide estimates of Resting Metabolic Rate, but
only estimate an animal’s average metabolic rate over a period of time, known as its field
metabolic rate (FMR), which may be affected by parallel changes in activity, thermoregulation, and other factors.