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Marine Mammal Physiology: Requisites for Ocean Living
3.3.2 Indirect methods for determining energetic costs
Except in a few unique circumstances, it is not possible to directly measure the oxygen consumption of wild marine mammals. However, by pairing direct measures of metabolism
in controlled circumstances with other physiological or biomechanical parameters, it is
possible to estimate nearly instantaneous to long-term energetic costs for a wide variety of
behaviors. For these measurements calibrated instrumentation is deployed on wild marine
mammals, and records respiration rates, heart rates, acceleration, swimming stroke rates,
or speed. Because each of these parameters is correlated to the rate of oxygen consumption
(e.g., Figures 3.1 and 3.3), they can be used to determine free-ranging energetic costs as the
animals move through the environment. The energetics of swimming, diving, migrating,
and foraging of pinnipeds (Boyd et al. 1999; Butler et al. 2004; Williams et al. 2004) as well
as small (Williams et al. 1992, 2015b) and large (Goldbogen et al. 2008, 2011, 2012) cetaceans
have been estimated using this indirect method.
Recently, the use of tri-axial accelerometers has enabled energetics to be measured on
individual behaviors, and even on an instantaneous basis. Accelerometry-based methods
rely on a quantified relationship between movement in three dimensions and the metabolic power required to fuel that movement. One method, termed overall dynamic body
acceleration (ODBA), integrates the dynamic acceleration of the animal’s body in each of the
three movement vectors. Higher levels of acceleration indicate movements requiring more
metabolic power (e.g., Fahlman et al. 2008; Skinner et al. 2014). A second method involving
accelerometry uses information from only one movement axis to identify individual swim
strokes, the total number of which can be used to estimate overall energy expenditure on a
cost-per-stroke basis (e.g., Williams et al. 2004; Maresh et al. 2014). To be successful, both of
these methods require species-specific calibration of the acceleration–energetics relationship, for which respirometry or doubly labeled water can be useful. While there remains
some uncertainty regarding the use of accelerometry to measure the field energetics of
free-ranging marine mammals (e.g., Dalton et al. 2014), these methods are increasingly
popular as they can provide data at a high temporal and behavioral resolution over long
periods of time (Fahlman et al. 2008).
3.3.3 Measuring field metabolic rates
While the measurement of oxygen consumption makes respirometry the gold standard for
estimating metabolic rates and energetic costs, it is generally limited to captive settings. Of
the various methods available for measuring FMR, the doubly labeled water (DLW) method
has been used most often in free-ranging, foraging pinnipeds (Boyd 2002). Like respirometry, the DLW method is based on the principle that the bodies of living animals are always
in a state of change, constantly exchanging materials such as oxygen, carbon dioxide, fuels,
and water with the external environment. The rate at which these various materials turnover
in the body is proportional to the animal’s metabolic rate (Speakman 1997). DLW specifically
measures the rate of carbon dioxide (CO 2 ) produced in expired gases as a proxy for metabolism and energy expenditure. In this method, water labeled with heavy isotopes of oxygen
(O 18 ) and hydrogen (deuterium D 2 or tritium H 3 ) is injected into the animal, and the rates
of isotope dilution in the body over time are used to determine CO 2 production. Doubly
labeled water performs best over prolonged periods, from hours to days, depending on the
size of the animal, and ultimately provides a single value that represents energy expenditure summed over the entire measurement period (Costa 1987). Researchers interested in
measuring the energy costs of specific, discreet behaviors should consider other methods.
Marine Mammal Physiology: Requisites for Ocean Living
3.3.2 Indirect methods for determining energetic costs
Except in a few unique circumstances, it is not possible to directly measure the oxygen consumption of wild marine mammals. However, by pairing direct measures of metabolism
in controlled circumstances with other physiological or biomechanical parameters, it is
possible to estimate nearly instantaneous to long-term energetic costs for a wide variety of
behaviors. For these measurements calibrated instrumentation is deployed on wild marine
mammals, and records respiration rates, heart rates, acceleration, swimming stroke rates,
or speed. Because each of these parameters is correlated to the rate of oxygen consumption
(e.g., Figures 3.1 and 3.3), they can be used to determine free-ranging energetic costs as the
animals move through the environment. The energetics of swimming, diving, migrating,
and foraging of pinnipeds (Boyd et al. 1999; Butler et al. 2004; Williams et al. 2004) as well
as small (Williams et al. 1992, 2015b) and large (Goldbogen et al. 2008, 2011, 2012) cetaceans
have been estimated using this indirect method.
Recently, the use of tri-axial accelerometers has enabled energetics to be measured on
individual behaviors, and even on an instantaneous basis. Accelerometry-based methods
rely on a quantified relationship between movement in three dimensions and the metabolic power required to fuel that movement. One method, termed overall dynamic body
acceleration (ODBA), integrates the dynamic acceleration of the animal’s body in each of the
three movement vectors. Higher levels of acceleration indicate movements requiring more
metabolic power (e.g., Fahlman et al. 2008; Skinner et al. 2014). A second method involving
accelerometry uses information from only one movement axis to identify individual swim
strokes, the total number of which can be used to estimate overall energy expenditure on a
cost-per-stroke basis (e.g., Williams et al. 2004; Maresh et al. 2014). To be successful, both of
these methods require species-specific calibration of the acceleration–energetics relationship, for which respirometry or doubly labeled water can be useful. While there remains
some uncertainty regarding the use of accelerometry to measure the field energetics of
free-ranging marine mammals (e.g., Dalton et al. 2014), these methods are increasingly
popular as they can provide data at a high temporal and behavioral resolution over long
periods of time (Fahlman et al. 2008).
3.3.3 Measuring field metabolic rates
While the measurement of oxygen consumption makes respirometry the gold standard for
estimating metabolic rates and energetic costs, it is generally limited to captive settings. Of
the various methods available for measuring FMR, the doubly labeled water (DLW) method
has been used most often in free-ranging, foraging pinnipeds (Boyd 2002). Like respirometry, the DLW method is based on the principle that the bodies of living animals are always
in a state of change, constantly exchanging materials such as oxygen, carbon dioxide, fuels,
and water with the external environment. The rate at which these various materials turnover
in the body is proportional to the animal’s metabolic rate (Speakman 1997). DLW specifically
measures the rate of carbon dioxide (CO 2 ) produced in expired gases as a proxy for metabolism and energy expenditure. In this method, water labeled with heavy isotopes of oxygen
(O 18 ) and hydrogen (deuterium D 2 or tritium H 3 ) is injected into the animal, and the rates
of isotope dilution in the body over time are used to determine CO 2 production. Doubly
labeled water performs best over prolonged periods, from hours to days, depending on the
size of the animal, and ultimately provides a single value that represents energy expenditure summed over the entire measurement period (Costa 1987). Researchers interested in
measuring the energy costs of specific, discreet behaviors should consider other methods.