51
Chapter three: Exercise energetics
Because the Kleiber standards were originally developed for terrestrial mammals
measured in air, applying them to semi- or fully aquatic mammals has proved challenging. This has led to differing opinions as to what represents resting in a marine mammal. It is unclear whether resting metabolic rates of sea otters, pinnipeds and cetaceans
measured in air is comparable to that measured in water. The dive response, described
earlier and in Chapter 2, complicates measurements as it can affect metabolic rate through
cardiovascular changes within individual tissues (Zapol et al. 1979). For some species,
face immersion is enough to trigger this response (Ridgway et al. 1975). Thus, metabolic
rates measured during submersion are considered by some to be confounded by the dive
response; others believe these represent true baseline energetic costs in marine mammals. The definition of maintenance metabolism is especially complicated for species
such as cetaceans and sirenians, for whom measurements on land are neither feasible nor
physiologically relevant. For deep-diving species, some non-essential tissues may enter a
hypometabolic state which reduces the rate of oxygen consumption while diving (Maresh
et al. in review) as well as provides a critical seasonal strategy for conserving energy
while fasting (Tift et al. 2013). For these reasons, comparisons of the baseline resting costs
between the various marine mammal groups or between marine mammals and terrestrial mammals are not always straightforward.
Despite the limitations, some general trends in resting metabolism are apparent for
the major marine mammal groups (Figure 3.2a). The relationship between body mass and
Resting Metabolic Rate (RMR MM ) for marine mammals is best described by
RMR
581mass
n 12 species r
66
MM
68
2
=
=
=
0
0
.
(
,
. )
(3.1)
where
metabolic rate is in kJ day −1
body mass is in kg (Maresh 2014)
From this relationship, it appears that with the exception of the sirenians and the tropical
Hawaiian monk seal (Monachus schauinslandi, Williams et al. 2011), marine mammals have
approximately twice the resting metabolic costs of similarly sized terrestrial mammals.
This trend is most consistent in otariids, cetaceans and sea otters, but more variable in
phocid seals. Notably, this allometric regression includes marine mammals ranging in
size from a 25 kg sea otter (Enhydra lutris) to a 5300 kg killer whale (Orcinus orca). Adult
mysticete whales far exceed this mass range (e.g., an adult blue whale, Balaenoptera musculus, is >180,000 kg; Folkens et al. 2002), and the Resting Metabolic Rates of the largest of
the mysticetes have never been directly measured. It is unknown how such extreme body
size will affect the allometric relationship for Resting Metabolic Rate in marine mammals.
Many explanations have been proposed for the observed elevation in Resting Metabolic
Rates of marine mammals compared to terrestrial mammals predicted by Kleiber (1975).
Currently, the most widely accepted hypothesis suggests that an elevation in metabolic
rate is necessary to offset the high cost of endothermy when in water (e.g., Speakman and
Król 2010; Hudson et al. 2013). Because thermoregulation may not be a problem for all but
the smallest marine mammals (Boyd 2002; Porter and Kearney 2009), many investigators
have proposed that other traits in marine mammals set metabolism (e.g., Liwanag et al.
2012; Heim et al. 2015). In these studies, high metabolic rates are considered an exaptation
rather than an adaptation per se, whereby the elevated metabolism of ancestral carnivores
that secondarily invaded the marine environment provided a competitive advantage over
Chapter three: Exercise energetics
Because the Kleiber standards were originally developed for terrestrial mammals
measured in air, applying them to semi- or fully aquatic mammals has proved challenging. This has led to differing opinions as to what represents resting in a marine mammal. It is unclear whether resting metabolic rates of sea otters, pinnipeds and cetaceans
measured in air is comparable to that measured in water. The dive response, described
earlier and in Chapter 2, complicates measurements as it can affect metabolic rate through
cardiovascular changes within individual tissues (Zapol et al. 1979). For some species,
face immersion is enough to trigger this response (Ridgway et al. 1975). Thus, metabolic
rates measured during submersion are considered by some to be confounded by the dive
response; others believe these represent true baseline energetic costs in marine mammals. The definition of maintenance metabolism is especially complicated for species
such as cetaceans and sirenians, for whom measurements on land are neither feasible nor
physiologically relevant. For deep-diving species, some non-essential tissues may enter a
hypometabolic state which reduces the rate of oxygen consumption while diving (Maresh
et al. in review) as well as provides a critical seasonal strategy for conserving energy
while fasting (Tift et al. 2013). For these reasons, comparisons of the baseline resting costs
between the various marine mammal groups or between marine mammals and terrestrial mammals are not always straightforward.
Despite the limitations, some general trends in resting metabolism are apparent for
the major marine mammal groups (Figure 3.2a). The relationship between body mass and
Resting Metabolic Rate (RMR MM ) for marine mammals is best described by
RMR
581mass
n 12 species r
66
MM
68
2
=
=
=
0
0
.
(
,
. )
(3.1)
where
metabolic rate is in kJ day −1
body mass is in kg (Maresh 2014)
From this relationship, it appears that with the exception of the sirenians and the tropical
Hawaiian monk seal (Monachus schauinslandi, Williams et al. 2011), marine mammals have
approximately twice the resting metabolic costs of similarly sized terrestrial mammals.
This trend is most consistent in otariids, cetaceans and sea otters, but more variable in
phocid seals. Notably, this allometric regression includes marine mammals ranging in
size from a 25 kg sea otter (Enhydra lutris) to a 5300 kg killer whale (Orcinus orca). Adult
mysticete whales far exceed this mass range (e.g., an adult blue whale, Balaenoptera musculus, is >180,000 kg; Folkens et al. 2002), and the Resting Metabolic Rates of the largest of
the mysticetes have never been directly measured. It is unknown how such extreme body
size will affect the allometric relationship for Resting Metabolic Rate in marine mammals.
Many explanations have been proposed for the observed elevation in Resting Metabolic
Rates of marine mammals compared to terrestrial mammals predicted by Kleiber (1975).
Currently, the most widely accepted hypothesis suggests that an elevation in metabolic
rate is necessary to offset the high cost of endothermy when in water (e.g., Speakman and
Król 2010; Hudson et al. 2013). Because thermoregulation may not be a problem for all but
the smallest marine mammals (Boyd 2002; Porter and Kearney 2009), many investigators
have proposed that other traits in marine mammals set metabolism (e.g., Liwanag et al.
2012; Heim et al. 2015). In these studies, high metabolic rates are considered an exaptation
rather than an adaptation per se, whereby the elevated metabolism of ancestral carnivores
that secondarily invaded the marine environment provided a competitive advantage over