57
Chapter three: Exercise energetics
that of a terrestrial mammal up to approximately 250 kg, above which a marine mammal’s
field metabolic rate becomes more economical than that of a similarly sized terrestrial
mammal. The diversity of marine mammal groups comprising the FMR relationship may
be a critical underlying factor driving this trend. Thus, Equation 3.4, while useful as a first
approximation of field metabolic rate, should be considered an estimate of the diverse
energy needs of the many marine species that make up this mammalian group.
Based on their taxonomic and ecological diversity, we might expect different metabolic adaptations for aquatic living by mustelids, otariids, phocids, odontocetes, and
mysticetes. Indeed, phylogeny does seem to explain much of the residual variation of
FMR after body size is taken into account. Among marine mammals, phylogeny is highly
correlated with activity levels, reproductive strategies, locomotor mechanics, and other
physiological and ecological drivers of overall energy expenditure. For example, among
the Pinnipedia, phocid seals tend to swim at more economical speeds and engage in
less energetically costly acrobatic maneuvers than otariids (Fish 1994; Chapter 1) which
is reflected in their FMR. Both athletic otariids and odonotocetes demonstrate higher
field metabolic rates relative to the general mammalian trend. Mysticete whales are more
likely to swim at low-cost cruising speeds rather than engage in costly sprints typical of
small odontocetes. To date, field metabolic rates have not been directly measured in adult
mysticetes. However, based on shared economical swimming patterns, it is reasonable to
predict that the FMR of this cetacean group may follow the trends of phocid seals rather
than those of otariids. Not surprisingly, sea otters, as small-bodied surface swimmers,
show higher than predicted field metabolic rates among the marine mammals (Thometz
et al. 2014). Using the same logic, we might also predict that Hawaiian monk seals and
sirenians, as moderately sized, slow, tropical swimmers, will have low field metabolic
rates compared to other marine mammal groups; this remains to be tested.
Because body size and phylogeny are correlated, it is possible to assign the major
marine mammal taxonomic groups to one of two distinct pace of life categories based on
these metrics. Species with a fast pace of life tend to be relatively small, highly active swimmers, and shallow diving, with a reproductive strategy that involves long lactation periods
and little-to-no separation of foraging from lactation (i.e., income breeders). Membership
in this group includes otariid seals, most odontocetes, and sea otters. Species with a slow
pace of life tend to be relatively large, economical swimmers, and deep diving, with a
reproductive strategy that involves short lactation periods and geographical and temporal separation of foraging from lactation (i.e., capital breeders). Membership in this group
includes phocid seals, and possibly some of the larger but inaccessible odontocetes and
mysticetes. As the only herbivorous marine mammal group, sirenians represent a special
case of a taxonomic group with exceptionally low energy costs below those of any similarly sized marine or terrestrial mammal. Tropical living, as exemplified by the Hawaiian
monk seal, may also contribute to the slower life pace of these marine mammals.
3.2.4 Behavioral strategies to reduce the energetic cost of swimming and diving
In contrast to the constant stroking of swimming humans, marine mammals display a
wide range of unsteady swimming behaviors when moving through water (Williams et al.
2015a). Both pinnipeds and cetaceans use extended glides, burst-and-glide swimming,
wave-riding, and for some species a roller-coaster pattern of movements while transiting
(Davis et al. 2003; Davis and Weihs 2007) and foraging (Williams et al. 2015b) that reduce
the energetic costs associated with locomotion. The obvious benefit of these behaviors is
the conservation of oxygen stores and prolongation of aerobic dives.
Chapter three: Exercise energetics
that of a terrestrial mammal up to approximately 250 kg, above which a marine mammal’s
field metabolic rate becomes more economical than that of a similarly sized terrestrial
mammal. The diversity of marine mammal groups comprising the FMR relationship may
be a critical underlying factor driving this trend. Thus, Equation 3.4, while useful as a first
approximation of field metabolic rate, should be considered an estimate of the diverse
energy needs of the many marine species that make up this mammalian group.
Based on their taxonomic and ecological diversity, we might expect different metabolic adaptations for aquatic living by mustelids, otariids, phocids, odontocetes, and
mysticetes. Indeed, phylogeny does seem to explain much of the residual variation of
FMR after body size is taken into account. Among marine mammals, phylogeny is highly
correlated with activity levels, reproductive strategies, locomotor mechanics, and other
physiological and ecological drivers of overall energy expenditure. For example, among
the Pinnipedia, phocid seals tend to swim at more economical speeds and engage in
less energetically costly acrobatic maneuvers than otariids (Fish 1994; Chapter 1) which
is reflected in their FMR. Both athletic otariids and odonotocetes demonstrate higher
field metabolic rates relative to the general mammalian trend. Mysticete whales are more
likely to swim at low-cost cruising speeds rather than engage in costly sprints typical of
small odontocetes. To date, field metabolic rates have not been directly measured in adult
mysticetes. However, based on shared economical swimming patterns, it is reasonable to
predict that the FMR of this cetacean group may follow the trends of phocid seals rather
than those of otariids. Not surprisingly, sea otters, as small-bodied surface swimmers,
show higher than predicted field metabolic rates among the marine mammals (Thometz
et al. 2014). Using the same logic, we might also predict that Hawaiian monk seals and
sirenians, as moderately sized, slow, tropical swimmers, will have low field metabolic
rates compared to other marine mammal groups; this remains to be tested.
Because body size and phylogeny are correlated, it is possible to assign the major
marine mammal taxonomic groups to one of two distinct pace of life categories based on
these metrics. Species with a fast pace of life tend to be relatively small, highly active swimmers, and shallow diving, with a reproductive strategy that involves long lactation periods
and little-to-no separation of foraging from lactation (i.e., income breeders). Membership
in this group includes otariid seals, most odontocetes, and sea otters. Species with a slow
pace of life tend to be relatively large, economical swimmers, and deep diving, with a
reproductive strategy that involves short lactation periods and geographical and temporal separation of foraging from lactation (i.e., capital breeders). Membership in this group
includes phocid seals, and possibly some of the larger but inaccessible odontocetes and
mysticetes. As the only herbivorous marine mammal group, sirenians represent a special
case of a taxonomic group with exceptionally low energy costs below those of any similarly sized marine or terrestrial mammal. Tropical living, as exemplified by the Hawaiian
monk seal, may also contribute to the slower life pace of these marine mammals.
3.2.4 Behavioral strategies to reduce the energetic cost of swimming and diving
In contrast to the constant stroking of swimming humans, marine mammals display a
wide range of unsteady swimming behaviors when moving through water (Williams et al.
2015a). Both pinnipeds and cetaceans use extended glides, burst-and-glide swimming,
wave-riding, and for some species a roller-coaster pattern of movements while transiting
(Davis et al. 2003; Davis and Weihs 2007) and foraging (Williams et al. 2015b) that reduce
the energetic costs associated with locomotion. The obvious benefit of these behaviors is
the conservation of oxygen stores and prolongation of aerobic dives.