58
Marine Mammal Physiology: Requisites for Ocean Living
Two key factors, buoyancy control and drag reduction through body streamlining,
facilitate these energy-saving strategies by marine mammals. As discussed in Chapter 1,
the streamlined shape and submerged swimming position of marine mammals reduce
the overall hydrodynamic drag encountered, and the associated energy required for moving through the water. Once again this is illustrated by the sea otter, a unique species of
marine mammal that can swim for extended periods either on the water surface or submerged. The oxygen consumption of sea otters swimming submerged at 0.80 m s −1 is 17.55
ml O 2 kg −1 min −1 , which is reduced by more than 40% if the animal simply changes to a
submerged mode of swimming at the same speed (Williams 1989). As discussed above,
this difference impacts the minimum cost of transport of sea otters, which is 69% higher
for the animals swimming on the water surface than when submerged (Figure 3.3c). In
view of this, it is not surprising that marine mammals spend 90%–95% of their time below
the water surface when migrating across ocean basins, moving between prey patches, or
even transiting short distances.
The necessity to breathe requires that marine mammals periodically surface, and
even here we find several behaviors that enable the animals to reduce the energetic cost
of swimming. High speed swimming near the water surface is exceptionally expensive
due to additional drag associated with the generation of waves (Chapter 1). Consequently,
surface intervals tend to be brief for marine mammals, ranging from <1 s for fast swimming dolphins and otariids (Hui 1989) to 3–5 s and longer for harbor seals (Phoca vitulina)
(Williams et al. 1991) and large whales.
Body streamlining facilitates other cost-efficient behaviors when fast-moving marine
mammals swim near the water surface. These include wave-riding and porpoising. As the
latter name implies these behaviors are most often equated with small cetaceans; however,
sea lions, harbor seals, and fur seals have also been observed to occasionally perform both
energy-saving strategies when swimming at high speed.
Porpoising enables fast-swimming mammals to avoid the problem of elevated drag
on the water surface by leaping into the air. By comparing the theoretical energetic costs
of dolphins swimming on the water surface to the costs associated with swimming submerged and leaping, Au and Weihs (1980) and Blake (1983) predicted the most economical
swimming positions based on the speed of forward movement. For all, speeds swimming
more than three body diameters below the water surface were the most economical position for a dolphin. When surfacing to breathe, the optimum swimming position changed
with speed. Theoretically, the cost of leaping is greater than the energy expended to overcome surface drag at slow speeds, so slow swimming dolphins should remain in the water
when breathing at the surface. At high speeds, the relative energetic costs are reversed as
wave drag increases exponentially. As a result, the most energetically efficient strategy is
to take to the air and porpoise when taking a breath.
Wave-riding by dolphins has been described by observers aboard ships since the
time of Greek mythology. Wild dolphins appear to surf effortlessly by positioning their
streamlined bodies in the bow or stern wakes of boats, matching the speed of the vessel
without propulsive movements by their flukes (Scholander 1959). Indeed, the physiological
responses of bottlenose dolphins trained to ride in the wake of a research boat demonstrated a significant energetic benefit to wave-riding (Williams et al. 1992). The heart rate,
respiration rate, and by inference metabolic rate were reduced when dolphins changed
from active swimming near the water surface to wave-riding. The resulting minimum cost
of transport for wave-riding at 3.8 m s −1 was nearly identical to that recorded for the same
dolphins freely swimming at 2.1 m s −1 outside of the wake zone of the boat. In this case,
wave-riding enabled the dolphins to move twice as fast for the same energetic expenditure.
Marine Mammal Physiology: Requisites for Ocean Living
Two key factors, buoyancy control and drag reduction through body streamlining,
facilitate these energy-saving strategies by marine mammals. As discussed in Chapter 1,
the streamlined shape and submerged swimming position of marine mammals reduce
the overall hydrodynamic drag encountered, and the associated energy required for moving through the water. Once again this is illustrated by the sea otter, a unique species of
marine mammal that can swim for extended periods either on the water surface or submerged. The oxygen consumption of sea otters swimming submerged at 0.80 m s −1 is 17.55
ml O 2 kg −1 min −1 , which is reduced by more than 40% if the animal simply changes to a
submerged mode of swimming at the same speed (Williams 1989). As discussed above,
this difference impacts the minimum cost of transport of sea otters, which is 69% higher
for the animals swimming on the water surface than when submerged (Figure 3.3c). In
view of this, it is not surprising that marine mammals spend 90%–95% of their time below
the water surface when migrating across ocean basins, moving between prey patches, or
even transiting short distances.
The necessity to breathe requires that marine mammals periodically surface, and
even here we find several behaviors that enable the animals to reduce the energetic cost
of swimming. High speed swimming near the water surface is exceptionally expensive
due to additional drag associated with the generation of waves (Chapter 1). Consequently,
surface intervals tend to be brief for marine mammals, ranging from <1 s for fast swimming dolphins and otariids (Hui 1989) to 3–5 s and longer for harbor seals (Phoca vitulina)
(Williams et al. 1991) and large whales.
Body streamlining facilitates other cost-efficient behaviors when fast-moving marine
mammals swim near the water surface. These include wave-riding and porpoising. As the
latter name implies these behaviors are most often equated with small cetaceans; however,
sea lions, harbor seals, and fur seals have also been observed to occasionally perform both
energy-saving strategies when swimming at high speed.
Porpoising enables fast-swimming mammals to avoid the problem of elevated drag
on the water surface by leaping into the air. By comparing the theoretical energetic costs
of dolphins swimming on the water surface to the costs associated with swimming submerged and leaping, Au and Weihs (1980) and Blake (1983) predicted the most economical
swimming positions based on the speed of forward movement. For all, speeds swimming
more than three body diameters below the water surface were the most economical position for a dolphin. When surfacing to breathe, the optimum swimming position changed
with speed. Theoretically, the cost of leaping is greater than the energy expended to overcome surface drag at slow speeds, so slow swimming dolphins should remain in the water
when breathing at the surface. At high speeds, the relative energetic costs are reversed as
wave drag increases exponentially. As a result, the most energetically efficient strategy is
to take to the air and porpoise when taking a breath.
Wave-riding by dolphins has been described by observers aboard ships since the
time of Greek mythology. Wild dolphins appear to surf effortlessly by positioning their
streamlined bodies in the bow or stern wakes of boats, matching the speed of the vessel
without propulsive movements by their flukes (Scholander 1959). Indeed, the physiological
responses of bottlenose dolphins trained to ride in the wake of a research boat demonstrated a significant energetic benefit to wave-riding (Williams et al. 1992). The heart rate,
respiration rate, and by inference metabolic rate were reduced when dolphins changed
from active swimming near the water surface to wave-riding. The resulting minimum cost
of transport for wave-riding at 3.8 m s −1 was nearly identical to that recorded for the same
dolphins freely swimming at 2.1 m s −1 outside of the wake zone of the boat. In this case,
wave-riding enabled the dolphins to move twice as fast for the same energetic expenditure.