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Marine Mammal Physiology: Requisites for Ocean Living
One major factor influencing the morphological design of marine mammals is how
much time they spend in water, and what they do while in it. Some animals have evolved a
body shape and propulsion system that not only minimizes drag, but also optimizes thrust
generation (i.e., whales and dolphins). Not surprisingly, these animals happen to travel over
very large distances in the water or chase very fast prey. Other animals have not optimized
their hydrodynamics but rather other anatomical characteristics that are more important
for other critical life functions, many of which occur on land, at the expense of increased
locomotor costs in water. In effect, many marine mammals are faced with conflicting physiological demands of an amphibious lifestyle (i.e., pinnipeds), while others have evolved fully
aquatic niches (i.e., cetaceans). In this chapter, we explore the mechanisms that marine mammals use to achieve different levels of swimming performance. Moreover, we will review
specific anatomical and behavioral adaptations in a comparative context to understand the
evolution of locomotor traits that characterize this extraordinary guild of oceanic predators.
1.2 Hydrodynamic forces at play during
locomotion: Drag, lift, and thrust
1.2.1 Flow structures around a body moving underwater
As seen from a distance, the flow of water moving past a marine mammal coasting or
gliding in water appears streamlined and orderly (Figure 1.1a). The incoming flows—or
freestream—part ways laterally and dorsoventrally at the nose (or rostrum), to accelerate
to speeds higher than the freestream’s until the body’s widest section is reached. Past this
point the flows begin to decelerate, and then rejoin to the speed of the freestream past the
body. At smaller scales near the animal’s body (millimeters to decimeters, Figure 1.1bc)
there is a thin layer of fluid “tucked” under the nearest of the fluid streamlines. Typically
moving at significantly slower speeds, this layer is called the boundary layer (Vogel 2003).
Such a thin sheet of water may also appear streamlined, or laminar, like the flows above
it; but in the right conditions, it will also appear turbulent, that is, populated with swirls
or eddies of all sizes spinning in all directions and at varying rates. The thickness of the
boundary layer generally increases posteriorly; along the body of a blue whale, for example, it is estimated to be only a few millimeters thick at the rostrum and about 0.2 m at the
end of the tail. Whether laminar or turbulent, the boundary layer at the tail of a gliding
animal then yields the turbulent wake past the tail. This wake is “wrapped” within the
streamlines of fluid that are rejoined behind the body (Figure 1.1c).
In contrast, the flow structure about actively swimming mammals is more complicated
(Fish 1993). Here, the animal’s appendages accelerate portions of the surrounding flows, to
add large-scale tornado-like vortical structures that may persist long after the animal has
passed by. These vortices can be seen on the surface of the water when paddling in a kayak
or canoe. Undulating hydrofoils—or flukes and fins—produce them as well. Generally vortices, and their small-scale eddy cousins within the boundary layer, tend to form whenever
a layer of fast fluid moves past a slow-moving layer, or one moving in the opposite direction.
The drag generated by marine mammals is related to the interaction between boundarylayer flows and large-scale flows. Such a link is crucial, as painfully realized by the aircraft
aerodynamicists of the early 1900s who, without knowing about, or in some cases not acknowledging the existence of the boundary layer, kept calculating the wrong drag (Bloor 2011). The
boundary layer arises from two basic forces: (1) shear forces in-between adjacent layers of
fluid are caused by viscosity, which tend to resist the layers’ relative motion as well as induce
Marine Mammal Physiology: Requisites for Ocean Living
One major factor influencing the morphological design of marine mammals is how
much time they spend in water, and what they do while in it. Some animals have evolved a
body shape and propulsion system that not only minimizes drag, but also optimizes thrust
generation (i.e., whales and dolphins). Not surprisingly, these animals happen to travel over
very large distances in the water or chase very fast prey. Other animals have not optimized
their hydrodynamics but rather other anatomical characteristics that are more important
for other critical life functions, many of which occur on land, at the expense of increased
locomotor costs in water. In effect, many marine mammals are faced with conflicting physiological demands of an amphibious lifestyle (i.e., pinnipeds), while others have evolved fully
aquatic niches (i.e., cetaceans). In this chapter, we explore the mechanisms that marine mammals use to achieve different levels of swimming performance. Moreover, we will review
specific anatomical and behavioral adaptations in a comparative context to understand the
evolution of locomotor traits that characterize this extraordinary guild of oceanic predators.
1.2 Hydrodynamic forces at play during
locomotion: Drag, lift, and thrust
1.2.1 Flow structures around a body moving underwater
As seen from a distance, the flow of water moving past a marine mammal coasting or
gliding in water appears streamlined and orderly (Figure 1.1a). The incoming flows—or
freestream—part ways laterally and dorsoventrally at the nose (or rostrum), to accelerate
to speeds higher than the freestream’s until the body’s widest section is reached. Past this
point the flows begin to decelerate, and then rejoin to the speed of the freestream past the
body. At smaller scales near the animal’s body (millimeters to decimeters, Figure 1.1bc)
there is a thin layer of fluid “tucked” under the nearest of the fluid streamlines. Typically
moving at significantly slower speeds, this layer is called the boundary layer (Vogel 2003).
Such a thin sheet of water may also appear streamlined, or laminar, like the flows above
it; but in the right conditions, it will also appear turbulent, that is, populated with swirls
or eddies of all sizes spinning in all directions and at varying rates. The thickness of the
boundary layer generally increases posteriorly; along the body of a blue whale, for example, it is estimated to be only a few millimeters thick at the rostrum and about 0.2 m at the
end of the tail. Whether laminar or turbulent, the boundary layer at the tail of a gliding
animal then yields the turbulent wake past the tail. This wake is “wrapped” within the
streamlines of fluid that are rejoined behind the body (Figure 1.1c).
In contrast, the flow structure about actively swimming mammals is more complicated
(Fish 1993). Here, the animal’s appendages accelerate portions of the surrounding flows, to
add large-scale tornado-like vortical structures that may persist long after the animal has
passed by. These vortices can be seen on the surface of the water when paddling in a kayak
or canoe. Undulating hydrofoils—or flukes and fins—produce them as well. Generally vortices, and their small-scale eddy cousins within the boundary layer, tend to form whenever
a layer of fast fluid moves past a slow-moving layer, or one moving in the opposite direction.
The drag generated by marine mammals is related to the interaction between boundarylayer flows and large-scale flows. Such a link is crucial, as painfully realized by the aircraft
aerodynamicists of the early 1900s who, without knowing about, or in some cases not acknowledging the existence of the boundary layer, kept calculating the wrong drag (Bloor 2011). The
boundary layer arises from two basic forces: (1) shear forces in-between adjacent layers of
fluid are caused by viscosity, which tend to resist the layers’ relative motion as well as induce