11
Chapter one: Hydrodynamics
1.3 Evolutionary biomechanics of marine mammal
locomotion: How different marine mammals achieve
different levels of locomotor performance
Thrust for swimming is generated from the interaction of the water with the movement of a
propulsive surface (e.g., paddles, flippers, flukes). Propulsors, therefore, are large in span and
area to increase the volume of water accelerated by the excursion of the propulsor (Blake 1981;
Fish 1993a). The propulsive movements can be classified broadly as drag-based oscillatory, liftbased oscillatory, and undulatory (Webb and Blake 1985). Oscillatory propulsion (both dragand lift-based) uses the motion of the paired appendages (e.g., feet) or a highly modified lunate
tail (e.g., flukes), whereas undulatory propulsion uses the movements of the body and tail.
1.3.1 Mechanics of drag-based paddling
Drag-based propulsion is used by semi-aquatic (e.g., muskrat, beaver, platypus, otter) and
fully aquatic mammals (e.g., manatee, humpback whale) for swimming and maneuvering
(Howell 1930; Fish 1996). Propulsion by drag-based oscillation is produced by the motion
of various combinations of the paired appendages (quadrupedal, pectoral, pelvic) either
alternately or simultaneously and oriented in either the parasagittal or horizontal planes
(Howell 1930; Fish 1996). The stroke cycle includes the power and recovery phases (Fish
1984; Fish and Baudinette 1999). In the power phase, the posterior sweep of the limb generates a large pressure drag, which provides an anterior thrust.
Maximum thrust is generated with a broad paddle area that is configured as a circle or
triangle with a constriction at the attachment point with the body (Fish 2004). The constriction minimizes interference drag with the body and provides a long lever arm to increase
the velocity of the paddle during the power stroke. Paddle area is increased by abduction
of the digits and by interdigital webbing or fringe hairs (Howell 1930; Fish 1984; Thewissen
and Fish 1997). The increased paddle area allows for the production of a high-pressure
drag on the paddle as it is swept posteriorly. The reaction force to the pressure drag is the
thrust that moves the paddling animal forward. The size of the paddle accelerates a large
mass of fluid to a low velocity, which is more efficient than accelerating a small mass of
fluid to a high velocity (Fish 2004). The recovery phase repositions the limb, incurring a
non-thrust generating drag. To limit the reduction in thrust during the recovery phase,
drag on the appendage is reduced by adducting the digits or rotating the appendage to
reduce the paddle area and by changing the timing of movement to reduce the relative
velocity (Fish 1984).
Drag-based oscillation has a low propulsive efficiency (thrust power/total mechanical
power output) of ≤0.33 (Fish 1984, 1992). This low efficiency occurs because thrust is generated through only half of the stroke cycle (Fish 1984). Energy is lost to increased resistive
drag as the foot is repositioned during the recovery phase. In addition, approximately
40%–50% of the total energy expended through the stroke is lost in acceleration of the
mass of the limb and the water entrained to the foot (Fish 2000). Propulsive efficiency for
the drag-based oscillation is highest at low speeds (Vogel 1994).
1.3.2 Mechanics of lift-based oscillation
Lift-based oscillation is a high-performance swimming mode (Lighthill 1969; Webb 1975;
Fish 1996). Several marine mammal taxa exhibit lift-based swimming modes including
cetaceans and sirenians (caudal flukes), otariid seals (pectoral flippers), and phocid seals
Chapter one: Hydrodynamics
1.3 Evolutionary biomechanics of marine mammal
locomotion: How different marine mammals achieve
different levels of locomotor performance
Thrust for swimming is generated from the interaction of the water with the movement of a
propulsive surface (e.g., paddles, flippers, flukes). Propulsors, therefore, are large in span and
area to increase the volume of water accelerated by the excursion of the propulsor (Blake 1981;
Fish 1993a). The propulsive movements can be classified broadly as drag-based oscillatory, liftbased oscillatory, and undulatory (Webb and Blake 1985). Oscillatory propulsion (both dragand lift-based) uses the motion of the paired appendages (e.g., feet) or a highly modified lunate
tail (e.g., flukes), whereas undulatory propulsion uses the movements of the body and tail.
1.3.1 Mechanics of drag-based paddling
Drag-based propulsion is used by semi-aquatic (e.g., muskrat, beaver, platypus, otter) and
fully aquatic mammals (e.g., manatee, humpback whale) for swimming and maneuvering
(Howell 1930; Fish 1996). Propulsion by drag-based oscillation is produced by the motion
of various combinations of the paired appendages (quadrupedal, pectoral, pelvic) either
alternately or simultaneously and oriented in either the parasagittal or horizontal planes
(Howell 1930; Fish 1996). The stroke cycle includes the power and recovery phases (Fish
1984; Fish and Baudinette 1999). In the power phase, the posterior sweep of the limb generates a large pressure drag, which provides an anterior thrust.
Maximum thrust is generated with a broad paddle area that is configured as a circle or
triangle with a constriction at the attachment point with the body (Fish 2004). The constriction minimizes interference drag with the body and provides a long lever arm to increase
the velocity of the paddle during the power stroke. Paddle area is increased by abduction
of the digits and by interdigital webbing or fringe hairs (Howell 1930; Fish 1984; Thewissen
and Fish 1997). The increased paddle area allows for the production of a high-pressure
drag on the paddle as it is swept posteriorly. The reaction force to the pressure drag is the
thrust that moves the paddling animal forward. The size of the paddle accelerates a large
mass of fluid to a low velocity, which is more efficient than accelerating a small mass of
fluid to a high velocity (Fish 2004). The recovery phase repositions the limb, incurring a
non-thrust generating drag. To limit the reduction in thrust during the recovery phase,
drag on the appendage is reduced by adducting the digits or rotating the appendage to
reduce the paddle area and by changing the timing of movement to reduce the relative
velocity (Fish 1984).
Drag-based oscillation has a low propulsive efficiency (thrust power/total mechanical
power output) of ≤0.33 (Fish 1984, 1992). This low efficiency occurs because thrust is generated through only half of the stroke cycle (Fish 1984). Energy is lost to increased resistive
drag as the foot is repositioned during the recovery phase. In addition, approximately
40%–50% of the total energy expended through the stroke is lost in acceleration of the
mass of the limb and the water entrained to the foot (Fish 2000). Propulsive efficiency for
the drag-based oscillation is highest at low speeds (Vogel 1994).
1.3.2 Mechanics of lift-based oscillation
Lift-based oscillation is a high-performance swimming mode (Lighthill 1969; Webb 1975;
Fish 1996). Several marine mammal taxa exhibit lift-based swimming modes including
cetaceans and sirenians (caudal flukes), otariid seals (pectoral flippers), and phocid seals