13
Chapter one: Hydrodynamics
1.3.3 Undulatory swimming
In terms of performance (i.e., speed, efficiency), undulatory swimming is intermediate between drag-based oscillation and lift-based oscillation. In undulatory swimming,
the body and tail are bent into a wave that travels backward at a velocity faster than the
animal is moving forward (Webb 1975). For otters and manatees, the undulatory wave is
generated by flexion and extension of the spine. As each section of the body accelerates
vertically, the wave faces caudally at an angle to the mean motion of the body. Fluid adjacent to the accelerated section produces a reaction force with a component in the direction
of thrust (Lighthill 1971). As the traveling moves along the body, its amplitude increases
to a maximum at the tip of the tail, where the velocity of the tip is high. For river otters
(Lontra canadensis), the tail tapers to a point reducing its effectiveness in thrust generation (Fish 1994). The giant river otter (Pteroneura brasiliensis) has a laterally expanded tail
that is undulated during simultaneous paddling of the hind feet (Fish 2001). The manatee
(Trichechus sp.) possesses a broad paddle-like tail, which is undulated in the vertical plane
(Kojeszewski and Fish 2007). The propulsive efficiency of the manatee is 0.67–0.81 with
the highest value at a swimming speed of 0.4 body lengths/s (Kojeszewski and Fish 2007).
1.3.4 Transition from drag-based to lift-based locomotion
The evolution of highly derived aquatic mammals (pinnipeds, cetaceans, and sirenians) represents the culmination of a sequence of transitional stages extending from terrestrial quadrupeds to fully aquatic mammals capable of high-performance propulsion
(Figure 1.5) (Howell 1930; Gingerich et al. 1990; Fish 1998a; Thewissen 2014). Semi-aquatic
mammals are in an evolutionarily precarious position, being unspecialized for locomotor performance in either terrestrial or aquatic environments. The energetic cost of being
semi-aquatic is higher than being adaptated for land or water (Williams 1999). Specialized
lift-based swimming modes that use oscillation of flippers or flukes have low minimum
costs of transport (i.e., metabolic energy consumed to move a unit mass a unit distance)
for aquatic mammals, whereas paddling has the highest minimum costs of transport and
undulation is intermediate (Fish 2000).
Selective pressures would have been high in the transition to a more aquatic lifestyle.
The evolutionary changes would have been directed by increases in swimming speed,
propulsive efficiency, energy economy, and dive time, while constrained by morphology
and neuromotor patterns. In recent years, fossil species have been discovered, which have
added to our knowledge of the transitional stages that evolved into the highly derived
aquatic mammals (Thewissen 2014). However, it has only been through observation and
experimentation on living analogs of the transitional forms that the selective pressures
have been determined directing the course to the pinnipeds, cetaceans, and sirenians.
As mammals became semi-aquatic, the earliest species, such as the pakicetids, would
have used a modified terrestrial, quadrupedal gait to move on along the bottom substrate
(e.g., hippopotamus; Coughlin and Fish 2009) or swim in a manner similar to the “dog paddle” (e.g., mink, Mustela vison; Williams 1983). The primitive quadrupedal gait would eventually be replaced with bipedal paddling, with alternating motions of either the pectoral or
pelvic extremities as exemplified by the polar bear (Ursus maritimus) and muskrat (Ondatra
zibethicus), respectively (Flyger and Townsend 1968; Fish 1984). Otters (L. canadensis) are able
to swim quadrupedally, but they generally swim with 2 feet (Tarasoff et al. 1972; Fish 1994).
Bipedal paddling avoids mechanical and hydrodynamic interference between the
ipsolateral limbs, increasing propulsive efficiency. This mode of swimming frees one set
Chapter one: Hydrodynamics
1.3.3 Undulatory swimming
In terms of performance (i.e., speed, efficiency), undulatory swimming is intermediate between drag-based oscillation and lift-based oscillation. In undulatory swimming,
the body and tail are bent into a wave that travels backward at a velocity faster than the
animal is moving forward (Webb 1975). For otters and manatees, the undulatory wave is
generated by flexion and extension of the spine. As each section of the body accelerates
vertically, the wave faces caudally at an angle to the mean motion of the body. Fluid adjacent to the accelerated section produces a reaction force with a component in the direction
of thrust (Lighthill 1971). As the traveling moves along the body, its amplitude increases
to a maximum at the tip of the tail, where the velocity of the tip is high. For river otters
(Lontra canadensis), the tail tapers to a point reducing its effectiveness in thrust generation (Fish 1994). The giant river otter (Pteroneura brasiliensis) has a laterally expanded tail
that is undulated during simultaneous paddling of the hind feet (Fish 2001). The manatee
(Trichechus sp.) possesses a broad paddle-like tail, which is undulated in the vertical plane
(Kojeszewski and Fish 2007). The propulsive efficiency of the manatee is 0.67–0.81 with
the highest value at a swimming speed of 0.4 body lengths/s (Kojeszewski and Fish 2007).
1.3.4 Transition from drag-based to lift-based locomotion
The evolution of highly derived aquatic mammals (pinnipeds, cetaceans, and sirenians) represents the culmination of a sequence of transitional stages extending from terrestrial quadrupeds to fully aquatic mammals capable of high-performance propulsion
(Figure 1.5) (Howell 1930; Gingerich et al. 1990; Fish 1998a; Thewissen 2014). Semi-aquatic
mammals are in an evolutionarily precarious position, being unspecialized for locomotor performance in either terrestrial or aquatic environments. The energetic cost of being
semi-aquatic is higher than being adaptated for land or water (Williams 1999). Specialized
lift-based swimming modes that use oscillation of flippers or flukes have low minimum
costs of transport (i.e., metabolic energy consumed to move a unit mass a unit distance)
for aquatic mammals, whereas paddling has the highest minimum costs of transport and
undulation is intermediate (Fish 2000).
Selective pressures would have been high in the transition to a more aquatic lifestyle.
The evolutionary changes would have been directed by increases in swimming speed,
propulsive efficiency, energy economy, and dive time, while constrained by morphology
and neuromotor patterns. In recent years, fossil species have been discovered, which have
added to our knowledge of the transitional stages that evolved into the highly derived
aquatic mammals (Thewissen 2014). However, it has only been through observation and
experimentation on living analogs of the transitional forms that the selective pressures
have been determined directing the course to the pinnipeds, cetaceans, and sirenians.
As mammals became semi-aquatic, the earliest species, such as the pakicetids, would
have used a modified terrestrial, quadrupedal gait to move on along the bottom substrate
(e.g., hippopotamus; Coughlin and Fish 2009) or swim in a manner similar to the “dog paddle” (e.g., mink, Mustela vison; Williams 1983). The primitive quadrupedal gait would eventually be replaced with bipedal paddling, with alternating motions of either the pectoral or
pelvic extremities as exemplified by the polar bear (Ursus maritimus) and muskrat (Ondatra
zibethicus), respectively (Flyger and Townsend 1968; Fish 1984). Otters (L. canadensis) are able
to swim quadrupedally, but they generally swim with 2 feet (Tarasoff et al. 1972; Fish 1994).
Bipedal paddling avoids mechanical and hydrodynamic interference between the
ipsolateral limbs, increasing propulsive efficiency. This mode of swimming frees one set