Propulsion

1. Adaptations of the Anatomy
   1. Tail Flukes
   2. Skin
2. Locomotion

The dolphin is a fast and active swimmer, capable of astonishing feats of speed and agility. Dolphins can travel on its belly, on its back or even on its side. It can make sharp turns easily and can jump to great heights. Swimming at about 35 km (22 miles) per hour, the dolphin seems to be indefatigable. The dolphin can even swim up to 50 km (30 miles) per hour. However, when physiologists studied its size and shape, they did not expect the dolphins to be so fast and agile in water. Why is it that dolphins are able to be so fast and agile?

To maintain the swimming speeds in dolphins, the tail would have to develop about ten times as much propulsive power as the muscles of other mammals. Therefore, it is seen that the dolphin's tail is extremely muscular and powerful.

Dolphins have a disproportionately large horizontal tail, known as a fluke. The flukes contain no bones and are mostly made up of connective and muscular tissues, forming a tough assemblage of muscles and fiber. Tendons of the muscles in the flukes are so developed that even the most violent muscular activity will not break the spinal column.

Not only is the tail stock a powerful propeller, it is also used in stabilization and steering. It allows the dolphin to vary its movement, to change direction or position. This explains why the dolphin is so agile, having the ability to turn easily, to swim on all its sides and to spin about freely.

Despite the powerful propulsive force produced by the tail, physiologists concluded that dolphins can never reach a speed of 50 km (30 miles) per hour. In 1963, P.E Purves introduced the assumption that dolphins swim as well as they do because their bodies generate almost complete laminar flow. In other words, the dolphins are able to eliminate frictional drag and turbulence. Significant reduction of frictional drag over the body of the dolphin results in a smaller propulsive force required for high speed swimming.

How the dolphin managed to reduce laminar flow is partly because of its streamlined body, allowing it to move through water easily. However, the main reason lies in the dolphin's silky smooth skin.

The water nearest to the skin creates the most frictional force. To minimize this, the skin secretes a high polymer of ethylene oxide that acts as lubricant, sloughing off skin cells. The skin cells shedds and renews every two hours, compared with every eighteen hours in human skin.

The dolphin's skin is formed of small folds, called dermal ridges. These are tiny ridges running parallel to the length of the dolphin. that are constantly moving. Usually, when a body passes through water, little swirls of water, known as eddies, are formed along the body's surface and these eddies create frictional drag. These tiny movements of the skins help to prevent the slowing effects of the water, by stopping the formation of eddies. The water around the body thus becomes calm, allowing the body to glide easily.

Dolphins often make use of the positive pressure field created by a object moving through the water, for example a ship, to be propelled forwards. It can be done so because there is a difference in the pressure on both sides of the flukes. In this way, the dolphins are enjoying a free ride in the pressure field and also saving some of the energy of locomotion.

However, dolphins do swim on their own most of the time. The main organ of propulsion is the flukes and the hind part of the body, which are moved up and down in a vertical plane, developing the thrust that propels the body along.

The flukes, attached to the vertebrae of the tail, are enveloped by bundles of ligaments that resist the bending of up or down. The fibres in the core of the flukes are pleated, with the lower surface having more pleats than the upper surface. If the flukes are raised upwards against the resistance of the water, the pleats on the upper side will cause the fluke to bend slightly. But if the flukes are moved downwards, the pleats on the lower side will open up and cause the fluke to bend considerably.

The beating of the tail is brought about by two pairs of muscles: the apaxial muscles and the hypaxial muscles. The epaxial muscles that lie above the vertebral transverse process, are much larger than the hypaxial muscles situated below the vertebral transverse process. This shows that it is the epaxial muscles, which are responsible for the upward movement of the tail that is the main driving force in swimming.

After the epaxial muscles have effected the power stroke, the tail then rises upwards, and the body moves forwards and downwards. The hypaxial muscles now pull the tail downwards, to get ready for the next power stroke. The buoyancy of the head and thorax, enhanced by the large amount of oil and fat contained in them, causes the front of the body to rise as the tail descends. The cycle then repeats itself, resulting in a upward and downward propulsive body movement.

Bibliography

Bonner, Nigel. Whales of the World. New York: Facts on File, Inc., 1989.

Carwardine, Mark. The Book of Dolphins. Dragon's World Ltd, 1996

Cousteau, Jacques. Whales. New York: Harry N. Abrams, Inc, Publishers, 1988.

Harrison, Sir Richard, et. al. Whales, Dolphins and Porpoises. New York: Facts on File, Inc., 1994.