Dolphin Adaptations for an Aquatic Environment

A. Swimming.

1. Swimming speed and duration are closely tied: high-speed swimming probably lasts only seconds while low-speed swimming may last for long periods of time (Goforth, 1990; Fish and Hui, 1991).
   2. Bottlenose dolphins routinely swim at speeds of about 5-11 kph (3 to 7 mph). Ergometric (exercise) studies indicate burst (maximum) speeds of 29-35 kph (18 to 22 mph) (Goforth, 1990; and Fish and Hui, 1991).

B. Diving.

1. Bottlenose dolphins generally do not need to dive very deeply to catch food.
   
   a. Depending on habitat, most bottlenose dolphins regularly dive to depths of 3.0-45.7m (10 to 150 ft.).
   
   b. They are, however, capable of diving to some depth. Under experimental conditions, the deepest trained dive is 547 m (1,795 ft.) (Ridgway, 1990).

2. It is possible for a dive to last eight to ten minutes.3. All marine mammals have special physiological adaptations used during a dive. These adaptations enable a dolphin to conserve oxygen while it is under water.
   
   a. Dolphins, like other mammals, have a slower heartbeat while diving.
   
   b. When diving, blood is shunted away from tissues tolerant of low oxygen levels toward the heart, lungs, and brain, where oxygen is needed.
   
   c. The muscle of bottlenose dolphins has a high content of the oxygen-binding protein myoglobin. Myoglobin stores oxygen and helps prevent muscle oxygen deficiency.

C. Respiration.

1. A dolphin breathes through a single blowhole on the dorsal surface of its head (Ridgway, 1972; Bryden and Harrison, 1986).
   
   a. The dolphin holds its breath while below water.
   
   b. It opens its blowhole and begins to exhale just before reaching the surface of the water.
   
   c. At the surface, the dolphin quickly inhales and relaxes the muscular flap to close it.
   
2. As a dolphin exhales, seawater that has collected around the blowhole is carried up with the respiratory gases. Seawater and the water vapor condensing in the respiratory gases as they expand in the cooler air form the visible blow of a dolphin.
   
3. During each respiration a dolphin exchanges 80% or more of its lung air (Ridgway, 1972). This is much more efficient than humans, who exchange only about 17% of their lung air with each breath.
   
4. Exhaling and inhaling takes about 0.3 seconds (Bryden and Harrison, 1986).
   
5. A bottlenose dolphin's average respiratory rate is about two to three breaths per minute (Ridgway, 1972).

D. Thermoregulation.

1. Bottlenose dolphins deposit most of their body fat into a thick layer of blubber that lies just underneath the skin. This blubber layer insulates the dolphin and streamlines the body. It also functions as an energy reserve. A bottlenose dolphin's body fat generally accounts for about 18% to 20% of its body weight (Bryden and Harrison, 1986).
   
2. A dolphin's core temperature is about 36.90C (98.4'F). There is a heat gradient throughout the blubber to the skin (Ridgway, 1972).
   
3. The dolphin's fusiform body shape and reduced limb size decreases the amount of surface area exposed to the external environment. This helps dolphins conserve body heat (Ridgway, 1972). Dolphins adapted to cooler, deeper water generally have larger bodies and smaller flippers than coastal dolphins, further reducing the surface area of their skin
   
4. A bottlenose dolphin's circulatory system adjusts to conserve or dissipate body heat and maintain body temperature (Ridgway, 1972).
   
   a. Arteries in the flippers, flukes, and dorsal fin are surrounded by veins. Thus, some heat from the blood traveling through the arteries is transferred to the venous blood rather than the environment. This countercurrent heat exchange aids dolphins in conserving body heat (Ridgway, 1972).
   
   b. When a dolphin dives, blood is shunted away from the surface of the body. This decrease in circulation conserves body heat (Ridgway, 1972).
   
   c. During prolonged exercise or in warm water, a dolphin may need to dissipate body heat. In this case, circulation increases to veins near the surface of the flippers, flukes, and dorsal fin, and decreases to veins returning blood to the body core. Excess heat is shed to the external environment (Ridgway, 1972).A countercurrent heat exchange system in the flippers, flukes, and dorsal fin help dolphins maintain body temperature.
   
5. In general, bottlenose dolphins have a higher metabolic rate than land mammals of similar size. This increased metabolism generates a great deal of body heat (Ridgway, 1972).
   
6. Mammals lose body heat when they exhale. because they breathe less frequently than land mammals, dolphins conserve a considerable amount of heat (Ridgway, 1972).

E. Sleep.

1. When studying sleep in bottlenose dolphins, researchers found that dolphins spent about 33% of each day sleeping (Ridgway, 1990).
   
2. Soviet researchers have shown that deep sleep in bottlenose dolphins may occur in only one brain hemisphere at a time. Research is ongoing (Ridgway, 1990).

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