Swimming
Commerson's are fast and highly maneuverable dolphins.
- They leap, spin, ride waves, swells, and breaking surf, and swim beside ships.
- They display an erratic swimming behavior and rarely swim in a straight line. This may be a specific foraging technique used to optimize food intake.
They routinely swim at moderate speeds of about 11 to 13 kph (7-8 mph) and stay below water for 15 to 20 seconds at a depth of about 1 to 1.5 m (3-5 ft.).
Commerson's dolphins often swim upside down and feed in this position as well.
- Swimming upside down may give the dolphins the advantage of swimming up on prey from below, trapping prey between themselves and the surface.
Swimming energetics
- Blubber smoothes the contour of a Commerson's dolphin and contributes to its characteristic fusiform shape, which is quite energy efficient for swimming. Compared to other body shapes, this body shape creates less drag (the opposing force an object generates as it travels through water).
- Commerson's dolphins and many other toothed whales sometimes "porpoise" at the surface: they swim fast enough to break free of the water, flying up and out and then back under in one continuous movement, which they generally repeat. Porpoising uses less energy than swimming fast at the surface.
- Wave-riding also saves energy. Commerson's dolphins and many other toothed whales sometimes ride ocean swells or a boat's bow wave or stern wake. Riding a wave or a wake, a dolphin can go almost twice as fast using the same energy cost.
Diving
All marine mammals have physiological adaptations for diving. These adaptations enable a Commerson's dolphin to conserve oxygen while underwater.
- Commerson's dolphins, like other mammals, have a slower heart rate while diving.
- When diving, blood is shunted away from tissues tolerant of low oxygen levels toward the heart, lungs, and brain, where oxygen is needed most.
- Certain protein molecules - hemoglobin and myoglobin - store oxygen in body tissues. Hemoglobin occurs in red blood cells. Myoglobin occurs in muscle tissue. The muscle of dolphins has a higher myoglobin concentration than the muscle of land mammals.
Unlike humans, marine mammals don't get "the bends" when they dive.
- As pressure increases with depth, the amount of gas that goes into solution in a diver's blood and body tissues also increases. At about 2 atmospheres of pressure (about 60 ft.), tissues are saturated. If a human diver returns to the surface too quickly, the gases, especially nitrogen, come out of solution and form bubbles in the muscles and blood. This painful and sometimes fatal condition is called "the bends."
- The bends is most common in scuba divers, but human breath-hold divers can also get the bends from deep diving. Human breath-hold divers dive on fully inflated lungs. Under pressure, a human's bronchioles collapse. Lung air is forced into the alveoli: the numerous tiny areas of the lungs where gas exchange takes place. Here gases are absorbed under pressure.
- Unlike human scuba divers, a whale doesn't breathe air under pressure. It inhales only at the surface and exhales before diving. Furthermore, in diving mammals, the alveoli collapse at about 3 atmospheres of pressure (about 90 feet), forcing air into the bronchioles (rigid air passages), a region where gases are not exchanged.
Respiration
A Commerson's dolphin breathes through a single blowhole on top of its head.
- The blowhole is relaxed in a closed position. To open the blowhole, a Commerson's dolphin contracts the muscular flap covering the blowhole.
- A dolphin holds its breath below water.
- A Commerson's dolphin opens its blowhole and begins to exhale just before reaching the surface of the water.
- At the surface, the dolphin quickly inhales and closes the muscular flap.
The visible spout of water that rises from a Commerson's dolphin blowhole is not coming from the lungs, which do not tolerate water.
- Water that is on top of the blowhole when the powerful exhale begins is forced up with the exhaled respiratory gases.
- Especially in cool air, a mist may form; it is water vapor condensing as the respiratory gases expand in the open air.
In comparison to a human, a Commerson's dolphin can hold its breath longer and exchange more lung air with each breath.
Thermoregulation
Like all mammals, Commerson's dolphins are warm-blooded. Living in the sea poses a particular challenge to marine mammals, because heat loss occurs about 25 times faster in water than in air at the same temperature.
Just under a Commerson's dolphin's skin lies a thick layer of blubber, composed of fat cells and fibrous connective tissue. Blubber helps insulate a whale from heat loss. There is a heat gradient from the body core, through the blubber, to the skin.
In general, Commerson's dolphins have a higher metabolic rate than land mammals of similar size. This increased metabolism generates a great deal of body heat.
Mammals lose body heat when they exhale. Because they breathe less frequently than land mammals, Commerson's dolphins conserve a considerable amount of heat.
A Commerson's dolphin's circulatory system helps maintain body temperature; it adjusts to conserve or dissipate body heat.
- Some arteries of the flippers, flukes, and dorsal fin are surrounded by veins. Thus, some heat from the blood traveling through arteries is transferred to venous blood rather than the environment. This phenomenon is called countercurrent heat exchange.
- When a Commerson's dolphin dives, circulation decreases at the skin, shunting blood to the insulated body core.
- During prolonged exercise or in warm water a dolphin may need to dissipate body heat. In this case, circulation increases near the surface of the flippers, flukes, and dorsal fin. Excess heat is shed to the external environment.
Sleep
Studies suggest that in many species of toothed whales, as well as gray whales, sleep probably occurs in only one brain hemisphere at a time. Studies also indicate that whales only experience slow-wave or non-rapid-eye-movement sleep. Unihemispheric slow-wave may be an aquatic adaptation that enables a whale to monitor its environment, keep swimming, and control respiration.
