Cetaceans

The sperm whale's deep dives raise interesting questions about cetacean anatomy and physiology. Even the shallower dives performed regularly by many cetaceans would jeopardize the health of a human diver. One important issue for a diving animal is keeping warm in the cold depths. All cetaceans have a thick layer of blubber insulating them from the frigid water; in addition, a diving cetacean is aided by the increasing pressure of the greater depths, which reduces blood circulation automatically. Blood flow to peripheral body areas is further reduced by proximity to the cold water, keeping the warm blood circulating to the internal organs. It is still a mystery that the cetacean brain can function normally at great depths; the heartbeat drops during a long dive, but somehow the brain maintains its normal temperature.

Another important issue to a diving animal is oxygen deprivation. Many rorquals routinely stay under for 30 minutes before surfacing for a breath. Longer dives are always followed by a certain amount of panting at the surface, so the whale can restore its depleted blood oxygen levels. This is not the whole story, of course, since no land animal could hope to match this feat of breath-holding. Cetacean muscle tissue contains much greater amounts of myoglobin, the oxygen-binding protein found in the muscles of all mammals. This means that ounce for ounce, cetacean muscle is capable of storing more oxygen where it is needed most, even when new oxygen is not being provided via the lungs. In addition, cetaceans apparently have a high tolerance for the waste products (lactic acid and carbon dioxide) that accumulate in working muscle in the absence of sufficient oxygen.

Of special interest to people who dive for recreation is the dangerous phenomenon known as the bends. Human divers take to the depths with a tank of compressed air, whose pressure equals or exceeds that of the surrounding water. Otherwise, our relatively feeble chests would collapse under the pressure of the surrounding water. The bends occurs when nitrogen gas present in the compressed air dissolves into our blood and tissues, forming bubbles when the pressure is reduced too rapidly upon ascending. How do cetaceans avoid this deadly condition? Upon diving, their remarkably flexible chests and small, elastic lungs collapse; the tiny pouches that absorb oxygen within the lungs (the alveoli) are forced shut and gas exchange ceases. This means that little or no nitrogen is transferred into the bloodstream, and the dangerous bubbling-up of dissolved gas does not occur upon resurfacing.

In a swimming cetacean, the tail is the main source of forward propulsion, pushing the animal forward by an up-and-out movement against the water (rather than side-to-side, like a fish). The tail is a flexible extension of the last vertebra, which supports the muscular flukes. Rolling and changing position in the water is accomplished by the flippers; the flippers and dorsal fin (which is not present in all species) act together as stabilizers. Cetacean bones are heavier than water, but the body floats easily owing to buoyant blubber, oil in the bones, and air in the lungs. Some great whale species have been observed to sleep for hours, usually at night, their heads passively rising clear of the water to expose the blowhole. Both eyes are closed, and the body floats motionless with tail and flippers hanging limply, while the whale breathes once or twice per minute with a brief, snorting exhale. Amazingly, the smaller dolphins, who have more to fear from sharks and other predators, appear to sleep with only half of their brain at a time; one eye remains open, enabling the animal to rouse itself should danger arise.