Evolution of Whales
To appreciate fully the biology of whales, one must know that their ancestors were terrestrial mammals. It is indeed impossible to account for all the steps which were necessary for this difficult reentry of the oceans. However, the fossil records for whales are numerous and permit at least a partial reconstruction of the evolutionary steps. Whales have obtained not only complete mastery of this difficult habitat, but also they have spread out to crop a variety of marine foods. Biologists have generally dramatized the earlier conquest of land by marine organisms, leaving the more recent and perhaps more difficult reentry of the marine world by whales to go unnoticed.
Breathing Adaptations
Not only have whales become completely aquatic, but they have been able to eliminate nearly all the design features which were necessary for life on land. Only the retention of air breathing remains, and this does not seem to be much of a hardship. There has been a great improvement in the conservation of oxygen so that really long dives are possible. The subtlety of this accomplishment is only partially understood by scientists. Apparently, diving mammals are able to shut down those bodily activities which contribute little to the diving mission. These activities can go on later when oxygen is available. It is also normal to incur an oxygen debt by borrowing from stockpiles present in the tissue fluids and muscles. After a long dive a whale will idle at the surface in order to completely free the body of the excess carbon dioxide, and to pay back the oxygen debt. The greater the debt the longer the surfacing, and the greater the number of breaths which must be taken.
When a whale surfaces to breathe, the act of exhaling is called “blowing.” Whenever a whale has been submerged for a normal dive, the air in the lungs becomes saturated with moisture from the blood. The exhalation of this spent air is accomplished very quickly by forcing the air out under pressure by the diaphragm, and the sudden expansion of the expelled air produces sufficient cooling to condense the moisture. This cloud or fog is the most conspicuous feature of a surfaced whale, particularly when the spout is 10 to 15 feet in height. Within a minute’s time the fog is usually scattered and heated enough to disappear. The duration of the spout depends principally on the temperature of the surrounding air, the amount of moisture condensed from the breath and the local surface wind. At the higher latitudes air temperatures are low enough that the spout may persist for several minutes. It is possible to recognize some of the whale species by the form and size of the spout.
Inhalation is accomplished very quickly. The breathing act is generally both visible and audible. The release of air produces a very loud “whoosh” which can be heard for quite a distance on a quiet day. The nostrils are called blowholes. In order to facilitate breathing, they have been moved from the tip of the snout to the top of the head (with the exception of the sperm whale), to prevent waves from flooding the lungs. During diving, the pressure of the water operates on the nostril in such a way as to close the nostril from the outside so that regardless of depth there can be no leak. The natural buoyancy of the animal exposes enough of the head to keep the nostrils clear of the waves.
The nostrils communicate directly to the lungs rather than share a portion of the throat as is customary in other air-breathing vertebrates. This means that the whale’s mouth and throat can be full of water without danger of flooding the lungs, and that it is unnecessary to empty such a spacious cavern prior to breathing. Furthermore, it is unlikely that a whale could keep his mouth closed enough to prevent flooding through the baleen because there is no upper lip over this device.
Although a whale can be sighted by the telltale spout, a frightened whale may elude detection by exhaling just before surfacing, so that nothing more than a foamy patch is produced. Under these situations the whale does not expose the usual amount of buoyant head, but only the nostrils. A disturbed whale can dive, and then surface a mile or two away, or it may not move at all, preferring to hide on the bottom or among rocky reefs or in the kelp. The California gray whale was judged by whalers to be the most wary and elusive of them all.
Swimming Adaptations
The most essential features needed for the successful invasion of the marine habitat were those necessary for efficient propulsion. Fish, eons before, had solved the hydrodynamic equations necessary for movement through such a resistive medium. This solution required a streamlined form with a tail for propulsion, placed at the very end of the body. Extra fins were employed for maneuvering and for balancing. Whales, too, have reached the same solution, and man, when he finally develops sufficient highspeed submarines, will employ the same solution, namely streamlining. As a consequence, all whales look alike, differing principally in the degree of streamlining, color or size. In a whale’s streamlined body there can be no sharp discontinuities to accommodate the head, the neck, the trunk, and lastly the tail. Instead these features must grade imperceptibly one into the other. The only allowable discontinuity is the end of the tail which is expanded into fanshaped lobes to function like a propeller.
Gray whale rolled over on side during courtship. Note outline under water of tail and tailflukes. Courtesy Scripps Institute of Oceanography.
These features, called tail flukes, are driven up and down in contrast to the tail of a fish which is driven sidewise. Whales have long banks of muscle along either side of the backbone which attach to the tail flukes by means of tendons. This makes it unnecessary to disturb the streamlined form by bending the hind part of the trunk as is necessary when fish drive with their tails. It also makes it possible to devote a great deal more muscle to the task. The power developed by these muscles is prodigious, capable of driving a 100-ton body through the water at speeds up to 20 knots. Wounded whales can smash a 20-foot whaleboat to bits with a single slap of the tail.
The hind limbs which were useful on land have been eliminated and all that persists are vestigial bones or cartilages which are buried deep below the surface of the body. The forelimbs have undergone reduction and modification into flippers which assist in the turning and diving. The flippers are useful in other ways, providing a platform on which the baby may stay when danger threatens. They are also useful during courtship and mating, but not for combat. The toothless whales do not have too much to fight with. They may strike an adversary with the powerful tail flukes, and during courtship the males jostle and bump each other.
Whales are almost completely hairless, save for a few bristles on their heads. Certainly the elimination of hair has improved streamlining, and has reduced the frictional drag. Furthermore, continuously wet hair could not have been of much value in keeping the whale warm. It is also possible that a hairy whale would have been very much bothered by skin parasites which would have flourished in the quiet water between the hairs. However, if this prompted the loss of hair, it was in vain for now the streamlined bodies of humpbacks and right whales are marred by large encrusting barnacles. It is surprising that the barnacles do not completely cover the whales. Perhaps they are scraped off on the bottom, or they cannot flourish during the long migration or in plankton-impoverished waters of the winter quarters. At any rate the parasites are kept partially under control so that much of the streamlined surface is unblemished.
If the physical properties of water forced upon whales a common shape, they did at least, by the buoyant effect, free the animals of the need for structural and muscular developments to support themselves against the pull of gravity. Free of this structural problem, whales were able to evolve into the largest mammals which the world has ever known. As they became larger, they had to shift in their feeding to slower and less maneuverable prey. It would appear that the porpoises, which feed on the rapid-swimming, elusive fishes, are small in order to catch their prey. The whales, which have specialized to feed on the jet propelled squids, were able to evolve into much larger whales because they could capture the squid either by stealthy approach or by sucking the squid into the mouth, thus counteracting its jet.
Tailflukes extended for deep dive. Courtesy Scripps Institute of Oceanography.