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.
[Illustration: 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.
[Illustration: Tailflukes extended for deep dive. _Courtesy Scripps
Institute of Oceanography._]