Whales do not feed extensively while migrating. For the most part there
is not time enough, nor is the food plentiful enough to make it worth
the effort. However, in the polar seas the whale's food is plentiful
enough to discolor the water. On close examination, the discoloration
proves to be caused by thousands of tiny shrimps which are very slender
and less than one-half inch in length. These creatures congregate in
swarms near the surface to feed on microscopic plants known as diatoms.
The whales need only swim back and forth through these cloudlike
aggregations to fill their mouths quickly with water and shrimp. With
each mouthful, the water is expelled between the jaws through a mat of
fibers which hangs down from the upper jaw. The shrimps, which are
retained on the mat, fall down onto the tongue and are swallowed. The
work of pushing out several tons of water with each feeding is done by
the tremendous tongue.
The fiber mats are the frayed inner side of enormous hornlike plates
which grow down from the palate. The main body of each plate is placed
edgewise to the outgoing water so that many plates are required to
complete the mat which runs from the tip of the jaw to the corner of the
mouth. These plates vary in size and stiffness from species to species.
Some of the plates from the mouth of the bowhead whale are 12-14 feet in
length, whereas in the finback whale the plates are 2-4 feet. There can
be over 200 plates per side in the filtering structure. The frayed inner
edge is constantly breaking off and the plates keep growing and fraying
to provide the necessary thickness for the mat. Technically, these
plates are called _baleen_. They were called whalebone by the whalers,
and that is the name which is still used in commerce. The plates have no
relationship to bones, nor could they be mistaken for them. The
whalebone was assiduously collected and sold to be made into a variety
of objects such as umbrella stays, corset stays, buggy whips and other
articles which today are made of steel or plastic. There was a great
demand for the product and a bowhead whale produced over a ton and
one-half of whalebone valued back in the 17th century at over 400
English pounds, equivalent to about $10,000 today.
It has been observed that the coarseness and thickness of the baleen is
suited to the size of the food which is filtered. For example, the
rorqual or Sei whale, which feeds on tiny shrimp species, has a
filtering surface which resembles fine wool. The blue whale, which feeds
on the largest of shrimps and on fish, has the coarsest filter.
Generally, fish occur only in the diets of the blue, finback and other
rorqual which swim fast enough to engulf them. Here, the fish are
weak-swimming, schooling fishes. The gray whale, unlike the other filter
feeders, feeds on bottom-frequenting crustaceans known technically as
amphipods. These organisms, occurring principally in shallow water, keep
the gray whales close to the shores of Siberia and Kamchatka.
In order for a whale to be able to exist on a 3 to 4 month feeding
period, it must have not only ample food, but time in which to collect
it. Although the whales may not all be far enough north to have a
24-hour day, there is enough twilight to let them feed the clock around.
In order to take advantage of the prodigious amounts of food available,
whales have a huge four-chambered stomach. It is not at all uncommon to
find 5 to 10 wheelbarrow loads of shrimp in the stomach. No one yet
knows how fast the food passes through the alimentary tract.
The shrimps on which the whales thrive are not uniformly concentrated,
but occur at special places where oceanographic processes have enriched
the surface waters with cold, nutrient-rich, subsurface water. The
principal oceanographic action which enriches the water at the surface
is called upwelling. It can be induced by a number of physical
conditions. One of the best places to look for upwelling is along the
edge of the polar icecaps and along the junction of strong currents.
Upwelling is one of the ways nature refertilizes the surface waters
which are otherwise deficient in nutrients. Whenever the ocean surface
is fertilized, the microscopic plants begin to grow and multiply,
discoloring the water to a brownish or reddish color. If the subsurface
waters continue to be pumped to the surface, the growth continues and a
rich ocean pasture results. The animals which begin to crop this are the
shrimps, and when they have thrived and reproduced, there are enough for
the whales. The other all-important factor in the fertility of the polar
seas is the length of day, which provides ample time for the sun's
energy to be entrapped by plants. By autumn, the days have shortened and
night begins to predominate. At this time the microscopic plants stop
growing and form resting stages which protect them through the long
winter.
The whale has made such efficient use of its summer feeding period that
it is fat enough to live the other 8 months without danger of
starvation. Whales might be thought of as huge natural tankers carrying
enough oil to provide for long periods of active fasting. Unlike those
mammals which avoid starvation by winter hibernation, the whale is able
to migrate away from inhospitable seas into warmer waters where they can
then reproduce and care for the new young. The strain on the mother at
this time must be considerable for her baby grows at a prodigious rate.
It has been calculated that a blue whale baby grows about 10 pounds per
hour, gaining a ton every 9 days.
Although there are at least nine species of filtering whales, each seems
to be dependent on a different kind of shrimp which are seldom found
together. This prevents the various species of whales from being in
constant competition with each other. However, no one altogether
understands why there is no competition. Of course, structurally a
species may be more suited to feeding on one kind of food. The porosity
of the filter plate, which varies, may account for this. The actual
shape and size of the head varies from species to species and this may
be related to improving the efficiency of feeding. Right whales have
heads which are very large, permitting a large intake of water and the
accommodation of a tremendous set of filters. These whales have heads
which are enlarged both in width and length. In fact, the head may
account for nearly one-third of the total body. The rorqual group are
more streamlined and have proportionately smaller heads. To make up for
this limitation, the floor of the mouth is pleated on the outside which
permits the floor to balloon out like a huge scoop each time the mouth
is filled.
The right whales, incidentally, were so named by the early whalers to
apply to those species which were suitable for whaling. The majority of
whales were not molested because they sank as they died, or they were
fast-moving, wary species which could not be approached with the
whaler's lance.
[Illustration: Infra-red photograph of gray whale reveals heat of
the spout and also indicates the double source of the spout.
_Photograph by T. J. Walker._]