Feeding—Filter Whales
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.
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.