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._] 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.