Because the seasons are opposite in the Northern and Southern Hemispheres, whales in the two halves of the world do not mix. Feeding The baleen whales have several hundred elongated triangular baleen plates growing down on either side from the roof of their mouths. These plates are made of a horny material and not of bone, even though they are popularly called whalebone. The whale takes in a large mouthful of water containing the shrimps or other small crustaceans or fish on which it feeds. When the mouth is closed, the throat region tightened and the large tongue pushed up, the water is forced out between the baleen plates and the food is left trapped and can be swallowed.
Some whales can also swim with their mouths open, straining out food continuously.
Most baleen whales feed for only four or five months a year when they are in polar seas which are particularly rich in food. A large whale will swallow two tons of food a day, and it builds up a store of energy in the form of a thick layer of blubber under the skin to last it through the rest of the year when it feeds very little. The baleen whales migrate several thousand miles to their breeding grounds in the warm waters where they pair and mate. The females carry a single young for almost exactly a year. During this time they migrate to the summer polar feeding grounds and then return to calve in the warm water areas.