The main difference between toothed and baleen whales is that baleen whales have no teeth at all but instead have baleen plates they use for collecting food. Although the baleen whales are the largest animals on earth, they usually feed on very small creatures.Their diet consists mainly of krill, plankton and small crustaceans and also small shoaling fish. To catch these small animals, teeth are useless. The baleens form a large sieve-like structure with which the whales can filter them out of the water.
Baleens are thin, long, somewhat triangular plates of keratin (the same material human hair and finger nails are made of) that grow down out of the skin covering the upper jaw. In the edges, these plates have lots of loose threads of keratin (a kind of hair) that forms a fine-mazed filter. Each species has a unique type of baleen: you can identify the species by the colour, width and length of the baleen plates. The blue whale has over 500-800 usually black, relatively short baleen plates (less than 1 m long and usually wider than they are long). The gray whale has 280-360 yellowish white, rather thick baleen plates, which are up to 45 cm long and 23 cm wide. The bowhead whale has the largest baleen: 650-720 dark baleen plates which are up to 30 cm wide and up to 430 cm long. Before birth, baleen whale embryos have rudimentary teeth, but these are resorbed before birth. Out of the skin on the upper jaw, baleen plates start to grow. At birth these plates are still soft, but they become hard soon afterwards. The baleen plates continue to grow throughout a whale's life.
The way baleen whales feed differs per group. Rorquals usually have a gulping style of feeding: they take large mouthfuls of water with food items in it. In the process, the throat extends enormously. They then close the mouth and push the tongue up against the palate and press all the water out through the baleens. The small creatures in the water are caught in the baleen and are swallowed when all the water is gone. Right whales however collect their food by swimming at or near the surface with their mouths open. The water then washes through their mouth, entering through an opening between the rows of baleen at the front and leaving through the baleen. The animals in the water are trapped on the baleen and are swallowed. Gray whales are bottom feeders: they dive to the bottom, scoop up a mouth full of sand, animals and water and force out the sand and water through the baleen. The bottom of the feeding areas of gray whales is usually full of circular craters caused by this way of feeding.
The humpback whale is a rorqual, and has a gulping feeding still. But this species has developed a special technique for making its feeding more efficient. Some whales dive down near a school of fish and slowly circle around the school towards the surface. On their way up, they constantly blow bubbles. In this way, they trap the fish in a net of bubbles and the fish will be concentrated in the centre of the ring of bubbles. The whales then come up in the middle of the bubble ring with mouths wide open and scoop up water with very high concentrations of fish. This remarkable fishing technique is called bubble-netting.
The amount of food a baleen whale needs of course depends on its body size. A large blue whale would need about 4,000 kg of krill perday. The much smaller minke whale would need about 71 kg per day. A blue whale can take up to 1,000 kg of krill in one feeding session. However, most whales do not feed the whole year. A lot of species feed in summer in cold waters and spend the winter in warmer waters without an abundance of food. During the feeding period, the whales may increase 50-65% in weight.