Baleen whales

Baleen whales have baleen rather than teeth. Hundreds of long baleen plates grow down on either side from the roof of their mouths. Popularly called "whalebone", baleen are actually flexible strips of keratin, the same substance that is found in human hair and fingernails.

The baleen plates are 1-3cm apart and edged with hairy fibres that create a sieve to strain prey like krill, plankton, and other small organisms from the water.

The whale takes a mouthful of water containing its prey, closes its mouth and forces the water out between the baleen plates with its tongue. The food is trapped and swallowed. Some whales can also swim with their mouths open, straining out food continuously. Unlike teeth, baleen continues to grow throughout a whale's life, as the ends are always wearing out.

Most baleen whales feed for only four or five months of the year in colder, more productive seas. In these rich waters, a large whale can eat two tonnes of food every day. They feed like this to build up a thick layer of fat, or blubber, which is a store of energy to sustain them through the remainder of the year when they feed very little.

Many of the baleen whale species make annual migrations to breeding grounds in tropical or sub-tropical waters. For many whales, this means a journey of several thousand kilometres. In the spring, the whales migrate back to the polar seas to feed, returning the next season to calve in the warmer waters. Calves nurse on their mother's rich milk during the migration back to the feeding grounds, where they are weaned and learn how to feed as adults.

Depending on the species, the mother then has a five-month-long rest period in her breeding cycle, during which she migrates back to the warmer waters where she mates again, two years after the start of the previous pregnancy.