Cetaceans play an important role in the life of the ocean: they serve as flagships and sentinels for the health and well-being of the whole marine ecosystem. Cetaceans are air-breathing, warm-blooded mammals that bear live young and nurse them on milk. The study of fossils indicates terrestrial ancestors that made their way back to the seas around 55 million years ago. Cetaceans fall into two categories: baleen whales, or mysticetes (encompassing all of the so-called "great whales" except the sperm whale), and toothed whales, or odontocetes (including the sperm whale, smaller whales, and all dolphins and porpoises).
For more information on whales, please visit the Endangered Wildlife Section.