The main difference between them is their size but they also have other more distinctive features. For example, porpoises have spade-shaped teeth while most of the larger whales haves no teeth at all. Hanging down from their upper jaws with stiff overlapping hairs, the baleen plates form a sieve which enables a whale to filter thousands of small fish or shrimp-like creatures, known as krill, from the seawater. Toothed whales, which include all the dolphins and porpoises, sperm whales and beaked whales, feed mostly on fish and squid and normally capture just one animal at a time. Baleen whales have two blowholes side by side, and toothed whales have just one.
Strong muscles close the blowhole before a whale dives underwater.
However, similar to other mammals, cetaceans do nurse their young and are warmblooded animals. Many whales migrate as far as 4,000 miles each year between their summer and winter habitats. It is very difficult to study animals that spend most of their lives underwater and far from the shore. For many years all that was known about these animals was learned from dead animals that had washed ashore or from those that were killed by whalers. Although whales, dolphins and porpoises still remain very elusive, modern technological advances and an increased effort to study them in the wild has increased our understanding of their distribution, behavior and life history.