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Whales, dolphins and porpoises are all cetaceans- mammals with exquisite adaptations for life in the sea. They evolved from a common land-dwelling ancestor that lived about 50 million years ago. All three families of cetaceans (from the Greek for "sea monster") breathe air, give birth to live young, feed their babies milk, and live in social groups. These animals range in size from the 4-foot, 55-kilogram vaquita porpoise to the 100-foot, 136,000-kilogram blue whale- the largest animal ever to live on Earth! Sometimes, it's hard to tell which of these three families a particular cetacean belongs to. For example, killer "whales" are actually the largest of the dolphins. Many people use the words "dolphin" and "porpoise" interchangeably, but there are important distinctions between the two.

Cetaceans are divided into two main types: baleen whales, or mysticetes, and toothed whales, or odonticetes. Technically, dolphins and porpoises are subfamilies of the toothed whales, along with some larger 'true' whales. Only 'true' whales are included in the baleen group. How can we tell which cetacean belongs in which category?

PORPOISES

Six different species make up the smallest type of cetaceans, the porpoises, subfamily Phocoenidae. Although people often confuse porpoises and dolphins, there are physical and behavioral differences between the two. Porpoises tend to be short (less than 7 feet long) and rather stout; in fact, their name comes from the Latin term porcopiscis, meaning "hog fish"! Porpoises have pointed noises instead of beaks, and also have smaller, rounder heads than dolphins. Their flattened, "spatulate" teeth are useful for catching small fish such as sardines and mackerel; dolphins, in contrast, have conical teeth for seizing and holding larger fish or marine mammals.

Shy and elusive, porpoises stick close to coastlines, but aren't seen as often as dolphins. They usually don't surf bow-waves or perform other acrobatics. Pods (social groups) are quite small, usually 2-6 animals. Large gatherings of more than 100 porpoises can occur when they migrate. Porpoises have a short life-span, generally 12-18 years, so they reach sexual maturity quickly at 3-5 years old. The little cetaceans breed quickly, too- females have a calf every year or two. They have to reproduce efficiently, since porpoises are threatened by water pollution, fishing-nets, collisions with boats, and even murderous dolphins. It's believed that dolphins kill their smaller cousins to eliminate

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