The group of animals we call cetaceans represent but two-thirds of the orders of "whales" that have ever existed.

Modern whales belong to the orders Mysticeti (baleen whales) and Odontoceti (toothed whales, dolphins and porpoises), however, there used to be a third order, the Archaeoceti -- the ancient whales. These three orders evolved independently, the similarities between them being the result of convergent adaptations to a pelagic lifestyle.

Early fossil evidence is slim, but it seems that the ecological vacuum left by the extinction of the great marine reptiles at the end of the Cretaceous Period (65 million years ago) was a void into which proto-whales evolved. Further back in time still, their closest relatives were the carnivores and even-toed ungulates.

Adaptations to the marine environment are what make cetaceans astounding and tantalizing. Though largely hidden, the technological wizardry they embody is astonishing. They pack powerful sonar, can dive to almost unbelievable depths, follow traditional migration routes spanning thousands of kilometers and have sophisticated forms of communication we are currently incapable of understanding.

They are true mammals adapted to a marine life. Cetaceans have lost body hair and hind limbs, they have nostrils that have migrated onto the top of the skull and have enormously developed jaws. The Mysticeti have evolved two very different, but equally successful, ways of eating. One group of these filter-feeding baleen whales -- the family Balaenidae, including the right and bowhead whales -- have monstrous heads with exaggerated jaw arches carrying enormously enlarged baleen plates for filtering. Their fellow members of the baleen whale family, the Balaeopteridae -- including the blue and humpback whales -- have smaller filters, but are able to filter more water thanks to their massive throat pleats, which expand into a vast pouch.

And while the Mysticeti have refined baleen filter-feeding, the Odontoceti have refined the use of specialized organs that enable them to use sound both to perceive their environment, and their prey in it, as well as to communicate about it to each other.

Water, being denser than air, conducts heat more effectively -- hence it quickly sucks the heat out of uninsulated bodies. That simple fact has forced marine mammals that range into cooler waters to conserve heat energy by various means. Blubber, the subcutaneous fat layer covering the muscles of a cetacean, serves both as insulation and as an energy store. (Alas, as explained elsewhere on this page, blubber is also an effective store of toxins absorbed from their prey.)

The pelagic environment is an unforgiving one for a mammal. Not only does it rob the body of precious heat, but it provides only minimal support. From the moment it is born, a young cetacean must be able to swim, follow its mother and reach the surface to breathe. In such an environment, aquadynamic streamlining has been taken to extremes. All external protuberances, including the genitals, have been greatly reduced. Previously external features are now hidden, though they may be revealed when necessary.

Whales are record-breakers. Not only is the largest known creature that's ever lived on earth a whale (blue whales can weigh more than 100,000 kg and average 25 meters in length), but the largest penis on earth (though hidden internally most of the time) is also a whale's -- in some great whales it may be up to 3 meters long.

How long do they live? The first known data on longevity came from harpoon heads that were lodged in a whale but didn't kill it. Early modern whaling harpoons were marked with the year of their use.

Added to the harpoon data, the discovery that whales' waxy ear plugs contain lines corresponding roughly to annual growth made further estimates possible. The vagaries of the first method, and the inaccuracies of the second (both require killing whales), have now been overcome by research methods that track individual whales.

The smaller porpoises average relatively short lives of around 15 years, dolphins may live to 20 years, and great whales such as the sperm, fin and bowhead survive an average of 30 to 40 years. Tooth rings and earplug-lamination indicate that individuals may survive to be much older, 70 in the case of sperm whales and 100 for fin whales. Such long-lived creatures mature slowly, over five-10 years. Even once they become reproductively active, whales may only produce young once every two or three years. Extensive whaling has, however, impacted population structures considerably, with fewer individuals reaching old age and maximum size.

Cetaceans, though, have yet more claims to the record books. Consider their worldwide range. Different species range from the ice limits of the Arctic Ocean to the fringes of the Antarctic continent; from the most remote oceanic areas to some of the deepest; and into some of the world's longest river systems, including the upper reaches of the Amazon in Peru.

Then consider their physical achievements. A fin whale preparing to dive, for example, can in less than two seconds exhale and inhale a volume of air 3,000 times greater than a human's lung capacity. Diving whales contain huge reserves of oxygen, not in their lungs, but in the richly oxygenated myoglobin of their muscles -- hence their extremely dark, sometimes virtually black, meat.

Deep-diving sperm whales can remain submerged for up to 90 minutes, astonishingly reaching depths of up to 3,000 meters, while bottlenose whales, the most frequently deep-diving species, can withstand 300 atmospheres of pressure yet surface quickly without experiencing the bends. How? Their lungs collapse under pressure and any nitrogen is forced into the nonabsorptive windpipe and nasal passages.

Their intelligence suggests that cetaceans are adaptable, imitative, sentient creatures. They are capable of complex communication and social interactions. The marine soundscape consists of widespread low-frequency moans, thumps, knocks, chirps, whistles and clicks -- all sounds used in communication by cetaceans. Not only is the blue whale the largest living creature, but it also produces the most powerful sound -- a 188-decibel whistle (jet aircraft can only muster 140-170 decibels). The 20-Hz sounds made by the fin whale can travel underwater for up to 840 km. These ocean-spanning sounds provided continuous social contact, until the advent of propeller-driven ships, which produce high levels of interference.

Cetaceans are unquestionably a remarkable group of animals. But only now, with modern research techniques, are we beginning to understand them.