Cetaceans have shed most of the external traces of their terrestrial
ancestry and are supremely adapted to underwater life. Their body shape
has become streamlined and they have lost most of their body hair, to
improve hydrodynamic efficiency; they have short stiff necks, which are
essential for swimming at high speed; their front limbs have turned into
flippers and their hind limbs have disappeared; they have muscular tails
to provide a powerful means of propulsion; and the nostrils have moved to
the top of their head for easy breathing at the surface.
There are also many other, less obvious adaptations. For example, they
have excellent hearing, which compensates for a poor (or entirely lacking)
sense of smell, and for the uncertainties of visibility underwater; they
have a high tolerance to carbon dioxide, to help with lengthy dives, and
are two or three times more efficient than land mammals at using the
oxygen in inhaled air; their rib cages are collapsible for deep diving;
and they have layers of insulating fat to keep them warm.
