CETACEANS
Whales, Dolphins and Porpoises
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"Cetaceans" is a collective term
that includes the aquatic mammals: whales, dolphins and porpoises. There
are two types of cetaceans: those with teeth and those with baleen. The
order Cetacea is divided into three subgroups:
(1) Odontoceti or "toothed whales," including 65 or more living
species of dolphins, porpoises and whales (sperm whales, belugas, narwhals
and beaked whales). All of these species have teeth.
(2) The Mysticeti or "moustached whales" includes ten living
species of baleen whales including the magnificent blue whale -- the largest
animal on earth at 100 feet -- fin, humpback, right, minke and gray whales.
Instead of teeth, baleen whales have a series of plates that grow down
from the roof of the whale's mouth. Baleen is made of keratin, a material
similar to human fingernails and resembles the frayed teeth of a comb.
The outer edge of baleen is smooth, while the inner edge is bristly to
hairlike in structure. This unique structure allows the whales to act
as giant filter feeders. When the whale gulps a mouthful of seawater,
the baleen traps small ocean crustaceans. The whale then closes its mouth
and spits out the excess water, leaving filtered food of small animals
stuck in the baleen. Using its large tongue, the whale then scoops off
food from the inside of its mouth and swallows it.
(3) The Archaeoceti or "ancient whales" who are extinct.
ODONTOCETES (toothed) and MYSTICETES (baleen) WHALES DIFFER IN SOME OF
THE FOLLOWING WAYS:
(1) Toothed whales feed largely on fish and squid, while baleen whales
feed on smaller ocean animals: krill, copepods and small schooling fish.
Toothed whales have an external blowhole while mysticetes have two external
blowholes. Olfactory bulbs or nerves are absent in odontocetes and greatly
reduced in mysticetes.
(2) Odontocetes have no throat pleats, while many species of mysticetes
have pleats to allow throat expansion (like an accordion) when feeding.
Telescoping of a Whale Skull
Photo: Michele Levin-Cota
EVOLUTION AND ADAPTATION
About fifty million years ago, the first whale-type mammal entered the
seas. Archaeocetes were small in comparison to modern whales. The archaeocetes
evolved from terrestrial animals believed to have been a dog-sized mammal
with a tail. The question of whether archaeocetes gave rise to the two
existing suborders, Mysticete and Odontoceti, is still a matter of controversy.
Some scientists believe the two suborders may have risen from two different
sources rather than one common ancestor.
What we do know is warm-blooded whales, dolphins and porpoises evolved
superbly to inhabit the aquatic world. Their bodies became streamlined
to increase swimming speed and reduce drag. The nostrils or blowhole migrated
to the top of the head. An elongation or telescoping of the skull extended
the upper and lower jaws forward of the blowhole, enabling them to breath
without interrupting a smooth swimming motion. The two forelimbs became
paddle-like flippers or pectoral fins containing hand and finger bones.
Hind limbs exist only as small bones embedded internally in side muscle,
vestiges of the pelvic girdle. Propulsion is accomplished by the vertical
motion of the horizontally flattened tail flukes or tail.
Cetaceans are essentially hairless, although hairs can still be seen on
the snout of young animals and, in some adult species in specialized areas
on their head. To maintain body heat, cetaceans grew a fat layer called
blubber under their skin to replace their loss of fur.
Since sound travels four times faster in water than in air and can travel
great distances underwater, cetacean hearing became acute. Hearing became
their most important sense, enabling them to communicate, navigate and
find food. In contrast, light does not travel well in water, and vision
is not the most effective way to communicate through the world's oceans.
Photo: Michele Levin-Cota
SPECIFIC CHARACTERISTICS OF CETACEANS
Diving Capabilities
As humans, we can only marvel at the champion swimming and diving capabilities
of whales and dolphins. Physiological and anatomical features enable cetaceans
to dive for extended lengths of time. Compared to nondiving mammals, cetaceans
have a greater ability to transport oxygen across lung membranes, and
a higher percentage of oxygen carrying cells in their blood. During prolonged
dives, in order to conserve oxygen, their heart rate and peripheral blood
flow is reduced. Their muscles contain myoglobin, a substance which combines
with oxygen, allowing it to be stored in muscles. When crushing water
pressure becomes a factor, alveoli collapse at about 100m, preventing
nitrogen absorption into the blood which makes them immune to the "bends."
This diving ability is exhibited by sperm whales that have been found
entangled in submarine cables at depths exceeding 1,000m (over 3,000 feet).
Sperm whales have been tracked by hydrophones to 2,800m during dives lasting
an hour or more. A wild common dolphin that was captured, tagged, released
and tracked dove to a depth of 260m and remained submerged for more than
eight minutes.
Hearing Capabilities
Hearing is integral to the life of whale, dolphins and porpoises. There
are almost as many different kinds of sounds as there are species. Generally
speaking, the large whales make lower sounds, and the small whales and
dolphins make high sounds. Blue whales and fin whales emit very low frequency
sounds, often below the threshold of human hearing, called infrasonic
sounds. The infrasonic sounds produced by the fin whale have been recorded
at a distance of over 3,000 miles, detectible with special instruments.
In contrast, toothed beluga whales are called canaries of the sea because
of their "bird-like singing."
Echolocation
Toothed whales and dolphins use echolocation to locate their food and
navigate. By sending out directional sounds and clicks, the whale listens
to reflected echoes that bounce off objects in its path. The sound of
the echo and how fast it returns allow the whale to form a visual picture
of the object. The object's size, density, and shape all affect the echo
resonance. A large squid, for example, creates a different picture for
the whale than a small fish.
Whales and dolphins produce sounds in a variety of ways: in the nasal
passages of the head by varying the opening of the nasal plug; by forcing
air through a partially-opened blowhole; and from the larynx. Whales hear
by picking up sound waves in the water. These sound waves enter the whale's
head and may be carried as vibrations through the lower jaw to the inner
ear or bulla. The bulla is free floating, surrounded by a bubbly foam,
and is not attached to the skull. Hearing abilities differ among species.
Some hear in a similar range as humans; other can hear ultrasonic sounds
far above human range.
Thermoregulation
Cetaceans are able to regulate their body temperature through the following
mechanisms. A blubber layer in which blood supply is minimal reduces the
chances of heat loss at their body's surface. A counter-current system
is a circulatory adjustment to limit loss of heat. Veins found in the
body's periphery are surrounded by arteries, thereby ensuring that heat
given up by blood traveling from the warmer body core is partially reclaimed
by nearby inflowing blood. Decreased respiration rates ensure warm body
air is given up less frequently than land-dwelling mammals. An increased
metabolic rate and a low ratio of body surface to volume is heat conserving
due to elimination of hind limbs and a reduction of forelimbs.
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Birth
Cetacean calves are generally larger than babies of terrestrial mammals.
This large body size of cetacean young reduces the amount of relative
surface area and presumably helps minimize heat loss. It is believed that
most cetacean calves are born tail first, so that the blowholes are last
to emerge, in order that even in a complicated birth the calf is not likely
to drown. Laboring mother are closely attended by one or more adults.
When the calf is born, the mother or an "auntie" may push the
baby up to the surface for its first breaths.
Calves are closely bonded to their mothers and nearly inseparable. The
cow keeps her infant close and will retrieve it if it strays. She will
put herself between her calf and any danger to protect it. Mothers have
been known to support a wounded or dead calf for hours at the ocean's
surface. The social structure of killer whales (orcas) revolves around
the female. Full grown males and females remain with their mother in family
groups for their entire life, which spans 50 to 80 years. Only death or
capture breaks this bond.
Sometimes other adult whales will "baby sit" while the mother
searches for food. Cetacean young are dependent on their mothers for long
periods of time. Some of the toothed whales nurse their young for as long
as two years. This extended dependence on the mother ensures a lower rate
of calf mortality and compensates for long calving intervals and low reproductive
rates of cetaceans.
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