Aquatic mammals have evolved from terrestrial ancestors and retain adaptations for air-breathing, with categories including amphibious, fully aquatic, and marine mammals. Their adaptations for aquatic life include streamlined body shapes, modified forelimbs as flippers, and unique respiratory systems to facilitate life in water. Various structural losses, such as the absence of hair and reduced hind limbs, showcase their specialized evolution to thrive in aquatic environments.

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Aquatic Mammals

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• Mammals are primarily terrestrial animals. However, some of
them have adopted an aquatic mode of life.
• The aquatic mammals have evolved from terrestrial
mammals.
• The fact that all of them are not gill-breathers but breathe
air through lungs, indicate their original terrestrial mode of
life.
• All the aquatic mammals are really terrestrial lung-breathing
forms which have reverted to an aquatic life, and they have
done so with remarkable success, the whales being the most
successful.
• They have reverted to water probably because of extreme
competition on land for food and shelter.
Aquatic Mammals

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Aquatic Mammals:
• There are several aquatic mammals. Aquatic mammals belong to
several orders of Mammalia.
• Depending on the degree for aquatic adaptation the aquatic
mammals have been divided into the following categories:
1. Amphibious Mammals:
• These mammals do not live permanently in water. They live on land
but go into water for food and shelter.
• They show only partial aquatic adaptations such as:
(i) Small external ears
(ii) Webbed feet
(iii) Flattened nails
(iv) Subcutaneous fat
• The mammals of this category include the beaver (Castor), musk rat
(Ondatra), nutria (Myocaster), otter (Lutra), mink (Mustela) and many
others.
• The amphibious mammals belong to several orders of mammalia
such as Carnivora, Rodentia, Artiodactyla, Marsupialia, Monotremata,
etc.

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Monotremata (Egg laying mammals, Ornithorhynchus)

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2. Aquatic Mammals:
• The mammals under this category spend most of the time in
water and usually come to land for reproduction.
• The typical examples are seals and hippopotamus.
3. Marine Mammals:
• These mammals never come to land and are perfectly at home in
water. The typical examples are whales.
Aquatic Adaptations:
• The adaptations or specialisations of truly aquatic mammals
(Cetacea and Sirenia) are divided into 3 major categories:
(i) Modifications of original structures,
(ii) Loss of structures, and
(iii) Development of new structures.

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1. Body Shape:
• In aquatic mammals, body shape is of prime importance.
• The external fishlike form, elongated head, indistinct neck and
tapering streamlined body offers little resistance and swims
rapidly through water.
2. Large Size and Weight:
• In aquatic mammals, the large size and body weight help the
aquatic mammals.
• Whalebone whale may grow up to 35 metres in length and
weigh about 150 tons.
• Large size reduces skin friction and loss of heat, but creates no
problem for support in water due to buoyancy.
A. Modifications of Original Structures

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3. Flippers:
• In aquatic mammals, the forelimbs are transformed into skin-
covered, un-jointed paddles or flippers, having no separate
indication of fingers.
• These paddles or flippers can move as a whole only at the
shoulder joint.
• The broad and flattened paddles or flippers serve as balancers and
provide stability during swimming.
4. Hyperdactyly and Hyperphalangy:
• In aquatic mammals, extra digits (hyperdactyly) and extra
phalanges (hyperphalangy) up to 14 or more in some forms, serve
to increase the surface area of flippers for greater utility for
swimming in water.
5. High and Valvular Nostrils:
• In aquatic mammals, the nostrils are placed far back on the top of
head so that animal can breathe air without raising head much out
of water.
• The nostrils can also be closed by valves during diving under water.

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6. Mammary Ducts:
• In aquatic mammals, during lactation, ducts of mammary glands
dilates, which is pumped directly into young by the action of
special compressor muscle.
• This arrangement facilitates suckling of young ones under water.
7. Oblique Diaphragm:
• In aquatic mammals, oblique diaphragm makes the thoracic
cavity larger dorsal and barrel-shaped for providing more space
to lungs for expansion.
8. Large Lungs:
• In aquatic mammals, large unlobulated and highly elastic lungs
ensure taking down maximum air Lore submergence.
• Like swim bladders of fishes, the dorsal lungs also serve as
hydrostatic organs in maintaining a horizontal posture during
swimming.

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9. Intra-Narial Epiglottis:
• In aquatic mammals, elongated, tubular and intra-narial
epiglottis, when embraced by the soft palate, provides a
continuous and separate air-passage, thus, allowing breathing
and feeding simultaneously.
10. Endoskeleton:
• In aquatic mammals, the cranium becomes small but wider to
accommodate the short and wide brain.
• The facial part of skull projects forming elongated snout or
rostrum
• The zygomatic arches are reduced. Due to reduced neck, the
cervical vertebrae are fused into a solid bony mass.
Zygapophyses are reduced.
• Sacrum is also reduced.
• Ribs become arched dorsally to increase thoracic cavity.
• Bones are light and spongy. In Cetacea, bones are filled with oil.

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11. Teeth:
• In toothed whales, teeth are monophyodont, homodont and
numerous, as many as 250.
• This helps in capturing or seizing prey, prevent its escape and
swallowing it without mastication.
• Usually, the mobility of jaws is reduced as they have no
function in mastication.

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• In aquatic mammals, there is a loss of a few structures which
are usually present in other mammals.
• These are as follows:
1. There is a loss of hairs. Skin surface usually remains smooth
and glistening due to loss of hairs except for a few sensory
bristles on snout or lips in some cases.
2. Pinnae are also absent. Presence of hairs and pinnae may
obstruct or impede the ever flow of water over body surface and
interfere with the speed and elegance of movement through
water.
3. Nictitating membranes, eye cleansing glands, lacrimal glands
and all kinds of skin glands (sweat and sebaceous) are also
absent because they would have been useless under water.
4 Skin losses its muscles and nerves due to thickening and
immobility.
B. Loss of Structures

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5. Hindlimbs are represented only by button-like knobs in the
foetus but disappear in the adult.
6. Pelvis is also rudimentary.
7. Fingernails are absent except for traces in foetus.
8. Scrotal sacs are also absent and testes remain inside
abdomen.

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C. Development of New Structures
1. Tail Flukes:
• In aquatic mammals, some large, lateral or horizontal expansions of
the skin develop on tail. These expansions are called tail flukes.
• These are not supported by fin-ray.
• Their up and down strokes not only propel the body through water
but enable rapid return to the surface for breathing after prolonged
submersion.
2. Dorsal Fin:
• In most Cetacea develop an unpaired adipose dorsal fin without
internal skeletal support.
• It serves as a rudder or keel during swimming.

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3. Blubber:
• In aquatic mammals, the blubber is the thick subcutaneous layer
of fat, which compensates for the lack of hairy covering.
• Blubber acts as a heat insulator. It not only retains the warmth
of the body but also provides a ready reservoir of food and water
during emergency.
• The fat also reduces the specific gravity of the animal, thus,
imparting buoyancy.
• Blubber also provides an elastic covering to allow changes in
body volume during deep diving and also counteracts the
hydrostatic pressure.
4. Baleen:
• In whalebone whales, teeth are absent.
• Instead, the upper jaw carries two transverse rows of numerous
triangular fringed horny plates of baleen or whalebone.
• These serve as an effective sieve for straining plankton (mostly
kril) which forms their chief food.

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5. Foam:
• Foam is a fine emulsion of fat, mucus and gas.
• Each middle ear cavity sends an inner pneumatic prolongation,
which meets with the fellow on the other side below the skull.
These extensions contain foam.
• It probably serves to insulate sound and improves audition or
hearing under water.
6. Melon:
• In some aquatic mammals, the melon is a receptor present in
front of nostrils It consists of a fatty mass traversed by muscle
fibres.
• It possibly serves to detect pressure changes in water.
7. Harderian Glands:
• In aquatic mammals, eyes under water remain protected by a
special fatty secretion of Harderian glands.

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chordata-zoology/8523