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:
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:
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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,
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(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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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 (Fig. 33.8).
A. Modifications of Original 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.
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.
6. Mammary Ducts:
In aquatic mammals, during lactation, ducts of mammary glands dilate to form large reservoirs of milk, which is pumped directly into mouth of young by the action of special compressor muscle. This arrangement facilitates suckling of young 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.
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.
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.
B. Loss of Structures:
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.
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.
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.
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.
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.