Mammalia: Aquatic Mammals | Zoology Optional Notes for UPSC PDF Download

Introduction to Aquatic Mammals

Terrestrial Lung-Breathing Forms

1. Original Terrestrial Mode:

   • Aquatic mammals, despite their aquatic lifestyle, breathe air through lungs.
   • This suggests their evolutionary origin as terrestrial lung-breathing forms.

2. Reversion to Aquatic Life:

   • Aquatic mammals, notably whales, have transitioned to water, possibly due to intense competition for food and shelter on land.
   • This reversion to aquatic life has been remarkably successful.

Categories of Aquatic Mammals

1. Amphibious Mammals

• Definition: Mammals not permanently residing in water but using it for food and shelter.
• Adaptations:
  • Small external ears.
  • Webbed feet.
  • Flattened nails.
  • Subcutaneous fat.
• Examples: Beaver, musk rat, nutria, otter, mink, etc.
• Orders: Carnivora, Rodentia, Artiodactyla, Marsupialia, Monotremata, etc.

2. Aquatic Mammals

• Definition: Spend most of their time in water but come to land for reproduction.
• Examples: Seals, hippopotamus.

3. Marine Mammals

• Definition: Never come to land; fully adapted to life in water.
• Examples: Whales.

Adaptations of Aquatic Mammals

1. Body Shape:
   • Fish-like form, elongated head, and streamlined body reduce resistance and enhance swimming.
2. Large Size and Weight:
   • Large size reduces skin friction, aids heat retention, and provides buoyancy in water.
3. Flippers:
   • Forelimbs transformed into skin-covered, un-jointed paddles for swimming stability.
4. Hyperdactyly and Hyperphalangy:
   • Extra digits and phalanges increase flipper surface area for efficient swimming.
5. Nostril Placement:
   • Nostrils far back on the head allow breathing without raising it much out of water.
6. Mammary Ducts:
   • Mammary ducts dilate to form milk reservoirs, facilitating underwater suckling.
7. Oblique Diaphragm:
   • Oblique diaphragm enlarges thoracic cavity for better lung expansion.
8. Large Lungs:
   • Large, unlobulated lungs aid in maximum air intake during submersion.
9. Intra-Narial Epiglottis:
   • Tubular intra-narial epiglottis allows simultaneous breathing and feeding underwater.
10. Endoskeleton Modifications:

    • Cranium becomes small and wider; cervical vertebrae fuse; ribs arch dorsally.

11. Teeth in Toothed Whales: 

    • Monophyodont, homodont teeth assist in capturing and swallowing prey without mastication.

      

1. Loss of Hairs: Smooth, glistening skin due to the absence of hairs, except for sensory bristles.
2. Absence of Pinnae: Pinnae absent to maintain streamlined body shape during swimming.
3.  Loss of Various Glands:Nictitating membranes, eye cleansing glands, lacrimal glands, and skin glands are absent.
4.  Reduced Hindlimbs and Pelvis:Hindlimbs represented by knobs in the fetus; pelvis rudimentary or absent in adults.

1. Tail Flukes: Lateral expansions on the tail called tail flukes for propelling through water.
2. Dorsal Fin: Unpaired adipose dorsal fin serves as a rudder or keel during swimming.
3. Blubber: Thick subcutaneous layer of fat (blubber) for insulation, buoyancy, and emergency food reservoir.
4. Baleen in Whalebone Whales: Absence of teeth replaced by baleen plates for filtering plankton.
5. Foam: Emulsion of fat, mucus, and gas in middle ear cavities for sound insulation and improved hearing.
6. Melon: Fatty mass in front of nostrils to detect pressure changes in water.
7. Harderian Glands: Secretion protects eyes underwater.

• Aquatic mammals showcase a variety of adaptations, including modifications of original structures, loss of certain features, and the development of new structures.
• These adaptations demonstrate the remarkable evolutionary transitions that have enabled mammals to thrive in aquatic environments.