This document summarizes the characteristics and adaptations of aquatic mammals. It describes two categories of aquatic mammals - amphibious mammals that live partly on land and partly in water, showing partial aquatic adaptations like webbed feet, and completely aquatic mammals that never come on land, showing complete aquatic adaptations like streamlined fish-like bodies. It also discusses the modifications, losses, and developments of new structures that aquatic mammals have undergone for an aquatic lifestyle, including modifications to their skeletons, lungs, teeth and other organs, as well as the loss of structures like hair and glands, and the development of new structures like tail flukes and baleen.

2. General characters of mammals
• Mammals are warm-blooded animals
• The skin is more or less covered with hairs.
• Sweat glands and sebaceous glands are present in skin.
• Mammary glands are present.
• External ears (pinnae) are present.
• Limbs are either plantigrade or digitigrade or unguligrade.
• Skull with two occipital condyles.
• Vertebrae are gastrocentrous composed of three pieces, the centrum
and two epiphyses.
• Mammals possess seven cervical vertebrae with few exception.
• Heart is four chambered.

3. Aquatic mammals
• Mammals are primarily terrestrial animals.
• Some of them have secondarily adapted an aquatic mode of life.
• Secondarily aquatic forms are lung breathers indicates their original mode of life.
• They have reverted to water probably because of competition on land for food and shelter.
Ref-www.edu.com Dolphin
Introduction

4. Types of aquatic mammals
• Aquatic mammals may be put under two categories depending on the degree of
aquatic adaption.
(1) Amphibious mammals (2) Completely aquatic mammals
Amphibious mammals
• These mammals do not live permanently in water, live partly on land and partly in
water.
• They go in water for food and shelter.
• They show only partial aquatic adaptation.
(a)Small external ears.
(b)Webbed feet.
(c)Flattened tail
(d)Subcutaneous fat.
e.g ; Ornithorhynchus ,Phoca ,Hippopotamus e.t.c. Phoca
Ref-Textbook of zoology by Parker & Haswell

5. Completely aquatic mammals
• Members of two orders,Cetacea and Sirenia are
completely aquatic forms.
• They never come to land and are perfectly
found in water.
• They show complete aquatic adaptation.
(a)Streamlined , fish- like forms.
(b)Forelimbs are modified to form paddle-like
flippers while posterior limbs are absent.
(c) Eyes small ,external ears absent ,mammary
glands well developed.
• e.g; Whales ,Dolphins,Porpoises,Manatees and
Manatee
Dugong

6. Aquatic adaptation
Adaptation of truly aquatic mammals fall into three categories
(1) Modification of original structure
(2) Development of Loss of structure.
(3) new structure.
Modification of original structure
Body shape
• The external fish –like forms.
• Elongated head ,indistinct neck.
• Tapering streamlined body.
• The tail enlarges.
Oblique diaphragm
• Oblique diaphragm makes thoracic cavity
• larger, dorsal and barrel shaped.
• It provides more space to lungs for expansion. Balaena mysticetus showing aquatic modification
Ref – Life of Vertebrate by J.Z Young

7. Endoskeleton
• Cranium becomes small but wider to accommodate the short
and wide brain.
• Cervical vertebrae are fused to a solid bony mass because of
reduced neck.
• Zygapophysis are reduced, sacrum is also reduced, bones are
light and spongy.
Large size and weight
• Whalebone may grow up to 35 meters in length and weigh
about 150 tons.
• Large size reduces skin friction and heat loss.
Flippers

8. Large lungs
• Lung is very elastic and extensible, respiratory surface
is small.
• Special cartilaginous rings and muscles are present in
bronchioles.
Intra – narial epiglottis
• Epiglottis is extended as a tube inserted into a
posterior narial cavity.
• It provides a separate air passage ,thus allowing
breathing and feeding simultaneously.
Teeth
• In toothed whales teeth are
monophyodont,homodont and numerous as many as
250.
• It helps in seizing prey ,swallowing it without
mastication.
• Ancient whale –like forms (zeuglodonts) had the
tooth differentiation characteristic of mammals.
• In baleen whales tooth are entirely absent there has
been developed the baleen.
Zeuglodon skull with teeth
Walrus teeth
Ref-Textbook of zoology by Parker & Haswell

9. LOSS OF STRUCTURES
• The whales and sirenians have lost all hairs except few bristle around mouth.
• Most of them are covered with foetal hair before birth ,which points to an
ancestrally hairy condition.
Loss of glands
• Lacrimal gland and all type of skin glands are absent.
• The salivary glands are also reduced.
Reproductive organs
• The testes abdominal, scrotal sacs are also absent.
• The placenta is diffuse ,uterus is bicornuate.
Girdles
• In pectoral girdles ,clavicles are absent.
• Pelvic girdles are greatly reduced, scapular fossa is extremely reduced.

10. Development of new structure
Tail fluke
• Tail develops large lateral or horizontal expansion of skin called flukes.
• They are not supported by fin rays.
Blubber
• It is a thick subcutaneous layer of fat ,acting as heat insulator.
• It reduces specific gravity of the animal, imparting buoyancy.
Baleen
• In a whalebone whale teeth are absent. Mouth with baleen
• Jaws carries horny plates of baleen ,serve as an effective sieve for straining plankton
Melon
• It is a receptor present in front of nostril
• It serve to detect pressure changes in water.
Harderian gland
• Eyes in water is covered by special fatty secretion of harderian gland.
Ref – Life of Vertebrate by J.Z.Young

11. Thank you