AQUATIC MAMMALS.pptx

about aquatic mammals

1. Aquatic mammals

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. Streamlined , fish- like forms. 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 Dugongs Ref-Textbook of zoology by Parker & Haswell Manatee Dugong

6. Aquatic adaptation Adaptation of truly aquatic mammals fall into three categories Modification of original structure Development of Loss of structure . new structure . Modification of original structure Body shape The external fish –like forms. Elongated head , indistinct neck. T apering 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 Entire skeleton of the limb enclosed by the skin ,no external division. The broad and flattened flippers serve as balancers and provide stability during swimming. Hyperdactyly and hyperphalangy Extra digits and extra phalanges enlarge the surface area of flippers . They have greater utility for swimming in water.

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