Introduction

INTRODUCTION

   

          Mammals are a special group of animals with a combination of characteristics that separate them from all other animals:They  are warm-blooded, have four chambered hearts, have hair or fur, breathe air through lungs, bear live young, and nurse their young with milk produced by mammary glands.

          Obviously there are a lot of examples of animals that live on land that meet these five qualifications, including dogs, horses, cats, rats, pigs and people.  

          For animals that live in the seas and oceans, those that meet all five criteria certainly include seals, sea lions, walruses, sea otters, polar bears and manatees.  

          But what about dolphins, whales and porpoises?  Hair?  Yes, very early in life these animals posses hair around their mouths which are usually lost soon after birth.  If you look closely at some of these adult animals you can still see the raised follicles were the hair used to be.  

          All marine mammals were originally land mammals and share common ancestry and characteristics with every other mammal.  Over time they became more and more specialized to meet the challenges of life in an aquatic environment.  For example, cetaceans, who live their entire lives in the water, have little need for hair.  In fact hair can be a problem by acting as a drag to slow them down as they speed through the water.  To be more efficient in the water they have lost almost all of their hair.  On the other hand, slower swimming marine mammals that still spend a lot of their time on land, such as polar bears and sea otters, still rely heavily on the protection and insulation that their hair or fur provide.  

 

         Marine mammals are vertebrates , adapted to live , feed and reproduce in the marine environment exclusively and they never visit terrestrial environment except few species of carnivores. There are about 120 species of marine mammals, those mammals that live in the sea for the greater part of their lives.

      They include whales (cetaceans), some species of which filter-feed directly on zooplankton and small fishes whereas other species feed high in the food web (e.g. killer whales), and seals/sea lions (pinnipeds), which tend to be high in the food web and feed mainly on fish. Marine mammals can be subdivided into four recognised groups; cetaceans (whales, dolphins and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and fissipeds, which are the group of carnivores with separate digits (the polar bear, and two species of otter). Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semi-aquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to ocean living. Now a days the members of the pinnipedia and the carnivoras are grouped into one order of carnivore for practical convenience. Hence,today the marine mammals are grouped  into mainly three orders viz. cetacea, sirenia and carnivora. As these are the warm blooded animals, they have the special adaptation to keep their internal body temperature warm and for this, most marine mammals have thick layer of blubber or fatty subcutaneous layer under the skin, which keep the body warm in the aquatic environment.

          There are about 78 species of cetaceans and 4 species of sirenians. In India marine mammals, are represented by all cetaceans and one sirenian species( Dugong). Among the marine carnivores , no seals or sea lions have been reported to be present in or near India and also no Indian carnivore is visiting marine or sea regularly.

          Many species can stay under water for a long time, but must come to the surface to breathe.  To enable then to stay under water for long periods, they store extra oxygen in their muscles and blood.  They also have more blood than land mammals in proportion to their body sizes, can direct their blood flow to only their vital organs (such as their heart and lungs), and can slow their heartbeat down so they are using less oxygen in a dive.

 
Last modified: Wednesday, 9 May 2012, 6:27 AM