UNIT 4 MAMMALS Structure Introduction Objectives General Characters and Classification Natural History MonotremataIPrototheria Marsupialia Eutheria Economic Importance Evolution and Affinities Threatened Species in India summary Terminal Questions Answers INTRODUCTION You have so far studied about vertebrates which lay eggs and take little or no care of their young ones. Now you will study a group of vertebrates that give birth to young ones and take care of them by nourishing them. They are called mammals. The name is derived from Latin word "mamma" meining breasts. Mammals are provided with mammary glands or breasts which secrets milk that is fed to young ones. The fertilized egg is retained within the uterus of the mother where it undergoes development in relative security. During development a structure called placenta is formed. It helps in supplying nourishment and oxygen to the developing embryo. It also helps the embryo in getting rid of nitrogenous wastes and carbon dioxide. All the four foetal membranes are formed during embryonic development. Allantois and the yolk sac establish close contact with the uterine wall and form the placenta. Mammalia represent the most successful and at present the most dominant group of tetrapod vertebrates on earth. There are about 5000 species of mammals alive today. As the body of reptiles is covered by scales and that of buds by feathers and the body of mammals is covered by hairs. Hairs also form an insulation to the body like feathers and helps in maintaining constant body temperature. So mammals are also warm blooded (Homeothermic) as birds. Mammals are widely distributed. They occur in the coldest regions of the earth like the arctics and the hottest regions like the sand deserts. They occupy all types of land habitats. They walk, run, swim, burrow and fly. There are mammals adapted to flying and aquatic life. The brain of mammals is well developed. They are more intelligent than other vertebrates. Mammals represent the zenith of vertebrate evolution at present. The present geological period is called the GOLDEN AGE OF MAMMALS. Man belongs to this group of vertebrates and stands at the pinnacle of evolution today. Objectives 0 After reading this unit you should be able to: explain important characters of mammals which have contributed to the success of their life on land, give an account of nonplacental mammals, describe adaptations in bats for flight and whales for aquatic mode of life, explain dentition in mammals, discuss the evolution of man, f)O list the names of threatened species of mammals in India. 4.2 GENERAL CHARACTERS AND CLASSIFICATION Mammals 1. The body of mammals is covered by hairs. Hairs and other structures like claws, hoofs and nails on the digits as also scales found in some mammals as on the tail of rats, form the epidermal exoskeleton of mammals. 2 Mammals are homeothermic. The hairy covering of the body and sweat glands found in the skin help in maintaining constant body temperature. 3. A number of glands like sebaceous, sweat, scent and mammary (milk) are present in the skin of mammals. 4. The skeleton of mammals is well ossified. The cranium inntheskull is large. The skull is bicondylar i.e., two occipital condyles. Teeth are heterodont (occurrence of different types of teeth), thecodont (teeth fitted into pits on the jaw bones) and Ossify : make or become hard like bone or change into diphydont (occurrence of two sets of teeth in the life of a mammal). The lower jaw bone. is made of only one bone, the dentary. The upper jaw is hsed with the skull and the lower jaw articulates directly with the skull. There are seven vertebrae in the neck of mammals irrespective of the length of the neck. An external ear opening and an external ear, the pinna are present. There are three ossicles in the middle ear (incus, malleus and stapes). They help in transferring sound waves to the internal ear. 5. The brain is comparatively large when compared to oher vertebrates. The cerebrum is well developed. There are four optic lobes (called corpora quadrigemina) as opposed to only two in the brain of other vertebrates. There are 12 pairs of cranial nerves. 6. The heart of mammals is four chambered. Both auricle and ventricle are completely divided into right and left chambers. The opening between the left auricle and the left ventricle is guarded by the bicuspid valve whereas the opening between the right auricle and the right ventricle is guarded by ths tricuspid valve (Fig.4.1). The presence of these valves is characteristic of mammals. There is only one aortic arch, the left aortic arch which curves around to the.dorsa1 side and forms the dorsal aorta. The red blood corpuscles are non-nucleated except in camels where they are nucleated. Superior vena cava \ Atrioventricular node veins \ Sinus node 'Ventricular septum Fig.4.I: Cutaway view oi the human heart showing bicuspid and tricuspid valves. Diversity in Chordates 7. There is a muscular diaphragm separating the abdominal cavity from the thoracic (Fig. 4.2). The diaphragm helps in respiration. Lowering and raising of the diaphragm sucks in and pushes out air from the lungs during respiration. ,!nciw'r teeth Sub-maxillary glands Fig.4.2: Dissection of rat showing diaphragm which sepatatw the abdominal cavity from thoracic. 8. Lungs are elastic and contain small air cavities called alveoli. There is a voice box called larynx in the trachea. It helps in producing sound. 9. The kidney of an adult mammal is formed fiom metanephros. A urinary bladder is usually present. 10. .Sexes are separate. There is sexual dimorphism. The male mammals are provided with copulatory organ called penis, It helps in transferring sperm (semen) to the female genital tract during copulation. Testes are usually present in extra abdominal sac like structures called scrotum. Fertilization is internal. Mammals are viviparous (having offspring which develops within mother's body). A placenta is formed during embryonic development. The anus and the urinogenital openings are separate and a cloaca is absent except in Monotremata. 4 Classification: Mammals are classified on the basis of the nature of teeth, limbs and the presence of claws, hoofs or nails on the digits. The class Mammalia is divided into three subclasses based on the nature of reproduction. Subclass I : Prototheria 1 Monotremata: It includes primitive reptile-like mammals. They lay yolky eggs yet secrete milk and are fundamentally different fiom all other members of the class mammalia. Testes are abdominal in position. They possess a cloaca. The external ear (pinna) is absent. They Mammals are restricted to Australia, Tasmania and New Guinea. Examples: The spiny ant eater (Tachyglossus~Echidna,(Fig. 4.3). ~ta& Fore i;mb GIG. (ly)lts Fig.4.3: Echidna, The mouth is small but it has a long sticky tongue to gather up its mites d and ants. The duckbill Platypus (Ornithorhynchus) (Fig. 4.4) is an egg laying mammal and exhibits a combination of reptilian and mammalian characters. Reptilian features are uninogenital system, precoracoids, absence of pinna etc. Mammalian features include hair, diaphragm, 4-chambered heart; 3 ear ossicles etc. Duckbill inhabits rivers, pools, and creeks and burrows to 40 feet long in river banks. The animal feeds on freshwater invertebrates, carried in cheeck pouches. Upper jaw forms a flattened beak covered with a smooth, hairless skin which forms a free fold at the base of the beak. Adult has no teeth and jaws are covered with horny plates. Limbs have 5-clawed and webbed digits. Tail happens to be flat and adapted for swimming. Mammary glands are without nipples. Female makes nest of roots and leaves during spring in burrows and lays 1-3 eggs. Young ones hatched out of eggs are nursed by milk secreted by scattered mammary glands on the abdomen of female. Fig.4.4: Ornithorhynchs. Subclass 11: Metatheria / Marsupialia There is a ventral abdominal pouch called marsupium in the females. The young ones are retained and protected in this pouch. The young pups born after a short gestation period (13 days in the kangaroo) are transferred to the marsupium, nourished and protected there until they are able to look after themselves. The animals are confined to Australia and America. Diversity in Chordates , , Fig.4.5: Macropus The Kangaroos (Macropus conguru) (Fig. 4.5) have become mostly terrestrial and developed a bipedal method of progression. Their bipedal locomotion involves modification of the ilia and thigh muscles for whose attachment the tibia bears a marked anterior crest. The foot gains increased leverage by elongation of metatarsal of digit 4. Digits 2 and 3 are very small and syndactylous. Due to hopping habit of kangaroos, it is no surprise to see that their hind limbs are long but it is somewhat surprising to find that their fore-limbs are quite short. Most of the length is provided by the long forearm bones which are considerably longer than the humerus. If the animal bends forward, its fore- limbs soon touch the ground. Do they have any function in locomotion? The fact is that they do, because kangaroos have a crawling gait in which the forelimbs and tail are used as props. Kangarocs (Marsupials) are pouched, ovoviviparous mammals that exhibit a second pattern of reproduction. These marsupials do have a primitive type of placenta, called a f choriovitelline or yolksac, placenta. The embryo or blastocyst of a marsupial is at first encapsulated by shell membranes and floats free for several days in the uterine fluid. After hatching from the shell membranes the embryo does not implant, or take root in A uterus as it does in euthqrians, but it does erode a shallow depression in the uterine wall in which it lies and absorbs nutrient secretions from the mucosa by the way of , vascularised yolk sac.