General Characteristics and Adaptations of Mammalia:
Mammalian skins have hair with two kinds of glands, which are sebaceous and sweat glands. Their skeletons are bony. There are 2 pairs of pentadacty-limbs. They do not build up gills from their visceral clefts. They contain external ear additionally to middle and inner ears. Just two of genera lay eggs. Embryo grows within mother. Mother has mammary glands that make milk for new born. Fertilization is internal. There are various kinds of teeth carrying different functions. There is muscular diaphragm between thorax and abdomen. Instances are man and dog. Mammals are more intensely transformed from original terrestrial vertebrate kind than birds; though birds are mainly specialized for flying. Mammals are independent of conditions in the external surroundings and keep the stable internal surroundings irrespective of environmental changes. Mammals can be stated as enhanced vertebrates. They are known as mammals as they have mammary glands with which their born alive, young are fed. Their growing embryo is nourished from placenta. All of these point to high level of specialization and organization in mammalian body.
Hair, Heat and Action Adaptations: Skin, Lungs and Heart
Mammals are warm blooded. This signifies they keep the steady body temperature that could even be higher than that of the environment apart from when in hibernation, mammals are prepared for action at all times. The given brief explains structure and function of skin:
Skin is composed of many layers:
i) Horny layer: composed of flat, dead cells similar to scales for protection
ii) Malpighian layer: of more rounded cells massed mutually to shape tongues that project into third layer. These cells have pigment for skin color. These 2 layers form epidermis that contains no blood vessels or nerves. Its thickness varies from part to part in body.
iii) The dermis: is true skin. Its cells are heaped in small projections (papillae) that fit in between tongues of cells sticking down from epidermis. It has blood vessels and nerves.
iv) Subcutaneous tissue: They are connective tissues. It has fat. Hair grows out of hair follicle and is oiled by sebum secreted by sebaceous gland. When muscles attached at base of hair follicle contract, they pull or else slanting hair to the right-angled position to skin. This traps the body of hair that forms the warm blanket which assists to keep body warm.
High body temperature is consequence of activity of muscle tissue that produces heat continually. Coat of hair on skin act as insulators stopping heat loss, same way as feathers perform in birds. Sweat glands in skin, secrete water into skin surface, that when evaporated causes cooling of high body temperatures. In this manner mammal regulates the body temperatures. With the steady body temperature and blood component, body can perform actions at all times irrespective of conditions in environment. Essence of mammals appears to be stability, energy and sensitivity. They exist with the steady internal environment all through life. This provides them best precondition for the free life.
Curiosity and explorations are thus strange only to mammals. The high activity of muscle and brain can only be maintained by the effective blood supply. Mammalian blood system is driven by the heart with 4 chambers - two in one half of heart. One half takes deoxygenated blood to lungs (to be oxygenated) while other gets oxygenated blood under high aerial pressure.
Food Intake Adaptations: Skull, Jaws, Ears
Mammals require nutrition to create energy in presence of oxygen. Nature of skull and jaws make sure that food can be taken in and ready for digestion. Skull bones have been decrease in number but elongated in the individual sizes. Original front side now has hole to accommodate powerful jaw muscles. The bony portion divides nasal and food passages to as far as throat. As mammal is always active, it should have constant supply of oxygen. Skull is mainly swollen to accommodate brain. Jaw bones are also decreased in number but increased in size (maybe from fussion of parts) to make sure increase in strength. All the adaptations of skull and jaw bones are to make sure that powerful muscles won't break them under stress of chewing. They support teeth that have been distinguished into different kinds for chewing (premolars and molars), for cutting canines and sharp pointed incisors for tearing. There are 2 sets of teeth in mammal -milk teeth and permanent teeth. Original primitive mammalian set of teeth (totaling 44) is adapted in several ways. Every half jaw (primitive) in adult has 3 shard-edged incisors in front, single large pointed canine, 4 premolars and 3 grinding molars. Number of teeth differs from mammal to mammal. The kind of dentition is determined by feeding.
Another advance in mammal is presence of 3 pieces of bone in ear that form the chain in articulation. These are known as stapes (or stirrup) that matches to columella of amphibians that transmit vibrations from center to inner ear. Outer two, anvil and hammer are thought to be derived from displaced jaw angle bones of reptile.
Adaptation for Running: Bones and Joints
One of first characteristics showing adaptation for fast movement is posture of foot. Animals that move on their whole feet are not fast movers. These that move on flat of their feet are explained as plantigrade. They are dissimilar from subplantigrades that rest with sole of their feet on ground and move on end joints of fingers and toes, like hare.
The joints in horse have shifted because of increase in length of bones. What looks like elbow is actually wrist because of increased length of bones of what must be palm. Elbow is inside body of animal. Sheep and cow are unguligrades but they have 2 digits (3rd and 4th) still functional. The nails of these two digits form cloven hoof. Vestiges of second and fifth are tiny projections behind hoof now. They don't touch ground. In bats (only mammals that can energetically fly) fore limb is altered to form wing. It varies from wing of birds as hind limb is also part of wing in bat. Wing is fold of skin supported by forearm and forefingers - spread out and lengthened. Short thumb has hooked claw and projects forwards from edge of the wing. Membrane of wing is joined to ankle of foot and extent between legs. It can only handle awkward ground movement as adaptation for flight. This locomotion is associated to flesh eating. This is in turn associated to increase in size of brain for increased balance, alertness, and welfare.
Maternity: Adaptations for care of young
Mammals show the high degree of care for the young. Only primitive mammals lay egg. In all other mammals, egg stays in body for much longer time. This retention assists to protect developing embryo and saves female trouble of having to guard eggs. It also increases their probabilities of survival for embryo. Some retiles (snakes) also give birth to the young alive similar to mammals. Birds though are recognized not to have such members who give birth to the young ones alive. This is possibly associated to their flying habit in which they can't afford to bear burden in flight. Mammals retain fertilized egg and nourish it with maternal tissues. Mammalian egg is much smaller than those of birds and reptiles. It is simply visible to naked eye. Egg and maternal system carrying egg has been significantly transformed. Developing egg becomes attached to wall of uterus or womb. It is housed in placenta and joined to uterus by umbilical cord. It is through placenta that embryo receives oxygen and nutrients from, and sends out wastes matter into maternal blood stream of components of amniotic egg, only amnion persist in the original state and provides as the protective water jacket around developing embryo. It forms foetal membranes that rupture during birth of young mammal.
Mammals suckle the young after they are born from mammary gland until they (young) are able to fend for themselves. So you see that whole process of reproduction from time of getting sexual maturity, to mating, birth and weaning of young become the complex issue involving physical, physiological and behavioral controls. Climax of the complexity is found in man.
The 2 major groups of mammals are pouched mammals and true placenta mammals. Pouched mammals (the marsupials) have been comparatively unsuccessful in joining placenta to wall of uterus. Young are born premature and are suckled with protective pouch known as marsupium. Microscopic shell and virtually yolk less eggs are kept in uterus to grow for about 2 weeks. Then they are born small and premature to suckle in marsupium for long time. All other mammals contain true placenta.
Diversity in Mammals:
Mammals are very varied in size. They range from small quarter-ounce shrew to 80-foot, hundred-ton whale. Mammals have become adapted to all mode of life - running, walking, swimming, burrowing, climbing and flying and feed in every likely ways. Few chew grass, tear flesh, sieve plankton, whereas others catch insects on wing. Each feeding habit is reflected in nature of the teeth. Original mammalian diet appear to have been small forms of animals such as worms, grubs, winged and wingless insects in spite of the variety they have kept some basic similarities. There are 7 cervival vertebrae in all (elephant, mice, man, giraffe etc). Class insectivores are prepared with long pointed nose and tapering tongue to lick up large number of minute insects. All hoofed forms feed on vegetation. Many ungulates have to be fast runners to run away being eaten by flesh eaters. This has required changes in skeleton of limb. They have grown hooves for gaining traction on ground as they run.
Mammals comprise; zebras, horses, deer and cattle. Rodents are vegetation. They are also known as knawing animals and explained as most successful of all mammals. Every rodents have effective grinding teeth. Most herbivorous mammals are the source of protein for man. Flesh eating mammals comprise cat's family great and small, bears, weasels, rancoons, dog and seal families. These all have kept their incisors for tearing and biting. Cetaceans (whales) and porpoises are air breathing warnblooded mammals which bear and suckle the young in typical mammalian fashion. They have though entirely readapted to life in water.
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