What Is A Mammal? (1962)

that the first large land animals to appear were reptiles reptiles invaded full environmental niche in water in the sky and on land although some reptilian types have survived other sorts of animals which had developed inconspicuously from a reptilian stock became better adapted to life on land these were mammals members of a group which includes almost all the larger animals on the surface of the Earth and in the sea when we compare a mammal's characteristics with those of a reptile we find much evidence for the superiority of the mammal a modern lizard and a hamster may be compared in a warm environment the reptile is quite active however its periods of activity may be brief in a cold environment its movements cease as the temperature decreases its rate of living slows down on the other hand mammals pursue alert continuously actives because of this activity the first mammals had an advantage over most reptiles in competition for food efficiency of movement depends partly upon Anatomy since reptil's legs are set out to either side a great amount of energy is expended to keep the body off the ground in contrast a mamals legs are set directly beneath the body bones not muscles support the animals weight specializations of this sort of support are common in modern mammals sense organs may also be compared The Reptilian ear lies close to the surface of the body it is not covered by skin or by any protective structures inside only one small bone conducts sound from the eardrum to the inner ear the situation is quite different in a mammal a Maman ear has an external flap for the concentration of sound waves the eardrum lies at the end of a long tube comparatively far beneath the surface of the skull it is connected to the inner ear by three bones which have evolved from Bones and in the old reptile jaw joint the three bones transmit vibrations with greater mechanical efficiency than a reptile's one bone the teeth in a reptil's mouth are almost exactly alike on the other hand each kind of mammal has specialized teeth suited to its particular eating habits a hamster's chisel-like front teeth are adapted for gnawing in a dog the arrangement is somewhat different the jaw is divided into incisors for nipping canines for attack and defense premolar and MERS for shearing food the skull of a reptile consists of a number of bones this jaw has seven bones it meets the skull far to the rear an arrangement that gives little mechanical advantage in biting in contrast a mammal's skull has relatively few Bones the Jawbone is a single bone that meets the skull relatively close to the front less energy is expended on a forceful bite what's more every mammal has a hard pallet which allows better separation of its breathing and feeding activities at the back of the reptilian skull is a comparatively small brain its structure is primitive for the cerebrum is concerned mainly with a sense of smell centers concerned with sight and behavior are located in a small brain region a mammal's brain is larger in comparison to the size of the animal than is the reptils the brain case is relatively larger the cerebrum Center for Learning and intelligent Behavior constitutes the bulk of the brain experiments reveal that a reptile has practically no mechanism for maintaining constant body temperature when a reple is subjected to cold its body temperature drops immediately and noticeably subjected to heat the reptile's body temperature rises a reptile has no no specialized mechanism to disperse excess heat within its body beats The Reptilian heart except for alligators and crocodiles no reptile has a double completely separate circulating pump one side pumps de oxygen ated blood from body to lungs the other returns oxygenated blood from lungs to body however the two systems are not completely separated oxygenated and deoxygenated blood may be intermixed in a mammal the situation is quite different normally the body temperature of a mammal is higher than the environmental temperature a major mechanism for maintaining temperature is the mammals body covering in a cold environment each hair of a thick furred animal is erected trapping a blanket of still air that provides insulation when warm conditions Prevail a mammal's body temperature does not change the reaction of the skin is reversed each hair is allowed to lie flat against the body reducing the insulating action and permitting heat to escape from the surface of the skin in a mammal the two sides of the heart are completely separated and there isn't the slightest chance that oxygenated blood will be mixed with deoxygenated blood this is an efficient pump though some reptiles give birth to live young nearly all deposit their eggs and desert them a developing reptile is nourished by yolk material in the egg after hatching there is no special mechanism to provide food for the Young on the other hand nearly all mammals bear their young alive born relatively helpless they depend on maternal care some small mammals produce many offspring at the same time as a litter in contrast large mammals generally produce only one or a few Offspring at a time more advanced animals retain their young until development is more nearly complete inside the mother's body they are nourished through an organ called the placenta the placenta is expelled at Birth among mammals an important aspect of maternal care is the feeding of young after their birth the term mammal is derived from the name of certain glands which secrete nutrient fluid for feeding these are the mamory glands their fluid is milk maternal care sometimes continues long after a young mammal ceases to require milk such unique adjustments to a complex way of life evolved over millions of years A Primitive stage in mammal development is represented by the egg laying mammals one is the duck bill platypus of Australia another egg laying mammal is the spiny anteater except for the reptilian characteristic of laying eggs the spiny and eater basically resembles modern mammals more advanced than the egg layers are marsupial such as the opossum marsupial are primitive primarily in their method of bearing young a marsupial is born at a very early stage in its development when it is quite immature after birth it lives under a specialized flap of skin on the mother's body known as a pouch during development in the pouch a young marsupial becomes firmly attached to a mamory gland as development continues it becomes increasingly independent until it eventually leaves the pouch the world's largest marsupial are the kangaroos of Australia modern mammals have successfully adapted to a wide variety of conditions the toothless an eater lives on ants and termites bats fly through the air fish-like mammals are at home in the Sea The sulfur bottom whale is the largest animal that now lives or has ever lived on earth other mammals are meat eaters still others are gwing vegetarians or cud chewing vegetarians one group is distinguished by its grasping trunk another by its intelligence man belongs to this group these are the mammals distributed over the Earth more wide than any other large animals descended from reptiles they have come to outnumber the reptiles filling environmental niches in water in the air and on land