Respiratory and Circulatory Systems.

Like birds, mammals are warm-blooded. To keep their body temperature at a constantly high level, mammals need to produce a great deal of heat. A high metabolic rate, in turn requires plenty of food for energy. Unlike reptiles, mammals can remain active even when environmental temperatures are low.

Mammals have efficient respiratory and circulatory systems to maintain their high metabolic rate. The four-chambered heart allows oxygen-rich blood to travel from the lungs to the heart without mixing with the oxygen-poor blood returning to the heart from the rest of the body. The relaxation and contraction of the diaphragm changes the volume of the lungs. The changed volume results in a change of air pressure inside the lungs, which in turn moves air in and out of the lungs. In mammals, the breathing and food passages are separate, allowing the animal to breathe and eat food at the same time. The air and food passages cross in a common area at the back of the mouth. When mammals swallow, a thin flap of cartilage, the epiglottis, moves downward and prevents food from moving into the respiratory passages.

Teeth are suited for different foods. The teeth of mammals have different shapes for doing different things. Your front teeth are sharp for biting, cutting an tearing tough foods. Your back teeth are flat and broad for crunching food. In this way you are suited for eating both meat and plant matter. Feel these differences in your teeth and observe them. A horse, which eats mainly a plant matter, has rough and wide teeth for crushing and grinding plant material. A dog, which eats animal matter, has pointed teeth that tear meat and the large, rounded teeth that crush bones. Scientists can tell much about the kind of food an animal eats by looking at the structure of its teeth.

Nervous System. In proportion to mammal’s body size, its nervous system is far larger than that of any other vertebrate. In addition, a large proportion of a mammal’s brain correlates and integrates incoming information. In mammals, the cerebrum is the dominant as well as the largest part of the brain. The brains of fish, amphibians, and reptiles are organized differently. In mammals, a part of the cerebrum, the cerebral cortex, has expanded to form a layer covering most of the forebrain. The cerebral cortex has become the major coordinating center of the brain. The exceptional learning ability and memory of mammals are attributed to the well-developed cerebral cortex.

Reproduction.

All mammals reproduce through internal fertilization. The male releases the sperm inside the body of the female where it fertilizes one or more eggs. After the eggs are fertilized, they are nurtured within the mother’s body for a time. Male mammals can mate at any time after they reach maturity. In most species, however, a female will not mate except during her fertile period, called estrus. Some species have estrus periods several times a year. In other species, estrus occurs once a year, timed so that the young’s birth takes place during the season most favorable for survival.

One of the most important and distinctive features of mammalian reproduction is that the young have a childhood, a distinct developmental period. When a young mammal is born, it is unable to survive in the world on its own. A young mammal needs a certain amount of parental care while it is learning how to obtain its own food and to defend itself against predators. The care and protection given to the mammalian young greatly increase their chances to survive. Thus mammals bear relatively small numbers of young, compared with vertebrates such as frogs and fishes.

Behavior

Many mammals live together in social groups for the purposes of hunting or defense. A zebra herd consists of one adult male and several females and their young, while a wolf pack is made up of several males and females and their young. Some hunting animals live solitary lives. Adult tigers, for example, come together only to mate. Other patterns of behavior also differ among species. Certain mammals exhibit territoriality. Others migrate or respond to cold weather by hibernation.

Territoriality. Many mammals exhibit territoriality – that is, they claim a particular area as their own and defend the area against other animals of the same species. Fur seals establish a territory only during the breeding season. At this, one male tries to prevent other males from entering his area. At the same time, he tries to move as many female seals as possible into his area. Other species establish territories as a means of protecting a food source. Many mammals mark their territories by leaving scents of body wastes. Not all mammals defend territories, but most have a home range – that is, an area over which they travel during normal activities.

Migration.

Many mammals move regularly over large distances in search of more food, better environmental conditions, or more suitable places to bear the young. Gray whales travel each autumn from arctic waters to waters off the coast of Mexico. There the young of the herd can be born in warmer waters. They return north in the spring.

Hibernation.

Migration is not the only way mammals solve problems of temperature extremes. Some species of bats, insect eaters, and rodents go into hibernation during winter months. Their metabolic rate decreases. Body temperature falls and heartbeat and breathing rates are slow. In such a state, the animals need little energy and can live off stored body fat. When the outside temperature increases, they slowly wake up and resume normal activities. Bears, skunks, and some other mammals spend the coldest part of winter in a den asleep in a state of dormancy. Unlike true hibernators, their body temperatures decrease only slightly and they may move in and out of sleep. A few mammals, mainly rodents, avoid the extremes of summer heat or drought by moving into a state similar to hibernation called estivation.

Classification of mammals

Biologists classify the 4,500 species of mammals alive today into 18 main orders on the basis of structural differences and development of the unborn. Members of these orders live throughout the world. Mammals differ in the way their young develop before birth. Mammals can be divided into three groups: egg-laying mammals, pouched mammals, and placental mammals.

Monotremes are mammals that lay eggs. Young monotremes develop within a protective shelled egg much as reptiles do. Pouched mammals are known as marsupials. Young marsupials develop within the mother’s body for only a short time. After birth, they complete their development inside a special pouch, or marsupium, located on the mother’s abdomen. Most modern mammals are placental mammals. In placental mammals, the young remain within the mother’s body until they are able to maintain life independently. A special organ called the placenta connects the unborn young to the mother.

Monotremes

Only two types of monotremes are alive today, and they live only in a few isolated regions of the world. The duck-billed platypus lives only in Australia. The spiny anteater lives in some parts of Australia and New Guinea.

Monotremes are considered mammals because they have hair and produce milk for their young. In other ways, the monotremes are fundamentally different from all other mammals.

Monotremes show a curious mixture of traits. They have a cloaca, as do the lower vertebrates. Their limbs are attached to the sides of the body like the limbs of reptiles, and their feet have claws. The platypus has webbed feet and a flat tail that it uses for swimming. Unlike most other mammals, monotremes do not have true teeth. The platypus uses its flat bill to probe in the mud for worms, snails, and shellfish. The spiny anteater uses its beak to probe into anthills for food.

The major reptile-like feature of the monotremes is that they lay eggs. Monotremes produce two to three eggs at a time. After fertilization the eggs are kept inside the mother’s body and are nourished for a short period before they are laid in the nest. The female incubates her eggs by curling her body around them. The anteater has a special brood pouch on her lower abdomen in which she keeps the eggs until they hatch.

After the young monotremes hatch, they are fed with milk secreted by their mother. The milk is produced by more than 100 specialized sweat glands on the lower abdomen. Unlike other mammals, young monotremes do not suckle. Instead, they lap up the milk as it oozes onto the mother’s belly.

Marsupials

Kangaroos, koalas, and almost all other marsupials live in Australia. A few species of marsupials live in South America. North America has only one marsupial, the opossum. The one major difference between marsupials and other mammals is that young marsupials complete their development inside their mother’s pouch.

The fertilized egg develops into an embryo, which remains in the female marsupial’s body for only a short time. The embryos feed from the yolk in the egg and do not attach to the uterine wall. In some species, internal development lasts only a matter of days. At birth, young marsupials are still in an incomplete stage of development. They are blind, helpless, and extremely small.

Once inside the pouch, the young marsupial fastens its mouth onto a nipple. The nipple swells, and the young becomes firmly attached and begins to suckle. Immature marsupials are fed and protected inside the pouch until they are able to feed and care for themselves. The hind legs of the newborn marsupial are poorly developed; actually, they are a little more than embryonic buds. The front legs, however, are more fully developed and tipped with claws. A young marsupial uses its front limbs to pull itself up the mother’s abdomen and into her pouch.

Date: 2014-12-22; view: 1029