Mammals

The mammals are the most advanced of all vertebrates and are the dominant land animals all over earth. There are a number of features that distinguish mammals from other animals. The most obvious are the possesion of hair, and of mammary glands with which the female suckles her young. These features are not shared by any other class of animals. Mammals and birds are the only animals which maintain a constant body temperature. They are aided in this by their fur and feahers. The majority of mammals retain their young in the womb until a fairly late stage of development. Parental care of the young is another important feature of mammalian life.

Guanaco
Photo by Laura Johnston

Modern mammals have only a single bone in the lower jaw. During their evolution from reptilian ancestors the mammals gradually lost the other bones from the jaw. Fossils indicate that the first mammals appeared in Triassic times, nearly 200 million years ago. The early mammals were about the size of small rats.

Present-day mammals are arranged in 18 orders. The order Monotremata contains three Australasian animals, the duck-billed platypus and two spiny anteaters. These have hair and feed their young on milk but they lay eggs and are not completely warm-blooded. They branched away from the main line very early in mammalian evolution and they are included in a separate sub-class-the Prototheria.

The order Marsupialia ("pouched mammals") give birth to their young at an early stage but keep them in a special pouch and feed them on milk for two months or more. There are marsupial dogs and mice and even moles and anteaters. Kangaroos, koala bears and opossums are all marsupials.

The great majority of mammals are placentals, which means they retain the young in the womb, nourishing until birth through a special attachment, the placenta. The most primitive living placentals are the shrews, moles and hedgehogs (order Insectivora). They have the typical five digits on each limb and they walk on the soles of the feet.



Bats (order Chiroptera) are the only truly flying mammals. Their wings are formed by thin membranes stretched between the fingers and the hind limbs. Bats feed mainly on insects but some of the larger ones eat fruit. The ears are always large, to pick up the high-pitched sounds by which bats find their way in the dark (echo-location).

The order Edentata contains three South-American groups: armadillos, anteaters and sloths. Their teeth are reduced or absent, consistent with a diet of soft-bodied invertebrates, especially ants.

Man, monkeys and lemurs are all members of the order Primates. They are mainly tree-living and have retained the primitive five-fingered limb which is adaptable for grasping. Development of the eyes and brain are important features of primates. The eyes are placed at the front of the head giving binocular vision. The teeth are not highly specialized for the diet is omnivorous.

The Rodentia are the commonest mammals. The important factors in their success are their small size and rapid breeding. The incisor teeth (one pair in each jaw) are very sharp and grow continuously to compensate for the continual wear during gnawing. Beavers, rats, and mice, guinea pigs and squirrels are examples of rodents. Rabbits and hares are similar but are grouped in a separate order, the Lagomorpha.

Whales (order Cetacea) are completely marine. They have reverted to a fish-shape, losing their limbs in the process. There are two groups: the toothed whales which eat squids and fishes, and toothless Whalebone whales which feed entirely on the minute organisms of the plankton.

The Carnivora is a larger order containing a number of distinct forms. The sub-order Fissipeda contains the wolves and dogs, bears, weasels and badgers and the cat family (Felidae). The teeth are normally adapted for tearing flesh. Bears are more omnivorous and lack the characteristic carnassial teeth. The carnivores frequently have binocular vision and are often camouflaged; for example, the leopard, in order to remain concealed from their prey. They usually walk on their digits. Seals and walruses, of the sub-order Pinnipeda. are also carnivores/ Their limbs are flippers and they come on land to breed. Family associations are well developed among the carnivores.

Barbary sheep

Ungulates are a large group of animals with a number of orders which include the elephants, hyraxes and sea-cows and the hoofed mammals. They are herbivorous and have large grinding teeth. There are two orders of hoofed mammals : the Perissodactyla and the Artiodactyla. They walk on the tips of the digits (unguligrade manner) giving them longer legs and therefor increased speed. The perissodactyls include the horse and rhinoceros. The latter have four digits but in the horse only the middle one is well developed. The artiodactyls, cows, deer and pigs, normally walk on two digits. They are the "cloven-hoffed" mammals. Many are ruminants, which menas they chew the cud, and they are frequently horned. The herding instinct is highly developed in these mammals as a protection against predators and as a regulator of grazing. Camouflage and swift running also help them to escape from enemies.

The class Mammalia is a relatively small one, especially when compared to the insects (it contains some 4,000 species, whereas insects comprises nearly a million). The mammals contain 19 orders, which include rodents (Rodentia), which make up about half of the mammals—and bats (Chiroptera), which account for about one quaater of mammals. Mammals are one of the most diverse groups, varying remarkably in structure and size, and in the habitats in which they are found. The smallest living mammals are the shrews (family soricidae); the pygmy shrew weighs less than 1 ounce. The largest mammal is the blue whale, with a weight of up to 145 tons.

Features

The main distinguishing feature of mammals are the presence, in the female, of mammary glands (mammae), a body covered in hair, warm-bloodedness, and a large brain. The dependence of the young on the mother means that there is a period during which it learn from the experience of its mother, and so become more efficient in dealing with the problems of survival.

In most mammals, hair or fur covers the entire body. Its purpose is to act as an insulator, because mammals are warm-blooded and need a means of preventing the loss of body heat. Body hair is one factor in enabling a mammal to maintain a fairly constant temperature, normally between 96.8 degree F (36 degree C) and 102 degree F (39 degree C). But in addition, the mechanism (the hypothalamus) situated in the brain regulates body temperature. In most mammals, this mechanism works together with sweat glands which are found in the skin and distributed over much of the body. Dogs have them only on their feet and keep cool mainly by panting. Heat induces hypothalamus to become active which causes blood vessels to dilate and give off the heat of the blood. The sweat glands exude liquid onto the skin so that it is cooled by evaporation. When temperatures are low, the blood vessels contract and sweat glands dry up. In addition, reflex shivering occurs which generates heat by successesive contraction of muscles.

Because the body temperature is kept fairly constant, mammals can usually remain active regardless of the external temperature, although in extreme conditions of cold or heat, some mammals hibernate, or estivate (sleep during the dry season). The body becomes torpid as the rate of the heart beat, breathing and other body processes slow down considerably. The animal then seeks shelter in a burrow or other safe place until the outside temperature increases.

Mammals have a four-chambered heart (also found in fishes, birds, and crocodiles), and a diaphragm which separates the chest and abdominal cavities so that the lungs can work more efficiently and therefore increase the amount of oxygen supplied to the blood.

Habitat

Mammals are regarded as extremely successful animals. One of the reason for their success is that they are highly adaptable and as such have been able to exploit an amazing variety of habitats. Some make their homes underground burrows, such as moles and many rodents, or among leaf litter on the ground (shrews); others lead semiaquatic life in fresh water, such as otters, or spend their whole life in the sea as whales do. Mammals also inhabit the tree canopies in forests (monkeys and others) and some live in the air at least part of the time, such as bats. Certain mammals are able to live in Arctic conditions without hibernating, as do Musk oxen, whereas others live successfully in desert conditions and can withstand the full heat of the Sun, such as camels.

Locomotions

Most mammals are quadrupeds, they move about on 4 feet. Some walk by placing the whole foot in the ground, when they are called plantigrade. This metod results in relatively slow locomotion and is seen in the polar bear. Many mammals, such as cats and dogs, lengthen their stride by walking on their toes; this is known as digitigrade locomotion and enables the animals to run at a high speed. Other animals walk on the tips of their toes which are normally protected by large nails or hoofs. This is called unguligrade locomotion and is found in deer, antelopes and horses. Humans are the only animals to walk bipedally in an erect position at all times, but some apes and monkeys move in this way for short periods.

Aquatic animals have limbs which have evolved into paddles, or fins. In some, such as the otter and the platypus, a membrane has evolved between the digits on the forelimbs, which aids them in swimming. In seals (Pinnipedia) all the flippers are used for moving through water, whereas in whales, the hind limbs have completely disappeared; locomotion is powered by movements of the back and tail whereas the function of the foreflippers is only to steer and balance.

The gibbons, orangutans and some other arboreal species of the Pongidae family have strong arms which they use to swing from when traveling from branch to branch. Some South American monkeys also have prehensile tails which operate as a fifth limb. The scaly-tailed squirrels, which also live in the tree canopy, have membranes which extend from the forelimb to the hindlimb and tail, on which they glide from tree to tree. They are sometimes called flying squirrels, but the only mammals that are true fliers are the bats. Their forearm is the major support of a wing membrane. The hindlimb and the tail often also support the wing.

Are There Venomous Mammals?

Besides well-known venomous snakes, lizards and fishes, venom systems can also be found in mammals. The mammalian animals known or suspected to be venomous Haitian solenodons (Solenodon paradoxurus), European water shrews (Neomys fodiens), American short-tailed shrews (Blarina brevicauda), platypus (Ornithorhynchus anatinus), vampire bats (such as Desmodus rotundus) and the slow lorises of Southeast Asia (Nycticebus species.)1

Mammalian platypus has the bizarre crural venom system. Rather than delivering venom through a bite, as do shrews and vampire bats, male platypuses have venomous spurs on each hind leg, which is connected via a duct to venom glands evolved from modified sweat glands. The study on the platypus reveals strong convergence between reptile and mammal venomous systems.

The slow lorises are the only primates, which harbor toxins. It has been proposed that the venom is a mixture of fluid of its brachial gland located in the ventral side of the elbow with saliva, and is applied to the top of the head for defense or kept in the mouth to bite.

References

  1. Why do we study animal toxins? Yun ZHANG




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