Mammals are warm blooded (maintain a constant body temperature) vertebrates (backboned animals) that raise their young on milk. Typically, mammals are covered in fur and give birth to live young but this is not strictly so. Mammals can be found in all climates and habitats of the world. Though most mammals live on land, many live in the sea (eg. Whales and dolphins) and one group, the bats, have mastered the skies just like the birds have.
Mammals have many unique features that have enabled them to dominate the world as they have.
Got milk?
The name, mammal, derives from a feature that all mammals share, the
mammary glands. These glands are present in female mammals and produce a nutrient rich substance (mostly fats, lactose sugars and protein) to feed their young. This milk is often more nutritious than the diet of the mother and so the mother often has to resort to taking nutrients from the tissues in her own body to produce food for the young.
The mammary glands are enlarged and modified sweat glands. In most mammals these glands are located in a swollen region called the
breast and the milk is secreted from a point called a
teat or
nipple. In the Monotremes (Duck-billed platypus and echidnas), however, the milk simply “sweats” through the skin in a particular region and is licked off the fur by the young.
Hairy beasts
All mammals have hair or really thick hair known as fur. Some adult mammals lose their hair (example whales and dolphins) but at some point in every mammals life they have hair. The hair of mammals is made from a protein called
keratin and it serves about four different functions.
- It slows the exchange of heat with the environment (insulation).
- It can have a sensory function like with whiskers.
- Different colours and patterns help with camouflage, warn away predators or to communicate to other animals.
- It also provides protection by simply providing an extra layer and the hairs in some species also take the form of spines such as in the porcupines.
Mammals descended from a group of reptiles called
therapsids. These have been termed as “mammal-like reptiles” and they lived on the planet around 260 million years ago. These reptiles had jaw bones that were similar to the mammals jaws of today, some may have had fur, been partially warm-blooded and some may have even produced milk.
However, the first true mammals arouse around 200 million years ago. These had fur, were warm-blooded but probably still laid eggs as the monotremes of today still do. The marsupials and placental mammals first appeared around 100 million years ago which gave rise to true live birth. During the reign of the dinosaurs, these animals were small rat-sized animals that were mostly nocturnal and spent most of their day underground.
Some orders of placental mammals appear to have arisen by around 70 million years ago - insectivores, rodents, ungulates, and possibly primates. But they were not able to expand into their modern ecological niches until after the extinction of the dinosaurs, 65 million years ago. After which, the ungulates became the major herbivores, primates became the arboreal (tree dwelling) insectivores and later the frugivores, the various carnivores, cetaceans (dolphins and whales) and maybe even bats were beginning to take hold of the various ecological niches available to them.
Throughout human history, mammals have played an important part in providing food and useful materials. At first, humans exploited them solely by hunting, but by about 14,000 years ago, humans started to domesticate certain species, controlling the way that they lived and bred. The earliest domesticated mammal was almost certainly the dog, which is descended from wolves. Domestication probably began when hunters adopted wolf cubs, and then used them as allies for scenting out game. Cats are thought to have been domesticated about 4500 years ago, and are descended from the African wild cat, an animal revered by the ancient Egyptians and widely valued for dealing with rodent pests.
Human hunters also discovered that herding mammals could be controlled. At first, they simply drove animals into places where natural boundaries such as cliffs and shorelines made them easier to attack, but over time, subtler techniques developed. People learned how to follow herds and manage them, first by traveling with them, and later by confining them in limited areas. Controlled breeding then produced distinct breeds, each with its own combination of useful characteristics. Horses, for example, were initially hunted for food, but later became valued as a means of transportation. Cattle were bred for meat, milk, and hides, while goats and sheep were raised for food and also for their wool.
Among the few non-hoofed mammals to have been domesticated, besides the dogs and cats, are the European rabbit and South American guinea pig, both of which provide meat, and the ferret. Descended from the European polecat, the ferret is sometimes kept as a pet, but was originally raised to drive rabbits from their burrows so they could be caught in hunters' nets.
Domesticated mammals now supply a large proportion of the meat humans eat and the fibers used for making clothes, although some wild animals are still hunted for food and for their fur. Until recently, mammals also supplied two other commodities—ivory and animal oils. Many kinds of mammals are also kept as pets, but some species are kept in captivity for other reasons, such as research.
In today's rapidly changing world, many wild mammals face an increasingly difficult struggle for survival. Data released in 1996 by the World Conservation Union (IUCN) indicates that of about 4600 species of mammal known to exist, over 1000 are classified as being at risk for extinction.
The number of mammal species becoming extinct has increased consistently over the last few hundred years. The
quagga, an African relative of the zebra, died out in 1883, following in the footsteps of the African bluebuck, which became extinct in 1800. During the preceding century, Steller's sea cow, an inhabitant of the Bering Sea, died out in 1768. This relative of today's manatees and dugongs could weigh over 5 metric tons, making it the largest animal to have become extinct in modern history.
Some of these animals vanished as a result of hunting, others through competition from other species. Several species of mammals have become extinct in the 20th century, including the Caribbean monk seal, which once inhabited the Bahamas as well as islands of the Caribbean, was last seen in Jamaica in 1952; at least four species of bats; and eight species of marsupials, including the thylacine, which disappeared when the last captive animal died in 1936.
Today hunting still poses a danger to some species, particularly animals such as elephants, rhinoceroses, and tigers, which fetch high prices on the black market for their tusks, horns, and other body parts. During the past 30 years alone, the population of black rhinos has dropped by over 95 percent, and the number of tigers has been reduced to about 5000. International efforts have helped to stem the trade in products from these animals, but without further intervention, their future as wild species remains in doubt.
For mammals as a whole, a more important threat is habitat change. Deforestation threatens a third of the world's primates, while urbanization, agriculture, and water pollution have had a serious effect on a wide range of mammals, from prairie dogs to aquatic species such as otters and river dolphins. In the drive to reverse this decline, some mammals have become the focus of preservation programs aimed at increasing stocks by habitat management, and also by captive breeding.
These programs have saved several species from almost certain extinction. Mammals rescued in this way include the giant panda, which has become a worldwide emblem of wildlife preservation, the white-throated wallaby, the
Arabian Oryx and also the North American black-footed ferret, which was once regarded as an agricultural pest. By the late 1970s, the black-footed ferret was believed to be extinct, but a small colony of survivors was discovered in 1981. Over 400 animals now live in captivity, and a number have been released into the wild.
After centuries of exploitation, some marine mammals have also been helped by recent protective legislation. The gray whale, for example, once seemed certain to become extinct, until hunting was banned by the International Whaling Convention in 1946. Since then, the numbers of gray whales in the Pacific Ocean have risen to about 20,000, which may be close to the population before hunting began. The Cape fur seal, which breeds on the rocky coasts of southern Africa, has shown an even more dramatic increase, growing from a few thousand in the 1930s, when it was first given protection, to over 1 million today. In the United States, the Endangered Species Act of 1973 prohibits federal projects from destroying the habitat of any endangered species.
