Mammal any of the members of the class Mammalia including humans; generally regarded as the most advanced of the vertebrates. The word is derived from the milk-producing mammary glands unique to the class. Their large and complex brains allow for learning, quick reactions and flexible behaviour. Mammals, like birds, are 'warm-blooded' or homoeothermal animals that is, they differ from 'cold-blooded' or poikilothermal forms, such as fishes, amphibians and reptiles, in their superior ability to regulate their own body temperature.

The smallest group of mammals, the monotremes, includes a single order with just three living species, all of which are confined to Australasia- the duck-billed platypus and the echidnas. These are distinguished from all other mammals by the fact that they lay eggs, and are known as prototherians (subclass Prototheria). Although now generally accepted as true mammals, they retain a number of primitive features. Even though they are furry and feed their young on milk, they lay eggs that are structurally very similar to those of a bird, and like birds they incubate them in a nest or burrow. The prototherians do not have breasts or teats but have special glands that ooze milk, which the young lap up from the fur, rather than suckling in the normal way. All other mammals bear live young and are known as therians (sometimes considered as the subclass Theria), but reproductive differences within this group allow further division into two subgroups. The first of these, Metatheria, again includes just a single order, which comprises the marsupial ('pouched') mammals. These too are best known from Australia, but they have a significant representation in South America, principally through the opossum family. Although marsupials give birth to live young, the young are born at a very early stage of development. The tiny offspring typically makes its way from the birth canal to the pouch, where it attaches itself to a nipple and remains for a protracted period until capable of feeding itself.

The great majority of living mammals belong to the second subgroup of therians, the eutherians or placental mammals. The name of the group is derived from the placenta, a special organ by which the embryo is fed directly from the mother's blood supply and by which waste products are removed. The length of time that the young grow within the mother (the gestation period) varies considerably from species to species, as does the degree of development at birth, but no placental mammal produces offspring as undeveloped as that of a marsupial. Bangladesh harbours a great diversity of placental mammals. Some species are large and spectacular, like the tiger and elephant.

Worldwide, some 4,500 species of mammals are known today, of which roughly one-tenth occurs within the Indian subcontinent. In Bangladesh there are 118 species of inland mammals in 13 orders and 35 families, and three marine mammals in one order and one family. They range in size from tiny shrews and pipistrelle bats, which weigh only a few grams, and measure a few centimetres, to elephants that stand over 3 metres at the shoulder and can weigh over 4 metric tons.

The largest mammal, the Blue Whale, is nearly 30 metres in length and weighs up to 150 metric tons. This is undoubtedly an extraordinary situation that such a great diversity still exists in a country with a very limited range of habitats. However, the country has lost the following 10 species of mammals during the last 100 years: 1. One-horned Rhinoceros, Rhinoceros unicornis; 2. Javan Rhinoceros, Rhinoceros sondaicus; 3. Asiatic Two-horned Rhinoceros, Didermoceros sumatrensis; 4. Gaur, Bos gaurus; 5. Banteng, Bos banteng; 6. Wild Buffalo, Bubalus bubalis; 7. Nilgai, Boselaphus tragocamelus; 8. Swamp Deer, Cervus duvauceli; 9. Hog Deer, Axis porcinus; and 10. Wolf, Canis lupus. Of the existing 110 inland mammals, 40 have come under different categories of threats: 21 critically endangered, 13 endangered, and 6 vulnerable; 53 species could not be evaluated due to paucity of data. At the moment only 17 are not threatened. The ten marine species of mammals that are found in the Bay of Bengal are cetaceans (family Balaenopteridae; whales locally known as Timi) and are globally threatened; of these the Blue Whale/Great Blue Whale (Balaenoptera musculus) and the Fin Whale/Common Rorqual/Finback (Balaenoptera physalus) are endangered, and the Humpback Whale (Megaptera novaeangliae) is vulnerable.

Among the 118 inland mammals, the order Chiroptera (bats) is the largest group with 29 species in 8 families. This is the least studied group in the country; very little or no data are available on 24 species. Carnivores are the second largest group with 27 species in six families. All the carnivores face some kind of threats: nine are critically endangered, seven are endangered and five are vulnerable. Family Felidae includes all the wild cats including Bengal Tiger, Leopard, Clouded Leopard, Golden Cat, Leopard Cat, Jungle Cat and Fishing Cat. Once found almost all over the country, Sundarbans is now the stronghold of the Bengal Tiger. This species is now endangered all over the world. Of the 10 species of primates (all are now threatened), the lesser ape (Bunopithecus hoolock), a critically endangered species, is found in the forests of northeastern and southeastern parts of the country. Though in a threatened condition, Bangladesh has still three species (2 critically endangered, 1 endangered) of bear in the forests of Syllhet, Chittagong and the Chittagong Hill Tracts. About 200 elephants roam in the forests of the southeast. Of the seven species of cetaceans, Ganges River Dolphin is widely distributed in the major rivers of the country; others occur in the Sundarbans and in the coasts. [Md Anwarul Islam]