Features of Mammals:

Characteristics of class Mammalia given below:

• Mammary glands in females for milk production for newborn. This milk is made by modified sweat glands known as mammary glands. It is from the glands that whole group takes its name, Mammals.
• Hairy skin with keratin (for insulation) having sebaceous and sweat glands. All mammals contain some hair at least at the starting of their lives - baby whales and dolphins are born with moustache.
• Single jaw bone on either side. In all other vertebrates, there is more than one bone on each side of jaw.
• Four-chambered heart with main artery leaving heart curves to left becoming aortic arch.
• Muscular diaphragm separates thoracic cavity from abdominal cavity.
• Warm-blooded (endothermic/homoeothermic - heat energy made from within to preserve constant high body temperature)
• Seven cervical vertebrae (neck bones) are there in most mammals
• Teeth (where present) are imbedded in jaw and in variety of forms for different functions (heterodontic).
• Mammals are separated in 3 main categories depending on how they are born. These categories are monotremes, marsupials, and placentals.

Except for 5 species of monotremes (that lay eggs), all mammal species give birth to live young. Most mammals also have specialized teeth, and largest group of mammals that is placentals, use placenta during gestation. Mammalian brain manages endothermic and circulatory systems, comprising four-chambered heart. Mammals contain a buccal cavity (the mouth cavity) with false palate as roof that means that nostrils don't lead directly in mouth. Body temperature of mammals is maintained at the constant level, meaning that mammals are endothermic that is they make heat within their bodies metabolically and also contain special cooling mechanisms.

Mammals are separated in 2 subclasses:

i) Subclass Prototheria: (monotremes: platypuses and echidnas)

Members of this subclass are extinct except order Monotremata that is represented by duck-billed platypus (flat footed) and echidnas - spiny anteaters. This group of mammals can be explained as having transited from reptiles as they show some reptilian characteristics as highlight below.

The monotremes contain the given characteristics:

• Shelled eggs like reptiles
• Single opening -cloaca (like reptiles) for both urine/faeces discharge and copulation
• Testes are not in scrotum but retained in body cavity
• Penis lies within cloaca and though homeothermic, their body temperature is low, from 30-33°C, so also is basal metabolic rate
• Mammary glands (though not truly mammary glands), which produce fatty sweat (milk) from glands in skin. Milk gathers and drips down tufts of hair in offspring's mouth.

Monotremes are found in Australia and Papua New Guinea. Animal keeps eggs and emerged young warm by curling around them. After hatching from eggs, platypus babies feed on milk from the mother. Spiny anteater lives on land and lays single egg in temporary pouch formed on female's belly. Egg has leathery shell. Egg hatches in 10 days and baby echidna is born blind and hairless. It gets milk from gland within mother's pouch. Body of spiny anteaters, with exception of underside, face and legs, is covered with cream colored spines.

ii) Subclass Theria:

(Live-bearing mammals) Members of this subclass, unlike monotremes, don't lay eggs but give birth to live babies. This signifies that there is period of development for embryo in mother before birth. Therians are further divided in two infraclasses like - Metatheria and Eutheria. Metatherians i.e. marsupials have pouch (marsupium) that is built-in baby carrier, where embryo completes its development.

Young of metatherians is called joey and is born in early phase of development (just after a month) similar to premature birth. Under this situation, joey can't survive on their own and so should be kept in pouch (marsupium - from which group derives its name) located on mother's belly. Members comprise opossums, kangaroos, koalas and possums. The newborn kangaroo stays in mother's pouch for about 6 months, where it feeds on milk. Koala babies are born with eyes closed and contain no ears or fur (body hair) so they stay inside the pouch on mother's abdomen for about 7 months while they grow fur and their eyes and ears grow. Kangaroos are found in Australia and Papua New Guinea. Opossums are marsupials dwell in North and South America.

Opossums (Didelphimorphia) make up largest order of marsupials in Western Hemisphere. Word opossum means white dog or white beast/ animal. Koala has large, sharp claws utilized in climbing tree trunks. Opossum is about size of the large house cat. It contains triangular head and long pointed nose. It contains grayish fur everywhere but on its ears, feet and tail. Its tail is prehensile. The prehensile tail is adapted for grasping and wrapping around things such as tree limbs.

Infraclass Eutheria (placentals): This group of mammals, also called as placentals, is characterized by having placenta - reproductive structure that is housed in uterus of female by which developing embryo connects to mother to get nutritive support. Offspring of eutherians gets all its nutritional requirements by placenta which links it to mother to develop in completely developed organism before birth. Period of development (gestation period) differs with species of mammal but certainly longer than in metatherians. For instance, gestation period in whale is 2 years, in mouse 21 days, and in humans nine months. These mammals are most abundant and diverse of class representing 94% of species of mammals.

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