Introduction: Marsupials or Metatherians are from the group of mammals that are diversified in the regions of Australia, South and Central America. They have evolved with great adaptations in their body structure, living and habitat. They are termed placental mammals as they follow parental care strategies after giving birth to their babies.  

Hindi meaning of marsupials and definition: Marsupials possess an extra abdominal pouch which is used to fulfil the absence of a placenta. After giving birth to infant marsupials, the mother marsupial keeps their babies inside the pouch to nourish them for their complete development by giving them parental care for several months. 

Characteristics of Marsupials: 

• Marsupials are viviparous.
• Marsupials lack placenta. 
• They have external body pouches.
• The external body pouch is used by the mother to keep their babies for providing parental care. 
• The mother marsupial possesses mammary glands to feed their infant babies for several months. 
• They have a resemblance to marsupium. 
• Marsupials are mammals, but they possess some characteristics which make them unique in comparison to other mammals like pouches which are not found in other mammals.
• The pregnancy of marsupial mothers is short as compared to eutherian mothers. 
• Reproduction in marsupials is unique from other mammals as the female marsupial possesses two vaginas and two uteri; on the contrary, the male marsupial possesses a bifurcated penis that has two split sides. The penis lies just behind the testicles. 
• The infant babies of marsupials develop themselves for a short period of time usually for a few weeks inside the uterus of their mothers. For further development of their body, they reside inside the pouches of their mothers, which is a specialised skin fold. It possesses a firm attachment of nipples which provide them nourishment for their growth for a few months. 
• Just like other vertebrate mammals, marsupials do not have complex neural structures called corpus callosum in their brain. The corpus callosum has a simple network of nerve fibres that connect the two sides. As compared to placental mammals, the size of the marsupial brain is also small and the versatility to perform a particular function is less than placental mammals, indicating that they are less intelligent than placental mammals.  
• Marsupials reside in a wide range of habitats. They can be found in dry scrublands, temperate deciduous forests and tropical rainforests.  
• Marsupials are found in the different regions of North, South and Central America, Australia, New Zealand and Tasmania.  
• Most of the marsupials reside in warmer climates. They have low body temperature and have a slow metabolic rate compared to mammals possessing placenta. The internal body temperature of marsupials lies in the range of 5.4-5.6 degrees Fahrenheit, which is 3 degrees lower than the temperature of placental mammals. 
• The first development of mammals took place around 175-200 million years ago (m.y.a). The three types of living mammals include Monotremes, marsupials and eutherians (placental mammals), whose common ancestor lived around 166 million years ago (m.y.a.).

Examples of marsupials: 

• Wombat.  The scientific name of the wombat is Vombatidae. The body structure of wombats is small and they resemble bears of small size. They live in burrows. They have powerful claws with rodent-like front teeth. Their pouches face the backside of the body. They are herbivorous in nature and feed only on grasses, barks and roots. Their body colour can be sand-like, brown, black or grey. After a gestation period of 20-30 days, they give birth to their infants. A group of wombats is known as Mob, colony or wisdom. Their metabolism is slow which takes around 8-14 days to complete. Other species of Wombat include Lasiorhinus krefftii, Lasiorhinus latifrons and Vombatus ursinus. 
• Marsupial mouse. The scientific name of Marsupial mouse is  Sminthopsis; they resemble a field mouse having a long tail and feed on insects. They belong to phylum chordata. They reside in the Simpson and Gibson deserts of Australia. Their habitat includes open shrublands, farmlands and sparse grasslands. They are nocturnal. 
• Numbat. It is found in specific regions of Australia.  It is an insectivorous feeder marsupial and is diurnal. It belongs to the family of Myrmecophagidae. Recently, they have been reintroduced into South America and New South Wales. They breed in the months of February and March and produce a litter once a year. They have a gestation period of about 15 days. Female numbats do not possess pouches like other marsupials. They carry out the process of weaning for the complete development of infant numbats. 
• Flying squirrels. The scientific name of flying squirrels is Pteromyini or  Pteromyini; they resemble conventional squirrels in their morphological features. They belong to the family of Sciuridae. They are monophyletic, omnivorous and nocturnal in nature. When the infants are born, they feel helpless but they get parental care for about five weeks, after which they leave the nest for their further survival. Their life expectancy is about 6 to 15 years.  Snakes, owls and raccoons are the predators of flying squirrels. They breed in the months of February and March. 
• Rock-wallaby.  The scientific name of Rock-wallaby is Petrogale; they resemble kangaroos. They are a smaller species than kangaroos. They belong to the family of Macropodidae. The word Petrogale has been derived from two words Petr meaning Rock and Gale meaning Weasel.  They are nocturnal species. They prefer loose piles, cliffs and isolated rocks to live in. According to scientists, red foxes along with rabbits and goats are the reason for the extinction of the Rock Wallaby species.
• Marsupial mole. Notoryctes typhlops is the scientific name of the Marsupial mole, which resides in the undergrounds of Australia. They have white fur on their body. The species includes Notoryctes typhlops and Notoryctes caurinus.  They belong to the family of Notoryctidae. These animals are small in size and live in digs, due to which they are considered fossorial animals.  They are the best examples of convergent evolution. 

Conclusion:

Marsupials are mammals that carry their young ones in a pouch formed by a fold of skin near the stomach. The pouches contain the mammary glands. Australia has about 150 species of marsupials that vary in size, from the giant kangaroo over two meters in height to the tiny insect-eating bandicoot. The wallaby, wombat, and koala are other examples of marsupials found in Australia.