Maarg Manthan · Topic 6.12

Marine Mammals, Egg-Laying Mammals and Marsupials

Environment & Ecology › Biodiversity Basics and Classification · Topic 6.12

Mammals are classed by how they reproduce into three groups. Monotremes or egg-laying mammals, the platypus and echidnas, lay eggs but feed their young with milk. Marsupials such as kangaroo, koala and wombat give birth to underdeveloped young that complete growth in a pouch. Placental mammals nourish the foetus through a placenta, and include most species, from bats to whales. Marine mammals are mammals adapted to life in the sea: cetaceans (whales, dolphins, porpoises), sirenians (dugong and manatees), pinnipeds (seals, sea lions and walrus), sea otter and polar bear. They breathe air with lungs, give birth to live young and nurse them. India's marine mammals include the dugong, finless porpoise, Indo-Pacific humpback dolphin, bottlenose dolphin, blue whale, Bryde's whale and the Ganges and Indus river dolphins in fresh water.

5 min read · UPSC Prelims · Environment & Ecology · Biodiversity Basics and Classification

At a Glance

Egg-laying mammals Platypus, echidna
Pouched mammals Kangaroo, koala
Sea cow Dugong
Whales Cetaceans
Seals Pinnipeds
Exam link UPSC Prelims, Environment & Ecology: Biodiversity Basics and Classification

Where Does It Fit?

This is the last topic of the macro on biodiversity basics. It covers three special mammal groups often asked about in the exam, and sets up the species lists and protection regimes in the next macro. Previous topic: Plant and Animal Kingdom Classification. Next topic: Causes of Biodiversity Loss.

How Do the Three Special Mammal Groups Differ?

Group	Features, examples and range
Monotremes (Prototheria)	Lay leathery-shelled eggs, have a single opening (cloaca), no teats but secrete milk through skin patches. Platypus (Ornithorhynchus anatinus) in eastern Australia and Tasmania, echidnas (spiny anteaters) in Australia and New Guinea
Marsupials (Metatheria)	Short pregnancy; young born tiny and crawl to a pouch (marsupium) with teats. Mostly Australasian: kangaroo, wallaby, koala, wombat, Tasmanian devil. Americas: opossums
Placentals (Eutheria)	Young nourished in the womb through a placenta; the great majority of mammals
Cetaceans	Whales, dolphins and porpoises. Fully aquatic; blowhole; no hind limbs. Baleen whales filter krill (blue whale), toothed whales hunt (sperm whale, orca, dolphins)
Sirenians	Sea cows: dugong in the Indo-Pacific and three manatee species in the Atlantic. Herbivorous; feed on seagrass
Pinnipeds	Seals, sea lions, walrus; flippers; come ashore to breed
Other marine mammals	Sea otter and polar bear are classed marine because they depend on the sea

Which Marine Mammals Live in India and How Are They Protected?

Dugong (Dugong dugon): Palk Bay, Gulf of Mannar, Gulf of Kachchh and Andaman and Nicobar; Vulnerable on the IUCN Red List; Schedule I of the Wildlife Protection Act; Dugong Conservation Reserve in Palk Bay (2022)
Ganges river dolphin (Platanista gangetica): India’s national aquatic animal since 2009; Endangered; Vikramshila Gangetic Dolphin Sanctuary in Bihar; Project Dolphin launched 2020
Indus river dolphin: a subspecies found in the Beas river in Punjab
Irrawaddy dolphin: Chilika Lake and Sundarbans; Chilika has the largest population in India
Finless porpoise and humpback dolphin: coastal waters
Blue whale and Bryde’s whale: Indian Ocean waters; blue whale is largest animal ever
Sperm whale and killer whale: visit Indian seas
Legal protection: all cetaceans and the dugong are in Schedule I of the Wildlife Protection Act 1972

What Else Should You Remember?

Platypus: has a duck-like bill and venomous spurs in males
Echidna: has spines and a long snout
Kangaroo: the largest marsupial; hops
Koala: eats eucalyptus leaves
Wombat: burrowing marsupial with a backward-facing pouch
Tasmanian devil: carnivorous marsupial under threat from a facial tumour disease
Opossum: the only marsupial in North America
Marsupials outside Australia: South American opossums and shrew opossums
Why marsupials are in Australia: isolation after Gondwana broke up kept placental competitors out
Convergent evolution: marsupial wolf (thylacine) resembled a dog, now extinct
Thylacine: last known individual died in 1936
Blue whale: up to about 30 metres and 150 tonnes
Sperm whale: deep diver, source of spermaceti and ambergris
Ambergris: a waxy substance from sperm whales; trade is banned in India
Humpback whale: known for songs and long migrations
International Whaling Commission: set up in 1946; a commercial whaling moratorium began in 1986
Whaling: Japan withdrew from the IWC in 2019 and resumed commercial whaling in its waters
Dugong threats: fishing nets, loss of seagrass, boat strikes and hunting
Dolphin threats: dams, pollution, entanglement and prey loss
Project Dolphin: to conserve river and marine dolphins, announced in 2020
Echolocation: toothed whales and dolphins use sound to find prey
Convention on Migratory Species: lists many cetaceans and the dugong; India is a party
Link to previous: see Plant and Animal Kingdom Classification
Link to seagrass: see Sea-Grass and Seaweeds
Link to protected species: see Wildlife (Protection) Act, 1972 Highest protection Schedule I Old vermin list Schedule V Old plant list Schedule VI…">Scheduled Animals Under the Wildlife Protection Act
Caution: whales and dolphins are mammals, not fish; sea cow is not a cow; platypus is not a marsupial
Exam hint: learn each group with one animal and the Indian marine mammals with their status

Why Does It Matter for the Exam?

Groups: monotreme, marsupial, placental
Marine: cetacean, sirenian, pinniped
India: dugong, river dolphin
Current affairs link: Project Dolphin and Dugong Conservation Reserve
Common errors: calling platypus a marsupial or dolphins fish

As an analytical point, these groups show alternate paths of mammal evolution and are mostly threatened by human use of seas and rivers. India’s dolphin and dugong programmes show how species-focused action can protect whole aquatic habitats.

Why Is This Topic Important?

Groups Three
Marine Adapted to sea
India Dolphin, dugong

What Came Next?

The next macro covers threats to biodiversity and conservation strategies, starting with causes of biodiversity loss. Next topic: Causes of Biodiversity Loss.

Key People and Terms

Monotreme Egg-laying mammal
Marsupial Pouched mammal
Cetacean Whale or dolphin
Sirenian Sea cow
Pinniped Seal group
Cloaca Common opening

Exam Corner

Points to Remember

Platypus and echidna lay eggs.
Marsupials raise young in a pouch.
Dugong is a sirenian.
Ganges dolphin: national aquatic animal.
All cetaceans are in Schedule I.

Do Not Confuse With

Monotreme and marsupial Egg layers versus pouch carriers
Dugong and manatee Indo-Pacific versus Atlantic sea cows
Cetacean and pinniped Whales and dolphins versus seals

Memory Hook

Eggs from monotremes, pouch from marsupials, womb from the rest; whales, sea cows and seals in the sea.

Mains Angle

Questions ask for examples and Indian species. These points help.

Groups Three
Marine Types
India Protection

A Question You May Face

An original practice question, not a past paper question.

“Distinguish monotremes, marsupials and placental mammals, and discuss the status and conservation of marine mammals in India.”

How to Answer

Groups Three.
Marine Types.
India Species.
Protection Schedule I.

GS Relevance

Prelims: three mammal groups, examples, Indian marine mammals, protection status. GS Paper 3: wildlife.

Frequently Asked Questions

What are monotremes?

Monotremes are mammals that lay eggs instead of giving birth to live young. They have a single body opening, the cloaca, and feed their babies with milk secreted from patches on the skin as they lack true nipples. Only five species survive: the platypus and four kinds of echidna, all in Australia and New Guinea.

What are marsupials?

Marsupials are mammals whose young are born at a very early stage of development and complete growth in a pouch on the mother's body where the teats are. Most live in Australia and New Guinea, such as kangaroos, koalas and wombats, while opossums live in the Americas.

What are marine mammals?

Marine mammals are mammals that depend on the ocean for food and living, and include cetaceans like whales and dolphins, sirenians like the dugong and manatees, pinnipeds like seals and walrus, and also the sea otter and polar bear. They breathe air, give birth to live young and feed them milk.

Which marine mammals are found in India?

India has the dugong, finless porpoise, Indo-Pacific humpback and bottlenose dolphins, Irrawaddy dolphin in Chilika and the Sundarbans, blue and Bryde's whales and sperm whales in its waters. The Ganges river dolphin and the Indus river dolphin live in fresh water in northern rivers.

Why is the dugong called a sea cow?

The dugong is called a sea cow because it is a slow, gentle, herbivorous mammal that grazes on seagrass meadows in shallow waters, much like cattle on land. It belongs to the order Sirenia and is in danger due to loss of seagrass, fishing nets and hunting. In India it is in Schedule I.

PYQ Practice — Statement Analysis

1 The platypus is an egg-laying mammal.

True

A monotreme.

2 Marsupials are placental mammals.

False

They are pouched mammals.

3 The dugong is a sirenian that feeds on seagrass.