Which of the following mammals lays eggs ?

Question

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Correct — D, Echidna. The echidna, or spiny anteater, is a monotreme — one of only two surviving groups of egg-laying mammals, the other being the platypus. The female lays a single small, leathery-shelled egg, which she transfers to a temporary pouch-like fold on her belly; the young hatches after roughly ten days and then feeds on milk that oozes from patches of skin, because monotremes secrete milk but have no nipples. Everything else about the echidna is mammalian: it is warm-blooded, it has hair modified into spines, and it suckles its young. Egg-laying does not disqualify it from being a mammal — it makes it the living reminder that mammals evolved from egg-laying ancestors, and monotremes are the branch that never abandoned the egg.

Why the other options are wrong — (a) The loris is a primate — a slow-moving, large-eyed, nocturnal one — and primates are placental mammals that give birth to live young. India has both the grey slender loris in the south and the Bengal slow loris in the north-east, which is why the name is familiar to Indian candidates, but nothing about its reproduction is unusual.  (b) The best trap on the list for a candidate who half-remembers the biology. The kangaroo is a MARSUPIAL, not a monotreme: it gives birth to live young, but the newborn is extraordinarily undeveloped — a few centimetres long — and crawls to the mother's pouch to complete its growth attached to a teat. Because the pouch is the memorable feature, and because Australia is the memorable place, kangaroo and echidna get conflated. The distinction to hold is exact: marsupials give live birth and finish the job in a pouch; monotremes lay an egg.  (c) The best trap on the list for a candidate who reasons from appearance. The hedgehog is covered in spines, rolls into a ball when threatened and eats insects — all of which the echidna also does — yet it is an ordinary placental mammal that gives birth to live young. The resemblance is convergent evolution: two unrelated insect-eating mammals arrived independently at the same spiny defence. Spines are not evidence of egg-laying.

Concept: Living mammals fall into three groups defined by how they reproduce. Monotremes lay eggs and are represented today only by the platypus and the echidnas, all confined to Australia and New Guinea; the name means 'single opening', from the cloaca they share with reptiles and birds. Marsupials, such as kangaroos, koalas and opossums, give birth to extremely immature live young that complete development attached to a teat, usually inside a pouch. Placental or eutherian mammals — the overwhelming majority, including humans, the loris and the hedgehog — nourish the embryo through a placenta and give birth to relatively well-developed young. What all three share, and what actually defines a mammal, is hair and the production of milk from mammary glands, not the manner of birth.

State PCS 2022 Q55 — Science & Technology PYQ with Answer

State PCS 2022 Q55 — Science & Technology PYQ with Answer