NCERT Solutions for Class 9th Science Chapter 11 .3 Sexual Reproduction in Animals — Pause and Ponder

Book page 21811 Updated on2026-09-08

Q4.

Why do animals with external fertilisation generally produce more eggs than animals with internal fertilisation?

Answer

Because in external fertilisation the chance that any one egg is fertilised and grows up is very small, so the number of eggs must be large enough to make up for it.

expected number of surviving young = N × p
N = number of eggs laid, p = chance one egg is fertilised and survives

external (frog): N = 5,000 – 50,000 at a time, survival low
internal (bird): N = 1 – 15 at a time, survival moderate to high (Table 11.4)

Three separate losses drive p down when fertilisation is external:

The gametes are released into open water. Sperm are diluted the instant they leave the male, and currents sweep both sperm and eggs apart. Many eggs are never reached by a sperm at all.
The eggs are exposed. As the chapter says, many are destroyed by water currents or eaten by other animals. Nothing shelters them.
The young are on their own. There is no shell, no womb, no parent guarding them; the larva must find its own food from the moment it hatches.

Why the mother cannot simply give each egg more: there is a fixed budget. The chapter puts it plainly — the mother's body cannot provide large quantities of yolk for so many eggs. So the strategy is to put in just enough yolk to produce a larva, which then hatches and feeds itself on organic waste, grows, and finally transforms into the adult, as in the butterfly of Fig. 11.17 and in the frog. Internal fertilisation reverses the choice: very few eggs, but each with enough yolk (in a reptile or bird) or a direct supply from the mother's body (in a mammal) to reach a well-developed young one.

Tip: read it as two ways of spending the same amount. External fertilisers spend it on many cheap eggs; internal fertilisers spend it on few expensive ones. Neither is better — each suits its habitat.

Q5.

In animals, which fertilisation method the gametes are more protected?

Answer

Internal fertilisation. The gametes meet inside the female's body, where they are shielded from everything that destroys them in open water.

Risk to gametes	External fertilisation	Internal fertilisation
Being swept apart	High — currents carry sperm and eggs away from each other	None — sperm are delivered directly into the female tract
Dilution of sperm	Very high — sperm spread through a large volume of water	None — sperm stay concentrated in a narrow tract
Being eaten	High — eggs and larvae are food for many animals	Very low — the embryo is inside the mother or a shell
Drying and temperature change	Not a problem in water, but it confines the animal to water	None — body fluids stay warm and moist, so the animal can live on land
Number of eggs needed	Thousands (Table 11.4)	A few — lizard 2 – 20, bird 1 – 15

Why the protection changes everything downstream: because fertilisation is almost certain, the animal does not need to make thousands of eggs. Freed of that, it can put a large investment into each one — a big yolk and a shell in reptiles and birds, or development inside the uterus in mammals. That is why the survival column in Table 11.4 rises from "low" for fish and frog to "moderate" for lizard and "moderate to high" for bird. Internal fertilisation is also what allowed vertebrates to leave the water altogether: a frog must return to a pond to breed, a lizard need not.

Was this helpful? Report an error