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💡 Many eggs or much care?
Some animals gamble on thousands of eggs and little care; others invest in a few young and protect them fiercely. Both strategies keep the species going.
Why do some animals produce a very large number of eggs?
- Fish, amphibians and insects lay hundreds to thousands of eggs with only a little yolk .
- The yolk is just enough to make a larva that then feeds itself and grows before transforming into an adult (e.g. butterfly, Fig. 11.17).
- Reptiles and birds lay fewer eggs with plenty of yolk ; in mammals the zygote grows inside the mother’s body — so fewer young, but higher survival.
| Animal | Habitat | Mode of fertilisation | Number of eggs produced | Estimated survival of young ones |
|---|---|---|---|---|
| Fish | Water | External | 100s–1000s at a time | Low |
| Frog | Water/land | External | 5,000–50,000 at a time | Low |
| Lizard | Land | Internal | 2–20 at a time | Moderate |
| Bird | Water/land | Internal | 1–15 at a time | Moderate to High |
⏸▶ Pause and Ponder
- 4. Animals with external fertilisation produce more eggs because many are destroyed by water currents or eaten, so extra numbers make up for the heavy losses.
- 5. Gametes are more protected in internal fertilisation , as fertilisation and early development happen inside the female’s body.
✅ Test Yourself
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Where does external fertilisation take place?
Show Answer
Outside the body, usually in water — as in frogs and most fish. -
Why do frogs and fish lay so many eggs?
Show Answer
Many are swept away by currents or eaten, so survival of each egg is low. -
Why do internally fertilised animals lay fewer eggs?
Show Answer
The egg or embryo is better protected, so the chance of survival is higher. -
What is the feeding stage between egg and adult in a butterfly?
Show Answer
The larva , which later becomes a pupa and then the adult.
Important Definitions
- External fertilisation — fertilisation that occurs outside the body, usually in water, as in fish and frogs.
- Internal fertilisation — fertilisation that occurs inside the female’s body, as in reptiles, birds and mammals.
- Yolk — the nutritive material in an egg that nourishes the developing embryo or larva.
