Reproduction: How Life Continues · Lesson 6 of 12
Sexual Reproduction in Animals
“From fish to butterflies, nature has never settled on just one family plan.”
• Compare external and internal fertilisation. • Relate habitat and protection to the number of eggs produced. • Explain the role of yolk and the larval stage. • Describe the butterfly life cycle. • Connect parental care with survival of young animals.
A frog releases thousands of eggs into water, while a bird may lay only a few eggs in a nest. Both reproduce sexually, yet the numbers and protection differ greatly. Reproductive strategies balance the chance that gametes meet, the nourishment available to embryos and the care young ones receive.
In many aquatic animals, including frogs and most fish, the female releases eggs into water and the male releases sperm over them. Fusion occurs outside the body. Water makes the meeting possible, but currents, predators and changing conditions destroy many eggs.
External fertilisation is the fusion of male and female gametes outside the female body.
Reptiles, birds and mammals use internal fertilisation. Sperm and egg meet inside the female body, where gametes and the early embryo receive greater protection. Internal fertilisation does not make survival certain, but it generally reduces exposure during the earliest stages.
Internal fertilisation is the fusion of male and female gametes inside the female body.
Problem
Why can producing thousands of eggs be useful when fertilisation occurs in water?
- 1.Eggs and sperm are released into an uncontrolled environment.
- 2.Some gametes fail to meet.
- 3.Some fertilised eggs are swept away or eaten.
- 4.Producing many eggs increases the probability that at least some young survive.
Variations in Reproduction in Animals
| Animal | Habitat | Fertilisation | Approximate eggs | Estimated survival |
|---|---|---|---|---|
| Fish | Water | External | Hundreds to thousands | Low |
| Frog | Water and land | External | 5,000-50,000 | Low |
| Lizard | Land | Internal | 2-20 | Moderate |
| Bird | Water or land | Internal | 1-15 | Moderate to high |
The table shows a broad pattern rather than an absolute rule. Species producing many exposed eggs often have low survival per egg. Species producing fewer, better-protected eggs often have greater survival per young one. Habitat, fertilisation and care act together.
Egg yolk contains nutrients for a developing embryo. Fish, amphibians and insects may produce hundreds or thousands of eggs, so the mother cannot provide a very large store of yolk in each. The available yolk supports development to a larval stage. The larva then feeds independently and accumulates material needed for later transformation.
A butterfly begins as an egg, hatches into a larva, forms a pupa and emerges as an adult. A frog also has an intermediate larval stage. The young form is not simply a miniature adult; it is adapted mainly for feeding and growth before transformation.
Reptiles and birds usually place enough yolk in each egg to nourish the embryo until hatching. Mammalian zygotes develop inside the female body. After birth, mammals generally feed their young with milk for a period.
Problem
Why might a bird lay fewer eggs than a frog?
- 1.Bird fertilisation occurs internally.
- 2.Each egg contains enough yolk to support development until hatching.
- 3.Eggs may receive protection and care in a nest.
- 4.Greater investment in each egg is linked with producing fewer eggs and a higher average chance of survival.
Some young animals can move or find food soon after birth or hatching. Others depend on adults for feeding, warmth and protection. Extended care requires time and energy from adults but can improve learning and survival.
Quiz
Which description best matches External fertilisation?
Which description best matches Internal fertilisation?
Which term matches this description: External fertilisation is the fusion of male and female gametes outside the female body.
Which term matches this description: Internal fertilisation is the fusion of male and female gametes inside the female body.
Which statement is a key takeaway from this lesson?
Practice Problems
- Distinguish external and internal fertilisation by location.
- Why is external fertilisation common in many aquatic animals?
- What relationship does the table suggest between egg number and survival?
- What is the function of yolk?
- Why is a larva described as a feeding stage?
- Arrange the butterfly stages in order.
- Give one advantage and one cost of extended parental care.
Key Takeaways
• External fertilisation occurs outside the female body and often involves many gametes. • Internal fertilisation provides greater early protection. • Egg number, yolk, habitat and care influence survival. • A larva feeds and grows before transforming into the adult form. • Parental care can increase survival of young ones.