Internal and External Fertilisation

HSC Biology | Free Study Notes 

In this lesson

• the key differences between internal and external fertilisation

• animal examples of each

• the survival implications of each method

• why each method suits different environments

What is fertilisation?

Fertilisation is the fusion of male and female gametes to form a zygote.

In animals, fertilisation can happen in two main ways:

• internal fertilisation

• external fertilisation

Internal fertilisation

Internal fertilisation happens when the fusion of gametes occurs inside the female body.

Key features of internal fertilisation

• fertilisation takes place inside the body

• fewer gametes are usually produced

• there is a higher chance of fertilisation

• gametes are protected from drying out

• common in terrestrial animals

Why internal fertilisation is useful

Internal fertilisation is especially useful on land because gametes are less likely to dehydrate.

External fertilisation

External fertilisation happens when the fusion of gametes occurs outside the body.

Key features of external fertilisation

• fertilisation happens in the external environment

• many gametes are usually released

• lower chance of each gamete being fertilised

• usually requires a moist or aquatic environment

• common in many aquatic animals

Why external fertilisation is useful

External fertilisation is well suited to aquatic environments because water prevents gametes from drying out and allows sperm to swim to the egg.  

Key differences

Feature	Internal fertilisation	External fertilisation
Where fertilisation happens	Inside the female body	Outside the body
Number of gametes	Usually fewer	Usually many more
Environment needed	Common in terrestrial animals	Usually aquatic or moist environments
Chance of fertilisation	Higher	Lower
Protection of gametes	Greater	Less

Animal examples

Examples of internal fertilisation

Common examples include:

• mammals

• reptiles

• birds

• many sharks

HSC materials specifically describe terrestrial animals as examples of internal fertilisation and even note snakes as producing relatively few eggs under this pattern. 

Examples of external fertilisation

Common examples include:

• many fish

• many frogs

• sea urchins

HSC materials specifically refer to aquatic animals with external fertilisation, including sea urchins, which may release very large numbers of gametes. 

Survival implications

Internal fertilisation and survival

Internal fertilisation has several survival advantages:

• gametes are protected from dehydration

• there is a higher probability of fertilisation

• fewer gametes are wasted

• fewer offspring may be produced, but each often has a better chance of survival

HSC marking guidance also notes that internal fertilisation leads to higher rates of survival of the fewer offspring produced. 

External fertilisation and survival

External fertilisation has different survival implications:

• many gametes must be produced because the chance of fertilisation is lower

• many embryos or eggs may be lost to predators or environmental conditions

• it may require less energy in gestation, because development occurs outside the body

HSC marking guidance notes one advantage of external fertilisation is that animals may expend less energy on gestation, as this occurs outside the body. 

Why each method suits different animals

Neither method is simply “better” in all situations.

Internal fertilisation suits:

• terrestrial life

• animals producing fewer offspring with greater protection

• environments where water is not available for gamete transfer

External fertilisation suits:

• aquatic life

• animals producing many gametes

• situations where releasing eggs and sperm into water is effective

Worked example

Exam-style question

Explain one advantage of internal fertilisation and one advantage of external fertilisation.

Worked answer

One advantage of internal fertilisation is that the gametes are protected from dehydration and the probability of fertilisation is higher. One advantage of external fertilisation is that it can occur with less energy spent on gestation, because development happens outside the body.

Why this works

This answer:

• gives one clear advantage of each type

• links each advantage to survival or reproductive success

• uses accurate biological language

Common mistakes

• Saying external fertilisation usually happens on dry land.

• Forgetting that internal fertilisation protects gametes from dehydration.

• Saying internal fertilisation always produces more offspring.

• Mixing up fertilisation with gestation or development.

• Giving examples without explaining why the method suits that organism.

Quick quiz

1. What is internal fertilisation?

2. What is external fertilisation?

3. Why is internal fertilisation common in terrestrial animals?

4. Why are many gametes usually produced in external fertilisation?

5. Give two animal examples of internal fertilisation and two of external fertilisation.

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