smc-3647-15-Internal vs. External

BIOLOGY, M5 2025 HSC 27

The table shows the number of eggs produced by females in TWO different groups of animals.

\(\textit{Group A}\)		\(\textit{Group B}\)
\(\quad\textit{Animal}\quad\)	\(\quad\textit{Number of Eggs}\quad\)	\(\quad\textit{Animal}\quad\)	\(\quad\textit{Number of Eggs}\quad\)
\(\text{Monotremes}\quad\)	\(1-3\)	\(\text{Crabs}\)	\(1000-2000\)
\(\text{Snakes}\)	\(1-100\)	\(\text{Sea Urchins}\quad\)	\(100\, 000\ \text{to 2 million}\)
\(\text{Birds}\)	\(1-17\)	\(\text{Squid}\)	\(2000-3000\)

Compare the types of fertilisation that occur in group A and group B animals with reference to the data provided. (4 marks)

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Group A Animals – Internal Fertilisation

Group A animals (monotremes, snakes, birds) use internal fertilisation where sperm meets egg inside the female’s body.
The data shows Group A produces few eggs (1-100), as the protected internal environment increases fertilisation success and offspring survival.
These animals typically provide greater parental care, further improving survival rates.

Group B Animals – External Fertilisation

Group B animals (crabs, sea urchins, squid) use external fertilisation where gametes are released into water.
The data shows Group B produces vastly more eggs (1,000-2 million) to compensate for low survival rates.
External fertilisation exposes gametes to predation, dilution and environmental factors, reducing fertilisation success.
These organisms provide minimal parental care, relying on high egg numbers for species continuity.

Comparison

The dramatic difference in egg numbers (1-100 vs 1,000-2,000,000) directly reflects the fertilisation strategy.

Internal fertilisation prioritises quality through protection and parental care, while external fertilisation relies on quantity to overcome environmental challenges.

Show Worked Solution

Group A Animals – Internal Fertilisation

Group A animals (monotremes, snakes, birds) use internal fertilisation where sperm meets egg inside the female’s body.
The data shows Group A produces few eggs (1-100), as the protected internal environment increases fertilisation success and offspring survival.
These animals typically provide greater parental care, further improving survival rates.

Group B Animals – External Fertilisation

Group B animals (crabs, sea urchins, squid) use external fertilisation where gametes are released into water.
The data shows Group B produces vastly more eggs (1,000-2 million) to compensate for low survival rates.
External fertilisation exposes gametes to predation, dilution and environmental factors, reducing fertilisation success.
These organisms provide minimal parental care, relying on high egg numbers for species continuity.

Comparison

The dramatic difference in egg numbers (1-100 vs 1,000-2,000,000) directly reflects the fertilisation strategy.

Internal fertilisation prioritises quality through protection and parental care, while external fertilisation relies on quantity to overcome environmental challenges.

BIOLOGY, M5 2023 HSC 2 MC

Which of the following is an advantage of internal fertilisation?

Decreases the risk of gamete dehydration
Increases the number of gametes released
Increases the number of zygotes at one time
Decreases the care provided to gamete and offspring

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\(A\)

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Internal fertilisation occurs in a moist environment, decreasing the risk of gamete dehydration.

\(\Rightarrow A\)

BIOLOGY, M5 EQ-Bank 27

Explain why internal fertilisation is a more robust process than external fertilisation in maintaining a species generation after generation. (5 marks)

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In order for a species to successfully continue and avoid extinction, a sufficient number of offspring must be able to survive until gestational maturity and produce offspring themselves, hence continuing the cycle.
External fertilisation is used mainly in sea animals such as fish where the survival rate of offspring is low due to the plentiful numbers of predators in such a vast environment.
In response to this environment, external fertilisation methods must produce large numbers of sperm and eggs to increase the number of fertilised embryos. This process requires of a significant amount of energy.
Fertilised embryos typically experience minimal or no parental care, further lowering the survival rate of offspring. Therefore, external fertilisation has numerous disadvantages to overcome in achieving the continuity of the species.
Internal fertilisation is generally more common amongst varying species (such as humans). Despite often only producing one child at a time, internal fertilisation as a process protects the developing embryo inside the female uterus.
Although the number of fertilised embryos is lower, greater parental care throughout most of early life and a significant decrease in predation creates a higher survival rate.
In this way, internal fertilisation is more advantageous to ensure the continuity of a species.

Show Worked Solution

In order for a species to successfully continue and avoid extinction, a sufficient number of offspring must be able to survive until gestational maturity and produce offspring themselves, hence continuing the cycle.
External fertilisation is used mainly in sea animals such as fish where the survival rate of offspring is low due to the plentiful numbers of predators in such a vast environment.
In response to this environment, external fertilisation methods must produce large numbers of sperm and eggs to increase the number of fertilised embryos. This process requires of a significant amount of energy.
Fertilised embryos typically experience minimal or no parental care, further lowering the survival rate of offspring. Therefore, external fertilisation has numerous disadvantages to overcome in achieving the continuity of the species.
Internal fertilisation is generally more common amongst varying species (such as humans). Despite often only producing one child at a time, internal fertilisation as a process protects the developing embryo inside the female uterus.
Although the number of fertilised embryos is lower, greater parental care throughout most of early life and a significant decrease in predation creates a higher survival rate.
In this way, internal fertilisation is more advantageous to ensure the continuity of a species.

BIOLOGY, M5 2019 HSC 9 MC

Which of the following is an advantage for animals using internal fertilisation rather than external fertilisation?

It prevents dehydration of gametes.
It involves large numbers of gametes.
It relies on adaptations such as mating rituals.
It allows gametes to combine to form unique offspring.

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`A`

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The damp environment utilised in internal fertilisation prevents dehydration of the gametes, which would lead to the death of the cells.

`=>A`

Mean mark 57%.

Students tested the hypothesis that the number of eggs/young produced was greater in animals using external fertilisation than those using internal fertilisation. They obtained the following data from secondary sources.

What conclusion can be drawn from the data? Justify your answer. (3 marks)

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Justify an improvement to the students' experimental design to test the same hypothesis. (2 marks)

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Explain ONE advantage for animals of using external fertilisation. (2 marks)

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a. Conclusion: There is little difference

The numbers of eggs laid/young born between different species was similar in both external and internal animals (43, 40).
The standard deviation (variability of the data) was significantly high (55, 32).
These two factors mean that students may conclude there is little to no difference between the number of young produced and the mode of fertilisation.

b. Potential adjustments to improve design

Students have selected only twelve species to use in their investigation, which has led to similar results in both internal and external fertilisation.
A much larger number of species should be included in a follow up investigation to increase the likelihood of yielding more accurate results.

c. Successful answers should include one of the following:

External fertilisation will expend less energy on gestation, as it occurs outside the body.
The large number of young produced in external fertilisation will ensure continuity of the species with minimal parental guidance.

Show Worked Solution

a. Conclusion: There is little difference

The numbers of eggs laid/young born between different species was similar in both external and internal animals (43, 40).
The standard deviation (variability of the data) was significantly high (55, 32).
These two factors mean that students may conclude there is little to no difference between the number of young produced and the mode of fertilisation.

Mean mark (a) 51%.

b. Potential adjustments to improve design

Students have selected only twelve species to use in their investigation, which has led to similar results in both internal and external fertilisation.
A much larger number of species should be included in a follow up investigation to increase the likelihood of yielding more accurate results.

♦♦ Mean mark (b) 38%.

c. Successful answers should include one of the following:

External fertilisation will expend less energy on gestation, as it occurs outside the body.
The large number of young produced in external fertilisation will ensure continuity of the species with minimal parental guidance.