Explain the biological advantages of internal fertilisation over external fertilisation in animals.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Internal fertilisation protects gametes from desiccation, allowing reproduction in a dry environment.
Because sperm do not have to travel through water, fewer sperm are required, reducing the energy cost of gamete production.
The fertilised egg can be retained inside the female or protected by a shell, giving the embryo physical protection from predators and harsh conditions.
Thus internal fertilisation is essential for colonising land, where water cannot serve as a medium for sperm to reach the egg.
Because sperm do not have to travel through water, fewer sperm are required, reducing the energy cost of gamete production.
The fertilised egg can be retained inside the female or protected by a shell, giving the embryo physical protection from predators and harsh conditions.
Thus internal fertilisation is essential for colonising land, where water cannot serve as a medium for sperm to reach the egg.
Examiner tips
- Use the exact terms "internal fertilisation", "desiccation", "energetic cost", "physical protection", "colonisation of terrestrial habitats".
- Show the logical link between each advantage and the question.
- Keep each point concise – 1–2 sentences per point.
Common mistakes
- Mixing up internal and external fertilisation in the same point.
- Failing to mention the energy savings from producing fewer sperm.
- Using vague words like "better" instead of specific advantages such as protection from desiccation.
Mark scheme (4 marks)
- Internal fertilisation occurs within the body of the female, protecting gametes from desiccation / drying out in a terrestrial environment.
- Fewer gametes (especially sperm) need to be produced because the probability of a sperm meeting an egg is much higher than in an aquatic / open environment, reducing the energetic cost to the organism.
- The fertilised egg / zygote / embryo can be retained within the female body (or protected by a shell / egg case), providing physical protection against predation and environmental hazards.
- Internal fertilisation is essential for colonisation of terrestrial habitats because water is not required as a medium for sperm to swim to the egg.
Key terms in this question
internal fertilisation · external fertilisation
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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