# Explain how internal fertilisation increases the likelihood of reproductive success in terrestrial animals compared with external fertilisation.

> IB DP Biology Standard Level (2023 syllabus) — D3.1 Reproduction · Explain · 4 marks

## Mark scheme (4 marks)

1. External fertilisation requires water as a medium for gametes to meet, which is unavailable or unreliable on land, whereas internal fertilisation occurs inside the body so does not depend on an aquatic environment.
2. Internal fertilisation protects gametes / the zygote from desiccation (drying out) within the reproductive tract of the female.
3. Fewer gametes need to be produced because the probability of a sperm reaching an egg is much higher / sperm are delivered directly close to the egg.
4. Internal fertilisation allows direct transfer of sperm into the female, enabling mating to occur independently of an aquatic habitat, so terrestrial species can colonise a wider range of environments / reproduce in dry conditions.

## Key terms

- [internal fertilisation](https://www.gradenine.co.uk/glossary/internal-fertilisation)
- [external fertilisation](https://www.gradenine.co.uk/glossary/external-fertilisation)
- [reproductive success](https://www.gradenine.co.uk/glossary/reproductive-success)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-internal-fertilisation-increases-the-0e723449) · Published by Druglandscape Ltd.