Question
Differentiate between internal fertilisation and external fertilisation with suitable examples for each.
Solution — Step by Step
Fertilisation is the fusion of the male gamete (sperm) with the female gamete (egg/ovum) to form a zygote. The site where this fusion occurs is the key difference between internal and external fertilisation.
In external fertilisation, the fusion of sperm and egg occurs outside the body of the female, typically in water or a moist environment.
How it happens:
- The female releases eggs into water
- The male releases sperm over the eggs
- Fertilisation occurs in the external medium
Examples:
- Fish (most species): Female goldfish or rohu releases eggs into water; male releases milt (sperm) over them
- Frogs and toads: Female lays eggs in water (spawn); male clasps female (amplexus) and releases sperm simultaneously
- Sea urchins and starfish: Release gametes directly into seawater
Features:
- Large number of eggs produced (millions) to compensate for huge losses
- No parental protection usually; eggs are vulnerable to predation, desiccation, UV damage
- Requires aquatic or very moist environment
- Offspring survival rate is low
In internal fertilisation, the fusion of sperm and egg occurs inside the body of the female, within the reproductive tract.
How it happens:
- The male introduces sperm directly into the female’s body (through copulation or equivalent)
- Fertilisation occurs in the female’s reproductive organ (e.g., fallopian tube in humans)
Examples:
- Humans and other mammals: Sperm travel through the uterus to fertilise the egg in the fallopian tube
- Birds: Internal fertilisation occurs before the hard shell is formed around the egg
- Reptiles (lizards, snakes): Fertilisation inside the female’s body
- Many insects: Butterflies, bees, mosquitoes
Features:
- Far fewer eggs produced (10s to 100s)
- High degree of parental protection (internal development in mammals)
- Offspring survival rate is much higher
- Does not require an aquatic environment
| Feature | External Fertilisation | Internal Fertilisation |
|---|---|---|
| Site of fusion | Outside female body | Inside female body |
| Environment needed | Water/moist medium | Not necessarily aquatic |
| Number of eggs | Very large (millions) | Smaller (few to hundreds) |
| Parental care | Usually absent | Usually present |
| Survival rate of offspring | Low | High |
| Examples | Frog, fish, sea urchin | Human, dog, birds, reptiles |
Why This Works
The trade-off is clear: external fertilisation releases enormous numbers of gametes to compensate for the low probability of any single sperm meeting an egg, and the high risk of egg loss to predators and environment. Internal fertilisation solves these problems by bringing the gametes together in a controlled, protected environment, so fewer gametes and eggs are needed.
This is an example of r-strategy vs K-strategy in evolutionary biology — external fertilisers are often r-strategists (many offspring, low investment), while internal fertilisers tend to be K-strategists (few offspring, high investment).
Common Mistake
Students often say “birds have external fertilisation because they lay eggs.” This is wrong. Birds have internal fertilisation — sperm fertilises the egg inside the female’s body, and then the fertilised egg is surrounded by the shell and laid. The act of laying eggs (oviparity) is separate from whether fertilisation was internal or external. The key question is WHERE the sperm meets the egg, not where the egg develops.