Internal & External Fertilization in the Animal Kingdom

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  • 0:00 Fertilization
  • 0:18 Internal Fertilization
  • 1:00 External Fertilization
  • 2:23 Pheromones
  • 3:02 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Although we tend to think of fertilization from the human perspective, there is actually more than one process of fertilization. There's the internal one that we already know, and a less familiar, external one.


In the animal kingdom animals reproduce by way of fertilization, the union of sperm and egg. That's pretty much the reason you are here, my friend. We are all familiar with this concept and the fact that this kind of fertilization, at least in humans, occurs internally. Of course, nature is quite varied, and you'll soon learn that fertilization can also occur externally.

Internal Fertilization

The more familiar form of fertilization in the animal kingdom is internal fertilization, a type of fertilization that occurs inside the female reproductive tract when sperm is deposited inside or near the female's reproductive tract. Again, this is the most famous type of fertilization since that's what humans use for reproduction. This kind of fertilization is actually a specific adaptation in the animal kingdom that allows sperm to be placed near an egg and reach it quite efficiently.

Another benefit to this kind of fertilization is that it can occur in a dry environment. To understand why this is important, let's explain and explore external fertilization.

External Fertilization

External fertilization is a kind of fertilization where eggs are released into the environment in order to be fertilized by the male. In this kind of fertilization, a moist, as opposed to dry, environment is necessary. This is true for two reasons: the first reason is that the gametes, which are just reproductive cells, the sperm and eggs, can dry out and become non-functional in a dry environment. The second reason is that a moist environment allows the sperm to move towards the eggs in order to fertilize them.

Basically, like you'd dry out in a desert and certainly wouldn't be able to swim, so with the sperm's struggle in a dry environment. That would be kind of bad for reproduction, wouldn't it?

External fertilization is also quite interesting in how it occurs. In many cases individuals of the same species found in the same area will release their gametes into the water simultaneously. This is known as spawning. In other instances this doesn't happen simultaneously. Instead, some individuals release their gametes after receiving chemical cues that others have already done so.

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