Advantages & Disadvantages of External Fertilization

By Lawrence Adams
Frogs and other amphibians use external fertilization.
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Animals use several biological processes that lead to sexual reproduction. Humans and most other land animals use an internal fertilization reproductive strategy in which the male deposits sperm directly inside the female. In external fertilization, on the other hand, the male deposits sperm into a body of water. The sperm travel until they reach an egg deposited into the water by a female.

Number of Gametes

Species that use internal reproduction produce relatively few gametes. Because the male directly deposits sperm into the female's body, fewer gametes are needed. External fertilization requires the male and female animals to produce larger numbers of gametes. Males must produce a large number of sperm to ensure that they travel throughout the body of water to meet an egg. Females also must deposit dozens or hundreds of eggs to ensure reproductive success. Producing a large quantity of gametes requires extra energy, which can be disadvantageous to an animal. However, the large number of gametes may result in a large generation of offspring, improving an organism's chance of passing on its genes.

Environment

External fertilization strategies require a body of water to be successful. Sperm have small tails that propel them through water; they would die on land. While depositing gametes in water does not pose a problem for fish, aquatic invertebrates and other animals that live in the water, it can be a disadvantage for other species. Amphibians and creatures that dwell on land must return to water to deposit their gametes.

Success of Fertilization

The close proximity of egg and sperm in internal fertilization increases the likelihood of a successful reproductive cycle. In external reproduction, animals disperse their gametes through a body of water. This dispersal reduces the likelihood that a sperm will find an egg. Many sperm and eggs die before achieving fertilization. The low success rate of external fertilization puts animals at a reproductive disadvantage compared to internal fertilization.

Animal Behavior

External fertilization strategies are behaviorally simpler than internal fertilization. A male and female may deposit their gametes at a slightly different time or place without jeopardizing their reproductive success. In contrast, animals using an internal fertilization strategy rely on hormones, mating rituals and behavioral factors to ensure that the male and female have sexual intercourse. The external fertilization strategy does not require these adaptations, making it a simpler reproductive strategy.

About the Author

Lawrence Adams' work has appeared in the "Marquette Literary Review" and "Broadview Press." He has a Bachelor of Arts from Marquette University in writing-intensity English and classical studies, with a minor in biology, and a Master of Arts in creative writing from the University of Illinois at Chicago.