Difference between Internal and External Fertilization
By Theydiffer - June 14, 2018

Because they bear strong associations, the terms “internal fertilization” and “external fertilization” are frequently used interchangeably. However, they greatly differ in a number of ways.

Summary Table

Internal FertilizationExternal Fertilization
The sperm fertilizes the egg cell inside the female’s bodyThe sperm fertilizes the egg cell outside the female’s body
Common among reptiles, birds, mammals and some fishCommon among mollusks, crustaceans, fish and frogs
A reproductive process used by angiosperms, bryophytes, gymnosperms and pteridophytesA reproductive process used by most types of algae
Reproductive processes include copulation and fertilization via a cloacal kiss or spermatophoreReproductive process includes spawning
Not affected by external factors like water temperature, daylight hours, heavy rainfall and floodsAffected by external factors like water temperature, daylight hours, heavy rainfall and floods
Modes of expulsion are classified into three: oviparity, viviparity and ovoviviparityUses no mode of expulsion
Sexual selection is a part of mating and reproducingSexual selection is not a part of mating and reproducing
Lower chances for genetic diversityHigher chances for genetic diversity
Zygotes and embryos have a higher survival rateZygotes and embryos have a lower survival rate


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Internal fertilization is the process of introducing the sperm cell into the female reproductive organ by sexual intercourse or other methods. During reproduction, the egg cell and the sperm cell unite inside the female’s body.

Getty Images/Science Photo Library/SEBASTIAN KAULITZKI

External fertilization, on the other hand, is a type of fertilization that occurs outside the body of the female. To increase the sperm’s mobility, external fertilization takes place in a moist or wet environment.

Internal vs External Fertilization

Contrary to popular misconception, there is a major difference between internal and external fertilization.

Site of Reproduction

During internal fertilization, the sperm fertilizes the egg cell inside the female’s body. External fertilization, on the other hand, is the process of fertilizing the egg outside the female’s body. Since the sperm needs external motility, this type of fertilization occurs in a moist or aquatic environment.

Animal Specie

Internal fertilization is most common among reptiles, birds, mammals and some fish, while external fertilization is a reproductive process used by mollusks, crustaceans, fish and frogs.

Plant Species

External fertilization is used by most types of algae, whereas internal fertilization is common among angiosperms, bryophytes, gymnosperms and pteridophytes.


Internal fertilization commonly takes place after the process of copulation, which pertains to the insemination of sperm into the female reproductive tract by vaginal penetration. Some animals that do not possess a copulatory organ, however, perform internal fertilization via a cloacal kiss or spermatophore. Common among birds, a cloacal kiss is the process of transferring the sperm to the female by initiating contact between the male and female cloacae. Meanwhile, fertilization via spermatophore, which is common among insects and arachnida, refers to storing sperm on the roof of the female’s cloacae until oviposition.

Unlike internal fertilization, external fertilization mostly occurs by the process called spawning. To successfully reproduce, the sperm and egg cells are released into the water. Spawning is affected by factors like the duration of daylight hours and the temperature of the water. Similarly, environmental problems like flood and heavy rainfall can greatly affect chances for fertilization.

Modes of Expulsion

Since the eggs are fertilized outside the mother’s body during external fertilization, there is no mode of expulsion necessary for the offspring to survive. This is in contrary to internal fertilization, where mature eggs need to be expelled.

During internal fertilization, the modes of expulsion are classified into three: oviparity, viviparity and ovoviviparity. Oviparity, which is common among reptiles and insects, is the process of laying and hatching eggs, while viviparity pertains to giving birth to live offsprings. Lastly, ovoviviparity, which is normal among vipers and snakes, resembles the two other classifications. During the process, the mother lays eggs as they are hatched.

Sexual Selection

During internal fertilization, sexual selection is crucial for finding a mate and eventually inseminating sperm into the female reproductive organ. External fertilization, by contrast, has low-to-nil chances for sexual selection. Even though females can lay eggs near the site of their preferred male partner, there is no guarantee that their eggs will be fertilized by their targeted male. Additionally, factors like competition in the aquatic environment can significantly lessen chances for sexual selection.

Genetic Diversity

Since external fertilization occurs outside the female’s body, the egg can be fertilized by species with different genetic characteristics, consequently leading to diversity. The opposite is true for internal fertilization, where the egg cell is fertilized by a selected mate.

Survival Rate

As compared to embryos produced during external fertilization, the zygotes and embryos during internal fertilization have a higher survival rate since they are implanted inside the female’s body and are therefore protected from the harsh outdoor environment. Likewise, an offspring from external fertilization has a lower chance for survival due to lack of parental care.