Internal Fertilization

Internal fertilization is a reproduction process that occurs inside the body of the female organism. This type of fertilization is mostly seen in land animals, bony fishes, birds, reptiles, and mammals. For internal fertilization to take place the male sperm has to fuse with the female ovary in the female reproductive tract.

In this article, we will read about the process of internal fertilization, the types and examples of internal fertilization, the difference between internal and external fertilization, and the advantages and disadvantages of internal fertilization.

What is Internal Fertilization?

Internal Fertilization takes place when the male’s sperm is deposited inside the female reproductive system where the egg is fertilized. Different animals have different methods of internal fertilization, in birds it is done by cloacal kiss while in mammals, reptiles, and certain fishes it is done by copulation. Most plants (bryophytes, pteridophytes, gymnosperms, and angiosperms) and higher animals (reptiles, birds, mammals, and terrestrial fungi) undergo internal fertilization.

How does Internal Fertilization work?

Internal fertilization is a complex process that is integral to the reproductive success of many animal species. There are various stages in this process:

Mating and Copulation

Initiation of Mating: The mating process begins with signals that can be visual, chemical (pheromones), auditory, or a combination of the three. The signals play an important role in attracting and selecting potential mates. Many species also engage in courtship behaviour like dance and displaying physical or acoustic demonstrations to attract the mate.

Copulation: During this process, the male sperm is transferred to the female which is done by special reproductive organs like a penis in mammals or an aedeagus in many insects. The copulatory mechanism varies among different species. In mammals, it involves physical penetration whereas in insects and birds, it involves a brief connection for the transfer of the sperm.

Sperm Transportation

After the sperm is deposited into the female reproductive tract it must travel to meet the egg. This process is complex and only a few sperm reach the egg to fertilize it. In mammals, the sperm travels through the cervix into the uterus and reaches the fallopian tube where the eggs are fertilized. In birds, the sperms are stored in special chambers before fertilizing the egg.

• Fertilization: Once the sperm reaches the egg it penetrates the outer layer of the egg and then fertilizes it for this process to take place several complex biochemical and mechanical interactions have to take place.
• Zygote formation: After fertilization, the genetic material combines to form a zygote and it undergoes cellular division to develop into an organism. In mammals, the zygote gets implanted in the uterus and it develops. In oviparous organisms, the zygote develops inside the egg and later hatches after incubation.

Types of Internal Fertilization

There are different types of internal fertilization among different organisms that are adapted according to the environment. There are three major types of internal fertilization:

Oviparous

In this process, the fertilization of the sperm and egg takes place inside the female’s body but the development of the embryo takes place in the outside environment when the female lays the egg. The eggs are encased inside a shell and are incubated until it is hatched. This type of fertilization takes place in birds and reptiles.

Viviparous

Viviparous species give birth to young ones and in this type of internal fertilization, the embryo develops inside the female’s body. This involves the placenta which is a special organ that facilitates nutrients and gas exchange between the mother and the developing embryo. Most mammals, including humans and some species of shark exhibit viviparity giving birth to live young ones.

Ovoviviparous

This type is a blend of oviparous and viviparous type of fertilization. Fertilization is internal and the embryo develops inside the female’s body while encased in an egg. In this type, the embryo gets its nutrition from the egg yolk and not from the mother. Certain types of fish like guppies and a few snakes and lizards exhibit this type of fertilization.

Internal Fertilization Examples

Some examples of organisms that undergo internal fertilization are:

Internal Fertilization Fish

When a fish undergoes internal fertilisation, the male inserts sperm into the female’s vaginal entrance via an intermittent organ. Only 500 of the more than 33,000 species of bony fish and basal cartilaginous fish exhibit this mechanism, making fish comparatively rare in general.

Internal Fertilization Mammals

All mammals like humans, dogs, elephants, and whales use internal fertilization. The mammalian reproduction involves the direct deposit of sperm into the female reproductive system during copulation and the fertilization occurs within the uterus or oviducts.

Internal Fertilization Reptiles

Reptiles like lizards, turtles, and snakes use internal fertilization, where the males of these species have hemipenis or a penis that transfers teh sperm into the female reproductive tract.

Internal Fertilization Amphibians

Most amphibians rely on external fertilization some of them like salamanders and caecilians use internal fertilization. The salamanders use spermatophores which are picked up by the females in the cloacal region to fertilize the eggs internally.

Internal Fertilization Birds

Most of the birds engage in internal fertilization but the method differs from mammals as the birds lack external genitalia. The birds perform a “cloacal kiss” where the male and female cloacal touch and allow the transfer of sperm.

Difference between External Fertilization and Internal Fertilization

Internal fertilization vs. external fertilization is given below:

Aspect	External Fertilization	Internal Fertilization
Definition	Fertilization occurs outside the female’s body.	Fertilization occurs inside the female’s body.
Environment	Occurs in the aquatic environment.	Usually occurs in terrestrial animals and some aquatic species.
Egg Protection	The eggs are exposed to the environment, predators, and v ariable condition.	The eggs are protected inside the female’s body by special organs.
Sperm Transfer	The sperms are released in the aquatic environment where they fertilize the egg.	Sperms are transferred directly into the female reproductive tract through copulation.
Gamete Quantity	A large number of sperm and eggs are released to increase the chance of fertilization.	Few eggs are produced and the quality of sperms varies among species.
Zygote Development	The development of the zygote takes place outside the parent body in the environment.	The zygote is developed inside the female body .
Parental Care	No parental care is seen after the eggs are released.	It is often associated with a high degree of parental care .
Selective Mating	Low chances of selective mating as a high number of gametes are produced.	High chance of selecting the mate as the female can control fertilization to a certain extent.
Examples	Fishes and amphibians.	Mammals, reptiles, birds, and some species of fishes and insects.

Advantages of Internal Fertilization

There are many advantages to internal fertilization which are the key to the survival and evolution of various species. The advantages are:

• Protection of gametes: The primary advantage of internal fertilization is that the gametes; sperm and eggs are protected from being destroyed by environmental factors like temperature, predators, and water currents.
• Reduced need for large amounts of gametes: Internal fertilization involves the direct transfer of sperm into the female and hence the need for large amounts of sperm is reduced.
• Selective breeding: The females have an opportunity to select mate and it leads to the production of stronger, healthier, or more genetically fit offspring.
• Enhanced survival of offspring: As the fertilization is internal the chances of the survival of the zygote are high as the female body is stable and nutrient-rich.
• Parental care: Species with internal fertilization exhibit parental care behavior by protecting the offspring from environmental changes and predators.
• Genetic diversity: Selective mating associated with internal fertilization leads to genetic recombination that contributes to genetic diversity and evolution.

Disadvantages of Internal Fertilization

The drawbacks of internal fertilization are :

• Challenges in proximity: Getting both the male & the female into close proximity becomes very challenging.
• Restricted production of offspring: The number of offspring that can be produced at any given time is restricted.
• Sexual Transmission of Diseases: The risk of sexually transmitted infections spreading is comparatively higher.

Conclusion – Internal Fertilization

It is a complex reproductive process that plays an important role in the genetic diversity and evolution of species. Internal fertilization advantages include the protection of gametes and the zygote that is formed after the fertilization process. In various animal phyla, including Chordata, Arthropoda internal fertilization can be seen. Internal fertilization occurs when sperm are transferred through insemination and then combine with an egg inside the body of a female creature.

FAQ’s on Internal Fertilization

What is the Difference Between Internal and External Fertilization?

In external fertilization, the fusion of the sperm and the egg occurs outside the female reproductive system. In internal fertilization, the sperm is delivered inside the female reproductive system through insemination.

What is an Example of Internal Fertilization?

In most of the birds the cloacal kiss is used for the transfer of the sperm and in slamanders, spiders and certain insects transfer the spermetosphere ( bundle of sperms) are transferred from males to females.

What is External Fertilization?

External fertilization is the process where the fusion of the male and female gamete takes place outside the female body in an external medium usually water. Fish and frogs are some of the organisms that perform external fertilization.

What is the Importance of Fertilization?

The fertilization process helps to retain the number of chromosomes in the organism and it leads to genetic variation due to the fusion of the gametes.

What are the Types of Fertilization?

The two main types of fertilization are external fertilization, which occurs outside the body in aquatic environments, and internal fertilization, which occurs inside the female’s body in terrestrial and aquatic environments.

Internal Fertilization Occurs in Which Phylum?

Internal fertilization occurs in several phyla, notably Chordata (including mammals, birds, reptiles, amphibians, and some fish) and Arthropoda (such as insects and arachnids), where mating and fertilization typically occur within the female’s body.

Can Internal Fertilization Occur in Plants?

No, internal fertilization does not occur in plants. Plants rely on external fertilization through pollination, where pollen is transferred to the female reproductive structures for fertilization to occur.