INTERNAL FERTILIZATION: PROCESS, ADVANTAGES, DISADVANTAGES, EXAMPLES - BIOLOGY - 2025

The fertilization or internal fertilization is the process during sexual reproduction where the male gametes (sperm) are released from the reproductive organ of a male to the reproductive organ of a female, within which the fusion of gametes and formation occurs the zygote.

Several authors consider that internal fertilization begins when the male releases the gametes inside the female during copulation and ends with the formation of the zygote, which is the cell resulting from the fusion of sex cells or syngamy.

Fertilization of the egg cell, sexual reproduction. Genomics Education Program

Although all viviparous animal species have internal fertilization (exclusively), this process also occurs in some oviparous and ovoviviparous species, and is not always correlated with the presence of copulatory or intrusive organs.

For animals whose reproduction is characterized by internal fertilization, this process represents an advantageous adaptation to the varied environmental conditions (which in many cases can be adverse) that gametes face during external fertilization, in addition to ensuring greater reproductive success.

Sexual reproduction with internal fertilization (Image by meineresterampe at www.pixabay.com)

Good examples of animals with internal fertilization, in addition to mammals, among which man is included, are birds that, despite being oviparous animals, join their cloacas so that the sperm of the male reach the egg cells of the female inside of the female.

Internal fertilization process

During the sexual reproduction of two animals, internal fertilization occurs when the male deposits his sperm cells inside a cavity of the female, within which the syngamy or fusion of the sex cells that give rise to the zygote occurs, of the that an embryo will then form and later a baby.

Although it is not included in the description of the process, internal fertilization implies that previously, in the two animals that reproduce, gametogenesis has taken place, that is, the formation of spermatozoa in the reproductive organ of the male and of the ovules or oocells in the female reproductive organ.

For internal fertilization to take place, it is then necessary for a male to come into contact with a female, for which there are usually different mating strategies whose success depends, many times, on various hormonal and / or environmental signals.

Sexual reproduction with internal fertilization (Image by Murad Swaleh at www.pixabay.com)

Not all species with internal fertilization have specialized copulatory organs, but in those that do exist, it is usually a penis with retraction capacity and a vulva, which has specific adaptations to receive the male organ in each species.

Depending on the type of animal, the development of its sexual cells may culminate during copulation, such is the case of humans, in which sperm require specific signals and factors found in the female's reproductive tract to complete their maturation.

In other cases, the oocells also require the presence of sperm in the female reproductive system to "activate" or "prepare" for the arrival of sperm.

Once the mobile sperm fuses with the egg cell inside the female, it "penetrates" the membranes that usually cover and protect it, and it releases its cytosolic content inside the egg. The following image shows how a human sperm reaches the egg.

Subsequently, the haploid sperm nucleus (with half the male's chromosomal charge) fuses with the haploid nucleus of the oocell (with half the female's chromosomal charge), forming a diploid structure called "zygote", in the which mixes the genetic material of both parents.

Advantage

Pigeons mating

Unlike external fertilization, the internal fertilization process does not merit the production and release of huge amounts of sex cells, especially from the male, which implies a metabolic advantage, since fewer resources are allocated for gametogenesis.

Since the contact and fusion of sex cells occurs in a closed space, under conditions of constant pH, salinity and temperature, internal fertilization can mean an advantage for the success or survival of the offspring, especially for those animal species with greater parental care.

Gestation (Image by redgular at www.pixabay.com)

Furthermore, the probability of contact between the reproducing male and female sex cells is much higher in a closed cavity within the female than in the aquatic environment where external fertilization occurs (which is typical of aquatic animals such as fish. and amphibians).

Disadvantages

One of the main disadvantages of the process of fertilization or internal fertilization is that the number of offspring produced is lower, which is evident from the point of view of the carrying capacity of the female, within whose specialized structures the process occurs. of syngamy and initial development of the offspring.

Likewise, and unlike what happens with external fertilization, this process implies a greater effort on the part of the parents to find a partner, since contact between a male and a female is essential.

Another of the disadvantages that can be pointed out with respect to internal fertilization is that the largest participation is of the females, since the nutrition of the offspring in a placenta (viviparous), the care of the eggs in a nest (oviparous) or the sustenance of the development of the eggs in its interior until the hatching of the young (ovoviviparous).

Parental care (Image by gilbertobal at www.pixabay.com)

The extensive requirement for greater parental care in many of the internally fertilized species can also represent a disadvantage, as the offspring are often unable to fend for themselves for quite long periods of time after birth.

Examples of internal fertilization

All mammalian animals, since they are viviparous (of those whose offspring develop inside the mother and are born alive) have internal fertilization. Examples of these animals are:

- The human being

- Whales and dolphins

- Cats and dogs (all cats and canids)

- Squirrels, mice, rats, rabbits, guinea pigs and other rodents

- Cows, pigs and horses

- Elephants, rhinos and giraffes

- Among others

Parental care (Image by Holger Detje at www.pixabay.com)

However, some oviparous and ovoviviparous animals also have internal fertilization and among these the most prominent group is that of birds and reptiles. Although external fertilization predominates in aquatic animals, some species of fish and amphibians are characterized by internal fertilization.

The main differences between these groups of animals with internal fertilization lie in the “method”, since not all species have specialized copulatory organs for this purpose (such as those of humans, for example).

Sexual reproduction with internal fertilization (Image by Elsemargriet at www.pixabay.com)

The sex cells of all birds and of some species of reptiles come into contact thanks to the "fusion" of their cloaca, while in other species of animals the males produce a kind of containers called "spermatophores", which fill with sperm and that they deposit in the cloaca of the females, where internal fertilization occurs.

In plants

Internal fertilization is also typical of most terrestrial plants. In flowering plants, the pollen grain germinates on the stigma, producing a tube inside the style that allows the emptying of microspores in the vicinity of the ovule (in the ovary).

These microspores are capable of fusing with the ovules contained in the ovary and thus produce the zygote that will give rise to the embryo, which will be "encapsulated" within the seed.

References

1. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). New York: McGraw-Hill.
2. Kardong, KV (2002). Vertebrates: comparative anatomy, function, evolution (No. QL805 K35 2006). New York: McGraw-Hill.
3. Moore, KL, Persaud, TVN, & Torchia, MG (2018). The Developing Human-E-Book: Clinically Oriented Embryology. Elsevier Health Sciences.
4. Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson.
5. Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.