We explain what the process of reproduction in animals consists of, its implications and sexual and asexual mechanisms.
What is reproduction in animals?
The reproduction is the set of biological processes through which living beings produce new individuals similar to themselves, members of their own species. The reproductive process can occur from one or two parent individuals (asexual and sexual reproduction respectively).
Reproductive processes vary enormously depending on the species and the kingdom to which it belongs. Thus, for example, plants and fungi reproduce by significantly different mechanisms than animals.
However, the reproduction of all living beings has one feature in common, regardless of the kingdom to which they belong: it is governed by the principle of self-perpetuation of species, that is, through the reproductive process the members of a species produce new offspring to preserve their genetic material and perpetuate the cycle of life.
There are two types of reproductive mechanisms, which are used depending on the species and the conditions in which the individuals find themselves: asexual and sexual mechanisms.
- Asexual mechanisms They are those that allow an individual to reproduce on their own, without the need for another parent or the intervention of germ cells (also called gametes) such as sperm and eggs. In asexual reproduction there are different types of self-replication mechanisms (such as binary fission or fragmentation) and all of them have in common the production of new individuals that are genetically identical to the parent, that is, clones. These mechanisms have certain advantages, such as how inexpensive and simple they are, and how quickly the division can happen. However, they do not provide genetic variability to the species (since all offspring are identical to each other and to the parent), which is why they allow very rigid and slow evolutionary margins.
- The sexual mechanisms They are those that involve the participation of individuals of both sexes since they require the union of reproductive or germ cells from a male and a female entity, to fuse their genetic materials and give rise to a zygote. When it develops, this new cell resulting from the fusion will give rise to a new individual, whose genetic code will be different from those of its parents. Although sexual reproduction has a much higher energy cost, requires more time and produces fewer individuals, it has a great advantage over asexual reproduction: it allows genetic variability, which is key to the evolutionary process and adaptation of species. to new living conditions. Without this variability, changes in species would take much longer, since it would require spontaneous DNA mutations, something that occurs very sporadically.
Depending on whether it is sexual or asexual, the mechanisms of animal reproduction will involve certain biochemical, cellular and even social processes. These necessary processes include the acquisition of sexual maturity of the individual and a suitable habitat for reproduction and, in the case of sexual mechanisms, obtaining a suitable partner for reproduction, and then proceeding to copulation, fertilization, gestation and birth.
Depending on the species and the mechanism, it may give rise to one or more descendants as the case may be.
See also: Reproduction of fish
Asexual reproduction in animals
Although most animals reproduce sexually, animal reproduction is not strictly sexual, and many species have asexual mechanisms (regular or emergency use) to reproduce. However, it is worth clarifying that this type of reproduction is observed in the most primitive and, generally, simple animal species since, although sexual reproduction is more expensive and demanding, it is much more convenient for most species.
asexual reproduction always generates individuals genetically identical to the parent and there are different types:
- Bipartition It occurs when the animal, usually single-celled, divides itself into two identical halves, each of which will give rise to a young offspring. This is the case, for example, of planarians.
- Cleavage This procedure also occurs when some individuals lose a limb or a region of the body, which when regenerated allows the emergence of two complete identical individuals, as occurs with the arms of starfish.
- Gemmation It consists of the mature individual growing a lump or “yolk” in some region of its body, which grows and develops until it forms a new and identical individual, and which can then detach itself from the parent's body and lead an independent life, or stay attached and form a colony. This is the reproductive mechanism of corals and marine sponges.
Sexual reproduction in animals
In this category we will see those mechanisms that involve the union of two different and separate parents (biparental procedures) and also those exceptional mechanisms that, in certain species, allow sexual reproduction from a single and same parent (hermaphroditism and parthenogenesis).
The common distinctive feature of all the previously mentioned mechanisms is that they imply the need to creation of gametes: differentiated sexual cells, that is, eggs in the case of the female and sperm in the case of the male, which when united inside or outside the maternal body (depending on the species), give rise to the formation of a new individual.
The fusion of an egg with a sperm gives rise to a zygote which gives rise to a new individual (although it is worth clarifying that in most animals, the female produces more than one egg, which allows the formation of more than one zygote).
There are different forms of sexual reproduction:
Depending on the place where the union of the gametes occurs:
- Mating by external fertilization It occurs when the encounter between the gametes does not occur inside the female's body, but in the environment. In that case, embryonic development is also external and the zygotes and embryos develop inside eggs, which are soft and must remain in water so as not to dry out. This mechanism is characteristic of ovuliparous animals (fish).
- Mating by internal fertilization It occurs when the gametes meet inside the mother's body, after intercourse during which the male physically introduces his sperm into the female's reproductive system. Within the female the zygotes develop to a certain extent, depending on the type of birth of the species:
- In oviparous animals. The fertilized female proceeds to deposit the eggs in the environment or in some type of nest, where they mature until the already formed offspring are released.
- In viviparous animals. There are no eggs, but the young develop inside the mother's body (in a uterine sac) until they are completely formed, and are finally expelled into the environment through birth.
- In ovoviviparous animals. The female carries the fertilized eggs inside her body until they hatch and then the young are released into the environment.
Depending on whether one or two individuals are involved:
- Biparental reproduction They are reproductive mechanisms that involve a male and a female, who have already reached sexual maturity, and who, through different procedures, bring their gametes into contact to give life to a new generation of members of the species. This is the case of the reproduction of mammals, including humans.
- Reproduction by hermaphroditism Some animal species have monoecious adults, that is, they have both sexes at the same time and, therefore, they can act as females or as males depending on the occasion, and can even carry out self-fertilization: the adult individual fertilizes its own female gametes, when the conditions are not met for another member of the species to do so. Only some invertebrates such as annelids, worms and snails, and some species of fish and frogs are capable of reproducing in this way.
- Reproduction by parthenogenesis Many species capable of reproducing biparentally can also do so through an alternative procedure, which is parthenogenesis and which is, strictly speaking, asexual. In this case, an unfertilized female gamete develops to form an embryo genetically identical to its parent, giving life to a new, clonal, but haploid individual (which has half of its parent's genome). Many arthropods, such as ants, reproduce in this way: the female queen is fertile, unlike the workers, and after mating with the male she can fertilize her eggs and produce new members of the colony, which are always female and diploid ( complete genome). The males, on the other hand, are produced sporadically and are haploid, since their only function is to fertilize the queen, which is why they are known as drones.
Alternate playback
Alternating reproduction is understood as a reproductive scheme characteristic of simple plants and certain primitive animals, in which generations that reproduce sexually and others that reproduce asexually alternate. Hence the name “alternating”.
This mechanism can be exemplified by the case of jellyfish, whose bodies form eggs and sperm that are expelled into the surrounding water, and there in the environment they meet and fertilize, generating a zygote that gives rise to a polyp.
This polyp is an intermediate life form that reproduces by budding, producing buds that completely separate from the body and eventually become adult jellyfish.
Very few known living beings possess this double type of reproductive mechanism.
Continue with: Plant reproduction
References
- “Reproduction” on Wikipedia.
- “Animal reproduction” at INSIBIO Conicet (Argentina).
- “Reproduction in animals” in the Biosphere Project of the Ministry of Education of Spain.
- “Reproduction in animals” in AboutCiencia.
- “Reproduction (biology)” in The Encyclopaedia Britannica.