Animal Reproductive Cycle

Life Cycle Definition

Let's start by looking at the definition of the life cycle of an animal.

Specifically, it describes an organism as haploid or diploid: haploid (n) means having one set of chromosomes, while diploid (2n) means having two sets of chromosomes.

The reproductive cycle of an organism can be:

• Haploid-dominant
• Diploid-dominant
• Alternate between haploid and diploid stages.

Organisms with a haploid-dominant life cycle spend most of their lives as haploid (n) gametes. The fusion of the haploid gametes forms a diploid zygote (a fertilized egg), which quickly undergoes meiosis to produce more haploid gametes.

• Many green algae and most fungi have a haploid-dominant life cycle.

Organisms with a diploid-dominant life cycle spend most of their lives as diploid (2n) individuals. Once they reach maturity, they undergo meiosis to produce haploid gametes. Then, the gametes fuse via fertilization, forming a diploid zygote. The zygote matures into a diploid adult, and the cycle continues.

• Most vertebrates (including humans) have a diploid-dominant life cycle. Other organisms include some fungi, some yellow-green algae, and some seaweeds.

Now, a life cycle that has both a haploid and a diploid multicellular stage is known as the alteration of generations.

• Examples of organisms in this category include mosses, liverworts, vascular plants, seaweeds, most red algae, and some fungi.

An organism goes through a reproductive life cycle to go from one generation (parent/s) to the next (offspring).

Types of Reproduction in Animals: Sexual and Asexual

Now, let's dive into the different types of reproduction in animals. Animals can reproduce sexually and/or asexually.

Asexual Reproduction

Binary fission, budding, fragmentation, and parthenogenesis are some ways by which organisms reproduce asexually.

The planarian flatworm is an example of an asexually reproducing species via binary fission. In this process, the individual tears itself into two pieces–a head and a tail–and then regenerates the missing half of each individual, resulting in two new worms! Pretty cool, right?

Budding is typically observed in invertebrate animals, including hydras and corals. In hydras, the bud forms and matures before breaking away from the main body. In corals, the bud matures, remains on the original organism, and multiplies as part of a new colony.

Fragmentation can be observed in sea stars. When an arm becomes detached from the parent, it can grow into a new individual. For some species, the arm must include a part of the central disk of the sea star. In others, the limb itself can develop into a new individual.

This can happen deliberately (the sea star may extend its limbs in opposite directions until it splits), by accident, or due to predation.

In most cases, an offspring produced parthenogenically arises from diploid eggs, meaning that these eggs do not require any genetic contribution from sperm. Parthenogenesis has been observed in water fleas, aphids, wasps, and also some species of honeybees, such as the Cape honeybees.

Sexual Reproduction in Animals

Sexual reproduction occurs differently across species. However, what is true for most sexually reproducing species is that at some point in their life cycle, meiosis leads to the formation of haploid cells, and upon maturity, a haploid cell of the individual will fuse with the haploid cell of another individual–a process called fertilization–producing a diploid zygote.

Once the haploid gametes are created, they won't be able to divide again. When two gametes, typically from distinct individuals, combine during fertilization, the diploid state is restored.

External fertilization

External fertilization means that the eggs and sperm are released into the environment, a process called spawning. Fertilization takes place once the sperm reaches the egg. This typically occurs in aquatic environments, where the eggs will not dry out.

External fertilization can happen in the following ways:

• Environmental cues like water temperature and amount of daylight or biological cues like pheromones can cause males and females to release gametes at the same time. Here, the males and females typically do not interact with each other as individuals, but are congregated in large numbers so that all gametes are in the same location. This is observed in some fish, crustaceans, mollusks, and invertebrates.

• An individual male courts an individual female to induce the latter to release its eggs. When the eggs are released, the male releases the sperm to fertilize the eggs. This is observed in some amphibians.

Internal fertilization

Internal fertilization means the sperm and eggs are joined within an animal’s body. This can occur by:

• The male depositing sperm in the female during mating; or

• The male depositing sperm in the environment which the female collects and deposits into its reproductive tract.

Internal fertilization typically occurs in land-based animals, but there are also some aquatic animals that use this method. Offspring are produced through internal fertilization in three ways:

• The female lays its fertilized eggs.

  • This is called oviparity. The fertilized eggs develop outside the female’s body and receive nourishment from the egg yolk.

    This can be observed in all birds and many bony fish, reptiles, and amphibians. It can also be observed in some cartilaginous fish and mammals.

• The female retains its fertilized eggs within its body.

  • The embryos take in nourishment from the egg yolk. The young are fully developed when the eggs hatch. This is called ovoviviparity.

    This process can be observed in some bony fish, sharks, lizards, snakes, vipers, and invertebrates.

• The female retains its fertilized eggs within its body, and the embryos take in nourishment from the mother through an organ called the placenta.

  • In this case, the offspring develops in the female’s body and is born alive. This is called viviparity.

    Viviparity can be observed in most mammals (including humans), some cartilaginous fish and several reptiles.

The Human Sexual Reproductive Cycle

Like most animals, humans have a diploid-dominant life cycle. This means that the multicellular diploid stage is the most dominant part of the life cycle.

When dealing with the sexual reproductive cycle in animals, we need to be familiar with somatic cells and germ cells.

Somatic cells are diploid (2n). The gonads also produce diploid (2n) germ cells, but when a human individual reaches maturity (or puberty) these cells undergo meiosis, producing haploid gametes.

• The male produces sperm in testes. The human male sperm is a small, mobile cell that occurs in large numbers

• The female produces an egg that matures in the ovary. The female egg is a large, non-mobile cell that requires a lot of energy to form.

During the reproductive cycle in humans, the sperm travels through the uterus to the fallopian tubes to fertilize the egg. The fusion of the haploid gametes results in a diploid zygote.

Sexual Reproduction Advantages and Disadvantages

Asexual reproduction is advantageous in a stable environment because the offspring will be adapted to that environment. However, offspring produced via asexual reproduction may be at a disadvantage in an unstable environment because due to the lack of genetic variation, these genetically identical offspring may not have the traits necessary to survive changes in the environment.

So, asexually reproducing species will have to rely on mutations for such traits to emerge. Fortunately, an asexually reproducing individual can produce large numbers of offspring quickly, allowing for a speedy response to environmental changes.

Another advantage of asexual reproduction is that it may make it easier for an individual to colonize new environments because it does not need to find a mate in order to reproduce.

On the other hand, sexual reproduction can be advantageous because the genetic variation among the offspring can give them better chances of survival in unpredictable or changing environments. However, a disadvantage of sexual reproduction is that it requires the species to maintain two different types of individuals, males and females, which both need to be present for them to successfully colonize new habitats.