Animal Reproductive Cycle

Did you know that when an “arm” is broken off a sea star, it can grow into a new individual? This reproductive process is called fragmentation, and a similar process can also be observed in other echinoderms, such as sea urchins and sea cucumbers. 

Animal Reproductive Cycle Animal Reproductive Cycle

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Table of contents

    There are many different ways by which animals produce offspring, and a species can also have more than one way of reproducing. Sea stars, for example, can reproduce sexually by releasing their sperm and eggs into the water where fertilization occurs or asexually by fragmentation. For other species, like humans, we can only reproduce sexually.

    Keep reading to learn more about animal reproduction and the animal reproductive cycle.

    • We will first define what a life cycle is.
    • We will then distinguish between sexual and asexual reproduction.
    • We will also describe how these occur in general and cite specific examples, including the human reproductive cycle.
    • Finally, we will look into the advantages and disadvantages of both types of reproduction.

    Life Cycle Definition

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

    A life cycle describes how many sets of chromosomes are present at different stages of an organism's life.

    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.

    Some textbooks might refer to these life cycles as haplontic, diplontic, and haplo-diplontic, respectively.

    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 results in offspring that are genetically identical to the parent. This means that the offspring are basically clones of the parent.

    Sexual reproduction involves the production of haploid gametes by meiosis and the combination of the genetic material of two individuals by fertilization, producing offspring that are genetically different from either parent.

    It is important to note that asexual reproduction does not involve a change in the number of chromosomes, whereas sexual reproduction does.

    Asexual Reproduction

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

    Let’s briefly discuss each one, starting with fission.

    Binary Fission is a process where an organism splits into two separate organisms. Here, organisms duplicate their genes, and then divide the cell into two identical cells.

    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?

    Next, we have budding.

    Budding is where an outgrowth of a body part “buds” off from the original organism, forming two individuals, one smaller than the other.

    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.

    Next up is fragmentation.

    Fragmentation is where fragments break off from an individual and develop into new individuals.

    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.

    The fourth type of asexual reproduction in animals is parthenogenesis.

    Parthenogenesis is a process where an embryo develops without fertilization.

    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.

    For most animals, their most prominent life stage is the diploid stage. The gonads, or the testes and ovaries, create diploid cells specialized for sexual reproduction called germ cells early in the embryo's development. The germ cells can undergo mitosis to maintain the cell line and meiosis to produce haploid gametes.

    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.

    Fertilization can occur externally or internally.

    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.

    For the sperm and the egg to be fertilized, they must be released at the same time and in the same location.

    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.

    While internal fertilization results in fewer offspring compared to external fertilization, their survival rate is much higher. This is because internal fertilization protects the fertilized egg from dehydration. It also limits predation on the young by isolating the embryo within the female.

    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.

    All cells of the body, except the germ cells (sperm and egg cells), are called somatic 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.

    Animal Reproductive Cycle - Key takeaways

    • A life cycle describes how many sets of chromosomes are present at different stages of an organism's life.
    • Animals can reproduce sexually and/or asexually.
    • Humans reproduce sexually and, like most animals, have a diploid-dominant life cycle.
    • Asexual reproduction can produce many offspring quickly, but genetic variation among offspring can come only from mutation.
    • Sexual reproduction results in genetic variation, giving offspring better chances of survival in changing environments.

    References

    1. Molnar, Charles, and Jane Gair, 13.1 How Animals Reproduce, Concepts Of Biology, 1st Canadian Edition, (n.d.).
    2. Organismal Biology, Animal Reproductive Strategies, (n.d.).
    3. OpenStax, 11.2: Sexual Reproduction, Biology LibreTexts, 2 Nov. 2015.
    4. Britannica Encyclopedia, Parthenogenesis | Definition, Types, and Facts, 9 Sept. 2022.
    5. Oney, Michael, Echinoderms Part Two, Organismal Diversity, 20 Apr. 2015.
    Frequently Asked Questions about Animal Reproductive Cycle

    What are the types of reproductive cycle?

    There are three types of reproductive cycles: haploid-dominant, diploid dominant, and alternation of generations (which alternate between haploid and diploid stages).

    What is a reproduction cycle?

    Reproduction occurs in a cycle which an organism undergoes to move from one generation to another. 

    What are the two types of reproductive cycles in mammals?

    The two types of reproductive cycles in mammals are estrous and menstrual. 

    What is animal reproductive system?

    An animal reproductive system is a collection of organs and tissues that work together to enable sexual reproduction in animals.

    Test your knowledge with multiple choice flashcards

    ___ means having one set of chromosomes.

    Most animals have a ___life cycle. 

    Which of the two involves meiosis and fertilization?

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