Reproduction in Animals - Class 8 Notes & Olympiad Questions

Reproduction in Animals

a) Reproduction in animals is the biological process through which new offspring are produced to ensure the continuation of the species.
b) There are two main modes of reproduction in animals: asexual reproduction and sexual reproduction.

Asexual Reproduction

Asexual reproduction is a mode of reproduction in which only one parent is involved in producing offspring. In this process, offspring are generated without the need for the fusion of specialised reproductive cells (gametes) from two parents. As a result, the offspring produced are genetically identical to the parent and to each other, creating clones. Asexual reproduction is common in many unicellular organisms and some higher plants and animals.

Methods of Asexual Reproduction

Some methods of asexual reproduction in animals include budding in hydra, binary fission in amoeba, and fragmentation and regeneration in planaria and starfish.

1. Budding

Budding is a form of asexual reproduction commonly observed in organisms like hydra, coral, and sponges. The process of budding involves the following steps:

a) A bud appears on the parent's body: A small outgrowth, or bud, forms on the body of the parent organism. This bud contains a portion of the parent's body cells.
b)  The bud grows into a full organism: The bud continues to grow and develop, eventually becoming a complete and independent organism. During this process, the bud receives nourishment and energy from the parent organism.
c)  Separation from the parent: Once the bud reaches a sufficient size and matures into a fully functional organism, it separates from the parent's body. This separation can occur in different ways, depending on the organism. In some cases, the bud breaks off entirely, becoming a new, individual organism. In other cases, like in certain corals and sponges, the bud remains attached to the parent's body, forming colonies of interconnected individuals.

Budding allows these organisms to reproduce rapidly and efficiently. It results in the production of genetically identical offspring, as the new organism originates from a portion of the parent's body. This form of reproduction is advantageous in stable and favourable environments where conditions support the growth and survival of the offspring.

2. Binary Fission

a) Binary fission is a method of asexual reproduction commonly observed in single-celled organisms like bacteria and protozoa. In this process, a single parent cell divides into two daughter cells, each identical to the parent. It is a simple and efficient way for these organisms to reproduce and increase their population rapidly.
b) Amoeba, a common microscopic organism, reproduces through binary fission.

During binary fission in amoeba:

1. The nucleus of the parent amoeba divides into two nuclei.
2. The other cell organelles in the cytoplasm also undergo division.
3. Finally, the cytoplasm splits, resulting in the formation of two separate amoeba cells.
4. Each newly formed amoeba is genetically identical to the parent cell, and they continue to grow and function as independent organisms.

Binary fission allows amoebas to rapidly reproduce and populate their surroundings, ensuring their survival and spread in various environments.

3. Fragmentation

a) Fragmentation is a mode of asexual reproduction where the parent organism breaks itself into multiple fragments or pieces.
b) Each fragment has the potential to develop into a new, complete organism.
c) This process is common in organisms like planaria, which are flatworms found in freshwater.
d) When a planaria divides itself into fragments, each fragment can regenerate the missing body parts and form a new planaria individual.

4. Regeneration

a) Regeneration is the ability of an organism to regrow or replace lost or damaged body parts.
b) In some organisms, like lizards, certain body parts can be lost due to predation or accidents.
c) Unlike fragmentation, where the entire organism breaks into fragments, in regeneration, only specific body parts or tissues are lost or damaged.
d) The organism then regrows these lost body parts over time. For example, lizards can regrow their tails if they lose them in a predator attack.

Sexual Reproduction

Sexual reproduction is a mode of reproduction in which two parents, typically one male and one female, are involved in producing offspring. This process involves the fusion of specialized reproductive cells called gametes from each parent, resulting in the formation of genetically diverse offspring. Sexual reproduction is the most common mode of reproduction in higher animals, including humans, and many plants.

Sexual Reproduction in Humans

I. Male Reproductive System

a) The male reproductive system is responsible for producing and delivering sperm, the male gametes required for sexual reproduction. The main male reproductive organs are the testes, epididymis, vas deferens, seminal vesicles, prostate gland, bulbourethral glands, and urethra.
b) Sperm from the testes move through the sperm ducts and mix with the secretions from the seminal vesicles, prostate gland, and bulbourethral glands to form semen. The semen is then ejaculated through the penis during sexual intercourse.
c) Each sperm cell is unicellular and consists of a head, a middle piece, and a tail. The head contains the genetic material (DNA), the middle piece is rich in mitochondria for energy production, and the tail allows the sperm to swim towards the egg for fertilisation.

Male Reproductive Organs

Testes: The testes are located in the scrotum and produce sperm and male sex hormones, such as testosterone.

Epididymis: The epididymis is a coiled tube attached to each testis where sperm mature and are stored.

Vas Deferens: The vas deferens is a muscular tube that carries mature sperm from the epididymis to the urethra.

Seminal Vesicles: The seminal vesicles secrete a fluid rich in nutrients that mix with sperm to form seminal fluid.

Prostate Gland: The prostate gland produces a milky fluid that helps neutralize acidity in the urethra and female reproductive tract.

Bulbourethral Glands: These glands secrete a lubricating fluid that aids in the passage of semen during ejaculation.

Urethra: The urethra is a tube that carries semen from the reproductive system and urine from the bladder out through the penis.

II. Female Reproductive System

The female reproductive system is a complex and intricate system responsible for the production of female gametes (ova or eggs), the nurturing and development of the embryo during pregnancy, and the production of female sex hormones. It consists of several organs and structures working together to facilitate reproduction.
The ovum, or egg cell, is the female gamete. It is a single cell containing a nucleus and cytoplasm. Once released from the ovary during ovulation, the ovum can be fertilized by sperm if sexual intercourse occurs around the same time.

Female Reproductive Organs

The female reproductive system consists of the following organs and structures:

Ovaries: A pair of ovaries are located in the lower abdomen on either side of the uterus. They are the primary female reproductive organs and are responsible for producing female gametes called ova (eggs). Ovaries also secrete the female sex hormones estrogen and progesterone, which play crucial roles in the menstrual cycle and pregnancy.

Fallopian Tubes (Oviducts): There are two fallopian tubes, one on each side, that extend from near the ovaries to the upper part of the uterus. These tubes serve as a pathway for the ova to travel from the ovaries to the uterus. fertilisation of the egg by sperm usually occurs in the fallopian tubes.

Uterus: The uterus, commonly known as the womb, is a hollow, muscular organ located in the pelvic cavity. It provides the environment for the developing embryo or fetus during pregnancy. If fertilisation occurs, the fertilised egg (zygote) implants itself into the lining of the uterus and begins to grow into a baby. If pregnancy does not occur, the uterine lining is shed.

III. Fertilisation and Development of the Embryo

Fertilisation is the process of fusing the male and female gametes (sperm and egg) to form a zygote, which is the first cell of the new organism.
Here's a summary of the subsequent events:

Fusion of Nuclei: During fertilisation, the nuclei of the sperm and egg come together, combining their genetic material to form a single nucleus in the zygote.

Cell Division: After fertilisation, the zygote undergoes multiple rounds of cell division through a process called mitosis. These divisions produce a cluster of cells, which eventually forms an embryo.

Embryo Development: As cell division continues, the embryo starts taking shape, and its cells differentiate into different types to form the basis of various tissues and organs.

Implantation: The developing embryo, now called a blastocyst, travels down the fallopian tube and reaches the uterus. It attaches itself to the uterine wall and starts embedding itself in the uterine lining through a process called implantation.

Growth and Development: Once implanted, the embryo continues to grow and develop inside the uterus. It receives nourishment and oxygen from the mother's bloodstream through the placenta, a structure that develops from the tissues of both the embryo and the mother.

Foetus Formation: At a certain stage of development, typically around the eighth week after fertilisation, the embryo is recognizable as a human, and it is then termed a foetus. During this foetal stage, all major organs and body systems form, and the foetus goes through various growth phases.

Birth: After a gestation period of around nine months, the fetus is fully developed and ready for birth. The process of childbirth, also known as delivery, occurs when the fetus passes through the birth canal (vagina) and is born into the world.

Types of Fertilisation

Fertilisation can be classified into two main types based on where it takes place: internal and external fertilisation.

Viviparous, Oviparous and Ovoviviparous Animals

Viviparous, oviparous, and ovoviviparous are terms used to classify animals based on their method of giving birth or reproduction. They refer to how the offspring are developed and born.

Viviparous Animals

a) Viviparous animals are those that give birth to live young ones.
b) In this reproductive strategy, the embryo develops and grows inside the mother's body, receiving nourishment directly from her through a specialized placenta or other means.
c) When the offspring have completed their development, they are born as live and fully-formed individuals.
d) The term "viviparous" comes from Latin and means "to give live birth."
e) Examples of viviparous animals:

1. Most mammals, including humans, dogs, cats, elephants, and whales.
2. Some reptiles, such as certain species of snakes and lizards.
3. Certain fish, such as some sharks and rays.

Oviparous Animals

a) Oviparous animals are those that lay eggs as part of their reproductive process. The embryo develops and grows within the protective environment of the egg outside the mother's body. The egg contains all the necessary nutrients and materials needed for the embryo's development. After a period of incubation, the egg hatches, and the young offspring emerge.
b) Examples of oviparous animals:

1. Birds are classic examples of oviparous animals, laying eggs in nests or suitable locations.
2. Many reptiles, including snakes, lizards, turtles, and crocodiles, lay eggs.
3. Amphibians, such as frogs, toads, and salamanders, typically lay eggs in water or damp environments.

Ovoviviparous Animals

a) There are also ovoviviparous animals, which produce eggs that are retained and hatch within the mother's body.
b) These animals give birth to live young, similar to viviparous species, but the live young result from hatched eggs inside the mother rather than direct internal development.
c) Great white sharks, rattlesnakes, and sea horses are a few examples of ovoviviparous animals.

Young Ones to Adults

The journey from young ones to adults is a process known as growth and development. It is a fundamental aspect of the life cycle of most organisms, including animals. Throughout this process, individuals undergo significant changes in their physical structure, behaviour, and reproductive capabilities. Below are some examples that illustrate this remarkable transformation.

Life Cycle of a Frog

In the life cycle of certain animals like frogs, there are distinct stages of growth and development, starting from the egg to the adult stage. In the case of frogs, the life cycle consists of three main stages: egg, tadpole (larva), and adult.

Egg: The life cycle begins with the laying of eggs by adult female frogs in water bodies like ponds or lakes. The eggs are usually laid in clusters and are protected by a jelly-like substance.

Tadpole (Larva): Once the eggs hatch, they give rise to tadpoles, which look very different from adult frogs. Tadpoles are aquatic, have long tails, and breathe through gills. During this stage, tadpoles feed on algae and other aquatic plants.

Adult: As the tadpoles grow and undergo a process called metamorphosis, they gradually transform into adult frogs. Metamorphosis is a series of drastic changes in the body structure and function, leading to the development of legs, lungs, and a more suitable body form for terrestrial life. Eventually, the tadpoles develop into fully-formed frogs capable of jumping and swimming on land.

Life Cycle of Insects

Insects, a diverse group of animals, also undergo remarkable transformations during their life cycle. This process is known as metamorphosis and can take two primary forms: incomplete metamorphosis and complete metamorphosis.

Incomplete Metamorphosis: In insects such as grasshoppers and cockroaches, the life cycle includes three stages – egg, nymph, and adult. Nymphs resemble miniature adults but lack certain mature features. As they grow, nymphs moult several times, each time resembling the adult form more closely. Eventually, they reach full maturity.

Complete Metamorphosis: Many insects, like butterflies and beetles, undergo complete metamorphosis with four distinct stages – egg, larva (caterpillar or grub), pupa (cocoon or chrysalis), and adult. The larval stage is markedly different from the adult and serves for feeding and growth. The pupal stage involves a transformation within a protective case, leading to the emergence of the adult insect with wings and reproductive capabilities.

Life Cycle of Humans

In contrast, human beings do not undergo the same type of metamorphosis as frogs or insects. Humans have a relatively gradual growth and development process, and their body parts, such as limbs and vital organs, are present from birth. Human growth involves a combination of cell division, tissue differentiation, and hormonal changes that lead to the development of various body systems over time.

While there are no drastic transformations like metamorphosis in humans, the process of growth and development is still critical for reaching adulthood and achieving the full potential of an individual's physical and mental capabilities. The human life cycle involves stages such as infancy, childhood, adolescence, and adulthood, each marked by specific milestones and changes in physical appearance, behaviour, and cognitive abilities.