Reproductive methods in animals

Reproduction in animals is either asexual or sexual. Asexual reproduction does not require the coming together of male and female gametes but sexual reproduction does.

Asexual reproduction

This form of reproduction is suitable for sessile or sedentary animals, which are unable to move about and seek mates. It is also common in animals that live in difficult environmental conditions where mortality is high and there is need for rapid reproduction and rapid growth to ensure the survival of the species.

Among the asexual reproductive methods are binary fission, budding, fragmentation, conjugation, regeneration and parthenogenesis.

1. Binary fission involves the splitting of the organism into two equal parts as in Amoeba, so that the resulting daughter cells have the same chromosome number as the mother cell and identical to it.

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Ciliates such as Paramecium have two nuclei, one large macronucleus and one small micronucleus. The macronucleus controls food utilisation and growth of the cell while the micronucleus is only concerned with reproduction.

Ciliates reproduce asexually by binary fission and occasionally by what is known as conjugation. During conjugation, two organisms align side by side. Then the macronucleus from each organism disintegrates while each micronuclei of each pair divides by meiosis giving rise to four haploid nuclei.

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Three of the micronuclei from each individual are eliminated leaving only one. The one micronucleus left in each pair duplicates. Then one of the two in each individual fuses with one of the other pair so that each organism has diploid chromosome number. Finally, the organisms separate.

2. Budding is an outgrowth of the parent animal that detaches itself and becomes a complete organism. This kind of reproduction is characteristic of the Hydra.

Budding in hydra

 3. Fragmentation is when animal breaks up into several parts and each fragment goes on to form another animal. An example this is the Spirogyra.

4. Regeneration is not strictly a reproductive method. For example if a sea star loses part of its body, the lost part is replaced by the growth of another one. It is a form of repair rather than reproduction.

Sexual Reproduction

In sexual reproduction, the male animal produces spermatozoa and the female animal produces eggs. The sperm fuse with the eggs in a process called fertilisation to form a zygote. Fertilisation can be external or internal. External fertilisation involves the deposition of sperm on eggs that have already been laid . . .

External fertilisation requires a moist wet environment to protect the embryo from desiccation or drying. It is therefore associated with animals that live in water such as fish or animals that go back to the water during at reproduction period such amphibians. Such animals lack parental care and produce large numbers of eggs in order to compensate for the large numbers of eggs and embryos that will be lost to predators and other environmental factors.

Hermaphroditism

In hermaphroditism, an organism has both sex organs. Included among hermaphroditic organisms are tapeworms and earthworms. Most of the time tapeworms occur singly in the intestine where it is impossible to find a mate leaving with self-fertilisation as the only available option. Consequently, they produce sperms are then used to fertilise their own eggs. Earthworms are free to move about and meet other earthworms. This is why they practise mainly cross-fertilisation.

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Reproduction in Insects

Insects have internal fertilisation. The female ovaries produce eggs that pass through the oviduct and uterus to the vagina where they are fertilised by the male’s sperm. In some insects, the sperm are stored in a sac in the female reproductive canal called the spermatheca until they are needed.

Reproduction in butterfly

 The change from a larva to an adult insect is referred to as metamorphosis. An insect exoskeleton does not grow, so it has to be shed from time to time to allow the insect grow. During metamorphosis, the exoskeleton is partly digested away and then split open. This act of shedding of the exoskeleton by the insect is called ecdysis.

The insect emerges with a soft new skeleton, which expands and hardness within one hour. The new insect is called an instar. Insect growth is therefore in bursts or discontinuous unlike that of humans and other animals which is continuous.

Insect Life Cycle

Insects must therefore undergo a series of development stages before they become mature. An insect, like the butterfly or mosquito, has a complete metamorphosis. This kind of life cycle involves the deposition of eggs, which hatch into larvae. The larvae feed and grow and then turn into pupae, non-feeding forms that are enclosed in a cocoon. The pupae develop into adult insects that are quite different in structure and appearance from either the larvae or pupae.

Incomplete Life Cycle

A grasshopper is an insect with an incomplete metamorphosis. Incomplete life cycle starts with the laying with eggs that hatch into tiny grasshoppers that look like adult grasshoppers except for their small size. The small grasshoppers feed and grow. After sometime they become too big for their exoskeletons which are then shed and new ones develop. This continues until the insect reaches maturity.

Reproduction in grasshopper

REPRODUCTION IN VERTEBRATES

Reproduction in Amphibians

Most amphibians lay large numbers of eggs in water where they are fertilised by the sperms shed by the male. The large numbers of eggs laid in masses held together by a gelatinous substance. The eggs hatch into tadpoles that have gills and tails but no legs. The tadpoles feed on vegetable matter as they grow. Then they undergo metamorphosis losing the tail and the gills and develop legs. From a herbivorous tadpoles the young frogs carnivorous, feeding on insects and other invertebrates

Reproduction in frog

 Reproduction in Reptiles

Internal fertilisation involves the deposition of sperm into the female reproductive canal so that sperm and eggs unite inside the female’s body. Internal fertilisation is not therefore wasteful since most of the ova produced are fertilised. It is suitable for animals that do not produce large numbers of eggs. Internal fertilisation is characteristic of land-based animals, such as reptiles, birds and mammals.

Bird's egg

The eggs of reptiles and birds are enclosed in a shell of calcium and protein that protects them from desiccation and from predators. They also have four different membranes in addition to the shell. These are the amnion, the allantois, the yolk sac and the chorion. The amnion is a chamber that is filled with an amniotic fluid in which the embryo is suspended. The allantois functions as a container for the urinary wastes produced by the developing embryo and its blood vessels, which lie near the shell. It also functions in the exchange of CO2 and O2. The yolk sac as its name indicates, encloses the yolk, which serves as food for the developing embryo. The chorion forms the outer membrane that surrounds the embryo and the other membranes. This reptilian egg is called a cleidoic egg and is responsible for the successful colonisation of the land by these creatures.

Lizard hatching from egg

Study Questions

1. List the different asexual reproductive methods used by microscopic organisms,

2. How is binary fission different from hermaphroditism?

3. Discuss the methods of reproduction in insects. What is metamorphosis?

4. In which ways are amphibians dependent on water?

5. Describe the reproductive changes that have enabled the reptiles to leave live and breed on land.