Reproduction in animals: sexual reproduction and reproductive strategies

Learning objectives

By the end of this lesson, you should be able to:

• define reproduction and distinguish sexual from asexual reproduction
• describe the roles of male and female gametes in sexual reproduction
• compare internal and external fertilisation
• explain reproductive strategies in mammals, birds, fish and insects
• relate reproductive structures and behaviour to successful reproduction
• distinguish fertilisation, development, hatching and birth

Why animals reproduce

Reproduction is the biological process by which organisms produce offspring of their own species. It ensures the continuation of a species from one generation to the next. Reproduction does not keep an individual animal alive; it maintains the population over generations.

In sexual reproduction, male and female gametes (sex cells) fuse during fertilisation to form a zygote. The zygote develops by repeated cell division and differentiation into an embryo and, eventually, a new individual. The male gamete is usually sperm; the female gamete is the ovum or egg cell.

Gametes contain half the normal number of chromosomes for the species. Their fusion restores the normal chromosome number in the zygote. In animals, gametes are produced by meiosis in the reproductive organs. Sexual reproduction produces genetic variation among offspring, which can help populations respond to changing environments.

Some animals can reproduce asexually, without fusion of gametes. Examples include budding in hydra and regeneration or fragmentation in certain simple animals. Asexual reproduction can be rapid and does not require a mate, but offspring are usually genetically very similar to the parent. This lesson focuses mainly on sexual reproduction in the major animal groups in the WASSCE syllabus.

Male and female reproductive roles

The male reproductive system produces and transfers sperm. Sperm cells are adapted for movement and fertilisation: a typical sperm has a head containing a haploid nucleus, an acrosome with enzymes that assist penetration of egg coverings, a midpiece rich in mitochondria, and a tail (flagellum) for movement.

The female reproductive system produces ova and provides a suitable environment for fertilisation and development. An ovum is larger than a sperm cell and contains stored nutrients in many animal groups. Its surface and surrounding coverings help regulate sperm entry.

The two gametes have complementary roles. Sperm delivers a paternal haploid nucleus to the egg; the egg supplies a maternal haploid nucleus and, in many species, cytoplasm and food reserves for early development.

Fertilisation: internal and external

Fertilisation is the fusion of the nuclei of male and female gametes. It forms a diploid zygote.

External fertilisation

In external fertilisation, eggs and sperm are released into the surrounding environment, usually water, and gametes meet outside the female's body. Many bony fish and amphibians use this method.

Successful external fertilisation often depends on:

• release of many eggs and sperm at approximately the same time
• a watery environment that allows sperm to swim
• suitable spawning sites and environmental conditions
• reproductive behaviour or signals that synchronise gamete release

A major disadvantage is that many gametes and developing embryos may be eaten, washed away or fail to meet. Fertilisation and early development are therefore less protected.

Internal fertilisation

In internal fertilisation, sperm is deposited in the female reproductive tract and fuses with an ovum inside the body. It occurs in mammals, birds, reptiles and many insects.

Internal fertilisation reduces the risk of gametes drying out and can increase the chance that sperm reaches an egg. It is an adaptation to reproduction on land, although internal fertilisation also occurs in aquatic animals. It often involves courtship and specialised reproductive organs.

These are broad patterns. Not every fish uses external fertilisation, and some aquatic animals use internal fertilisation.

Reproduction in fish

Many bony fish reproduce by spawning. The female releases eggs into water, and the male releases milt containing sperm over or near them. Fertilisation is usually external. The eggs develop into embryos and hatch into young fish.

Fish often produce many eggs because eggs and young may be eaten or lost. Some species provide little parental care, while others guard eggs, build nests or protect young. Reproductive strategies differ widely between species.

In a typical fish life cycle:

1. Adult fish become reproductively mature.
2. A female releases eggs and a male releases sperm.
3. Fertilisation occurs in water.
4. The embryo develops inside the egg.
5. The egg hatches and a young fish emerges.
6. The young fish grows into an adult capable of reproduction.

Reproduction in amphibians

Many frogs reproduce in water. During amplexus, the male clasps the female and releases sperm as she lays eggs. Fertilisation is generally external. The jelly-like egg coverings help protect the eggs and keep them together.

A typical frog develops through metamorphosis:

• eggs are laid in water
• embryos develop and hatch into tadpoles
• tadpoles have tails and gills and live in water
• limbs develop, lungs become functional and the tail is gradually absorbed
• the young frog becomes an adult that can reproduce

This is a life cycle involving a major change in body form and habitat. Not all amphibians have identical life cycles; some have direct development or different forms of parental care.

Reproduction in birds

Birds reproduce by internal fertilisation. During mating, sperm enters the female reproductive tract and fertilises an ovum. The developing egg passes through the oviduct, where albumen (egg white), shell membranes and a hard shell are added.

A typical bird's egg contains:

• shell: reduces physical damage and water loss while allowing gas exchange through pores
• shell membranes: provide additional protection
• albumen: supplies water and protein and cushions the embryo
• yolk: nutrient store for the developing embryo
• embryo: developing young bird
• air space: provides an air supply near hatching

The shell is porous, so oxygen can enter and carbon dioxide can leave. The embryo uses yolk as its principal food reserve. The egg is incubated at a suitable temperature, often by one or both parents. After hatching, many birds feed and protect their chicks.

Birds lay eggs with protective coverings and food reserves. This is an adaptation that allows embryonic development outside the mother's body.

Reproduction in mammals

Mammals generally use internal fertilisation. The male deposits sperm in the female reproductive tract. In mammals such as humans, fertilisation usually occurs in the oviduct (fallopian tube). The zygote divides as it moves towards the uterus and develops into an embryo.

In most mammals, the embryo develops inside the uterus. The placenta forms an exchange surface between maternal and fetal blood supplies. It allows oxygen and dissolved nutrients to pass to the fetus and carbon dioxide and other wastes to pass to the mother. Maternal and fetal blood normally do not mix directly. The umbilical cord connects the fetus to the placenta.

The young is born alive in most mammals. The mother commonly provides milk from mammary glands and may protect and care for the offspring. There are exceptions: monotremes such as the platypus lay eggs, while marsupials give birth to relatively undeveloped young that usually continue development in a pouch.

Reproductive success and parental care

Animals invest different amounts of energy in each offspring. Many fish release very large numbers of eggs and provide little care. Many mammals produce fewer young and invest substantial energy in pregnancy, feeding and protection. Birds commonly protect eggs through incubation and feed or defend their chicks.

Parental care can increase the chance that offspring survive, but it requires time and energy. Reproductive strategies reflect trade-offs between offspring number, offspring size, protection and parental investment.

WASSCE examination focus

• Fertilisation is the fusion of gamete nuclei; it is not the same as mating or egg-laying.
• A zygote is diploid because the haploid nuclei of two gametes fuse.
• External fertilisation usually occurs in water and commonly involves many gametes.
• Birds and most mammals use internal fertilisation.
• Do not state that all fish have external fertilisation or that all mammals give birth to fully developed young.
• For a comparison question, organise points under site of fertilisation, gamete number, protection and examples.
• Remember that the placenta is an exchange organ; maternal and fetal blood normally remain separate.

Quick check

1. Define fertilisation.
2. State two differences between internal and external fertilisation.
3. Why do many fish release large numbers of eggs?
4. What is spawning?
5. State two functions of the yolk in a bird's egg.
6. What is the role of the placenta in most mammals?
7. Explain why sexual reproduction produces genetic variation.

Answers

1. The fusion of the nuclei of male and female gametes to form a zygote.
2. External fertilisation occurs outside the female's body and is usually associated with water; internal fertilisation occurs inside the female and protects gametes during fertilisation.
3. Many eggs and young are lost through predation or environmental conditions, so producing many increases the chance that some survive.
4. The release of eggs and sperm into water by fish, usually for external fertilisation.
5. It stores nutrients for embryonic development and supplies much of the energy and material needed by the embryo.
6. It enables exchange of oxygen and nutrients from the mother to the fetus and removal of carbon dioxide and other wastes from the fetus.
7. Meiosis produces genetically different gametes, and fertilisation combines genetic material from two parents.