science

Internal fertilization

Internal fertilization is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Internal fertilization rather than just read about it. In short: Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction.

Key takeaways

Internal fertilization belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Internal fertilization to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Internal fertilization from memory before moving on to harder problems.

Reference excerpt

Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. Most taxa that reproduce by internal fertilization are gonochoric. Male animals inseminate females in order to internally fertilize their egg cells. Most reptiles and certain other vertebrates transfer sperm through a male intromittent organ into the female cloaca during copulation. Mammals ejaculate sperm and seminal fluid through the male penis into the female vagina. Most birds use the "cloacal kiss," pressing cloacas together to transfer sperm. Salamanders, spiders, some insects and some molluscs undertake internal fertilization by transferring a spermatophore, a bundle of sperm, from the male to the female. After fertilization, embryos develop in eggs in oviparous species or inside the mother’s reproductive tract in viviparous ones.

Evolution of internal fertilization

Internal fertilization evolved many times in animals. According to David B. Dusenbery all the features with internal fertilization were most likely a result from oogamy. It has been argued that internal fertilization evolve because of sexual selection through sperm competition. In amphibians, internal fertilization evolved from external fertilization.

Methods of internal fertilization Fertilization which takes place inside the female body is called internal fertilization in animals is done through the following different ways:

Copulation, which involves the insertion of the penis or other intromittent organ into the vagina (in most mammals) or to the cloaca in monotremes, most reptiles, some birds, the tailed frog, some fish, the disappeared dinosaurs, as well as in other non-vertebrate animals. Cloacal kiss, which consists in that the two animals touch their cloacae together in order to transfer the sperm of the male to the female. It is used in most birds and in the tuatara, that do not have an intromittent organ. Via spermatophore, a sperm-containing cap placed by the male in the female's cloaca. Usually, the sperm is stored in spermathecae on the roof of the cloaca until it is needed at the time of oviposition. It is used by some salamander and newt species, by the Arachnida, some insects and some mollusks. In sponges, sperm cells are released into the water to fertilize ova that are retained by the female. Some species of sponge participate in external fertilization where the ova is released.

Processes unique to internal fertilization Internal fertilization involves different processes than external fertilization. Some of these unique processes include the following: sperm capacitation, building reservoirs of sperm/ sperm pockets within the oviduct, sperm migration within the oviduct, acrosomal reactions, and sperm competition. Sperm capacitation has been studied primarily in mammals such as humans, rodents, and bovines. This is an especially important process for internal fertilization because it ensures that sperm are activated at the right time and place to promote the greatest success with fertilization The prevention of polyspermy is also an important process of internal fertilization. Internal fertilization utilizes a slow block, or cortical reaction, that uses different mechanisms and components than the mechanisms and components used during external fertilization. While external fertilization also utilizes a fast block system, little is known about a fast block response associated with species that fertilize internally.

Expulsion

At some point, the growing egg or offspring must be expelled. There are several possible modes of reproduction. These are traditionally classified as follows:

Oviparity, as in most invertebrates and reptiles, monotremes, dinosaurs and all birds which lay eggs that continue to develop after being laid, and hatch later. Viviparity, as in almost all mammals (such as whales, kangaroos and humans) which bear their young live. The developing young spend proportionately more time within the female's reproductive tract. The young are later released to survive on their own, with varying amounts of help from the parent(s) of the species. Ovoviviparity, as in the garter snake, most vipers, and the Madagascar hissing cockroach, which have eggs (with shells) that hatch as they are laid, making it resemble live birth.

Advantages to internal fertilization Internal fertilization allows for:

Female mate choice, which gives the female the ability to choose her partner before and after mating. The female cannot do this with external fertilization because she may have limited control of who is fertilizing her eggs, and when they are being fertilized. Making a decision for the conditions of reproduction, like location and time. In external fertilization a female can only choose the time in which she releases her eggs, but not when they are fertilized. This is similar, in ways, to cryptic female choice. Egg protection on dry land. While oviparous animals either have a jelly like ovum or a hard shell enclosing their egg, internally fertilizing animals grow their eggs and offspring inside themselves. This offers protection from predators and from dehydration on land. This allows for a higher chance of survival when there is a regulated temperature and protected area within the mother.

Disadvantages to internal fertilization Gestation can and will add additional risks for the mother. The additional risks from gestation come from extra energy demands. Along with internal fertilization comes sexual reproduction, in most cases. Sexual reproduction comes with some risks as well. The risks with sexual reproduction are with intercourse, it is infrequent and only works well during peak fertility. While animals which externally fertilize are able to release egg and sperm, usually into the water, not needing a specific partner to reproduce. Fewer offspring are produced through internal fertilization in comparison to external fertilization. This is both because the mother cannot hold and grow as many offspring as eggs, and the mother cannot provide and obtain enough resources for a larger amount of offspring.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Internal fertilization

Start with the simplest possible case. Write down what Internal fertilization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Internal fertilization before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Internal fertilization ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Internal fertilization

In research: Internal fertilization appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Internal fertilization in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Internal fertilization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Reproduction in animals, so understanding it makes those chapters shorter.
In everyday life: Look for Internal fertilization outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Internal fertilization in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Internal fertilization means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Internal fertilization out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Internal fertilization in simple terms?

Why does Internal fertilization matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Internal fertilization?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Internal fertilization.