A Hen's Egg Is Fertilised Inside and Then Developed Outside
Learn how external and internal fertilisation differ, what separates oviparous from viviparous animals, why some animals have thousands of young and others only one, and why variation matters.
Is a hen's egg fertilised inside the hen or outside?
Inside — and the confusion this causes is worth clearing up before anything else.
A hen's egg is fertilised internally, before the shell forms around it. Only afterwards is it laid, and the embryo then develops outside the mother's body, living on the yolk until it hatches.
So the hen uses internal fertilisation and is oviparous. Those are two separate facts about two separate stages.
- Where the gametes fuse is a question about fertilisation — internal or external
- Where the embryo develops is a question about oviparous or viviparous
A frog answers both questions with outside. A human answers both with inside. A hen answers one each way — fertilised inside, developed outside — and that is why the hen is the example every question on this topic uses. This page covers the third part of the CBSE Class 9 Science chapter on reproduction.
A hen's egg is fertilised internally, before the shell forms around it. Only afterwards is it laid, and the embryo then develops outside the mother's body, living on the yolk until it hatches.
So the hen uses internal fertilisation and is oviparous. Those are two separate facts about two separate stages.
- Where the gametes fuse is a question about fertilisation — internal or external
- Where the embryo develops is a question about oviparous or viviparous
A frog answers both questions with outside. A human answers both with inside. A hen answers one each way — fertilised inside, developed outside — and that is why the hen is the example every question on this topic uses. This page covers the third part of the CBSE Class 9 Science chapter on reproduction.
How does external fertilisation differ from internal fertilisation?
By whether the gametes fuse outside the body or inside it.
External fertilisation. The female releases her eggs into the surrounding water and the male releases sperm over them. Fusion happens outside both bodies.
- Examples: most fish, frogs and other amphibians
- It requires water, because sperm can only swim to an egg through a liquid, and eggs laid in air would dry out
- Very large numbers of eggs and sperm are released, because most gametes never meet, and many of the zygotes that do form are eaten or die
Internal fertilisation. The male deposits sperm inside the female's body, and fusion happens inside her.
- Examples: birds, reptiles, mammals, and also some fish
- No external water is needed, because the female's body provides the moist environment the sperm need
- Far fewer eggs are produced, because fertilisation is much more likely to succeed
Everyday evidence. During the monsoon a pond fills with masses of frog eggs held together in jelly — hundreds of them, laid and fertilised in the open water, with no parent anywhere nearby. A hen, by contrast, produces one egg at a time.
External fertilisation ties an animal to water. No land animal uses it, and it cannot be used away from a pond, river or sea. Internal fertilisation removed that restriction, and it is what allowed vertebrates to reproduce in dry places — which is why every animal living entirely on land fertilises internally.
The number of gametes follows from the risk. Where fusion is left to chance in open water, the only way to make it reliable is to release an enormous quantity. Where fusion happens inside the body, a few eggs suffice. The reproductive numbers are not arbitrary; they are set by how likely each gamete is to succeed — and the same reasoning governs the number of offspring, which is the subject two sections further on.
External fertilisation. The female releases her eggs into the surrounding water and the male releases sperm over them. Fusion happens outside both bodies.
- Examples: most fish, frogs and other amphibians
- It requires water, because sperm can only swim to an egg through a liquid, and eggs laid in air would dry out
- Very large numbers of eggs and sperm are released, because most gametes never meet, and many of the zygotes that do form are eaten or die
Internal fertilisation. The male deposits sperm inside the female's body, and fusion happens inside her.
- Examples: birds, reptiles, mammals, and also some fish
- No external water is needed, because the female's body provides the moist environment the sperm need
- Far fewer eggs are produced, because fertilisation is much more likely to succeed
Everyday evidence. During the monsoon a pond fills with masses of frog eggs held together in jelly — hundreds of them, laid and fertilised in the open water, with no parent anywhere nearby. A hen, by contrast, produces one egg at a time.
External fertilisation ties an animal to water. No land animal uses it, and it cannot be used away from a pond, river or sea. Internal fertilisation removed that restriction, and it is what allowed vertebrates to reproduce in dry places — which is why every animal living entirely on land fertilises internally.
The number of gametes follows from the risk. Where fusion is left to chance in open water, the only way to make it reliable is to release an enormous quantity. Where fusion happens inside the body, a few eggs suffice. The reproductive numbers are not arbitrary; they are set by how likely each gamete is to succeed — and the same reasoning governs the number of offspring, which is the subject two sections further on.
What is the difference between oviparous and viviparous animals?
Where the embryo develops — outside the mother's body in an egg, or inside it.
Oviparous animals lay eggs. The embryo develops outside the mother's body, nourished by the yolk stored in the egg, and protected by a shell or a membrane. The young hatch.
- Examples: birds, most reptiles, fish, amphibians, insects
Viviparous animals give birth to young. The embryo develops inside the mother's body, nourished through the placenta, and the young are born.
- Examples: most mammals — humans, cows, dogs, cats, elephants
The three standard cases, set out together.
- Frog: fertilisation external, development external — oviparous
- Hen: fertilisation internal, development external — oviparous
- Human: fertilisation internal, development internal — viviparous
Everyday evidence. A hen sits on her eggs to keep them warm, because the embryos inside are developing in the open air and need heat supplied from outside. A cow needs to do nothing of the kind — the calf develops at her own body temperature inside her.
Internal fertilisation does not mean viviparous. This is the single most examined point on the page, and the hen is the reason. Every bird and almost every reptile fertilises internally and lays eggs. The two terms describe different stages, and a question giving you one is not telling you the other.
What the two strategies trade. An egg laid outside is vulnerable — it can be eaten, or chilled, or dried out — but it costs the mother nothing to carry once laid, and several can be produced at a time. An embryo carried inside is protected and supplied continuously, and the mother bears the weight and the risk for the whole period, so only one or a few can be managed.
Protection has a price, and it is paid in numbers. That is the pattern the next section states as a general rule, and both the fertilisation and the development choices on this page are examples of it.
Oviparous animals lay eggs. The embryo develops outside the mother's body, nourished by the yolk stored in the egg, and protected by a shell or a membrane. The young hatch.
- Examples: birds, most reptiles, fish, amphibians, insects
Viviparous animals give birth to young. The embryo develops inside the mother's body, nourished through the placenta, and the young are born.
- Examples: most mammals — humans, cows, dogs, cats, elephants
The three standard cases, set out together.
- Frog: fertilisation external, development external — oviparous
- Hen: fertilisation internal, development external — oviparous
- Human: fertilisation internal, development internal — viviparous
Everyday evidence. A hen sits on her eggs to keep them warm, because the embryos inside are developing in the open air and need heat supplied from outside. A cow needs to do nothing of the kind — the calf develops at her own body temperature inside her.
Internal fertilisation does not mean viviparous. This is the single most examined point on the page, and the hen is the reason. Every bird and almost every reptile fertilises internally and lays eggs. The two terms describe different stages, and a question giving you one is not telling you the other.
What the two strategies trade. An egg laid outside is vulnerable — it can be eaten, or chilled, or dried out — but it costs the mother nothing to carry once laid, and several can be produced at a time. An embryo carried inside is protected and supplied continuously, and the mother bears the weight and the risk for the whole period, so only one or a few can be managed.
Protection has a price, and it is paid in numbers. That is the pattern the next section states as a general rule, and both the fertilisation and the development choices on this page are examples of it.
Why do some animals have thousands of young and others only one?
Because an animal has a limited amount of energy, and it can spend it on producing many young or on caring for a few, not both.
There is a consistent trade-off:
- Many offspring, little or no parental care. Most die, and the strategy works through sheer numbers
- Few offspring, a great deal of parental care. A high proportion survive, so fewer need be produced
Many with little care. A fish releases very large numbers of eggs into open water and swims away. A frog lays hundreds. Nothing is done to protect them, and the great majority are eaten or fail to develop. The survivors are enough to continue the species, and that is all the strategy requires.
Few with much care. A bird lays a small clutch, incubates the eggs to keep them warm, guards the nest, and then feeds the chicks until they can fly. A mammal carries one or a few young inside her, gives birth, and then feeds them on milk and protects them for weeks or months.
Everyday evidence. A stray dog has a litter of a few pups, keeps them in one place and guards them fiercely, and most of the litter survives. A fish in the same locality releases far more eggs than the dog has pups in a lifetime, and almost none of them reaches adulthood.
Survival rate is what links the two halves. If care raises the chance that each offspring survives, then fewer offspring are needed for the same result. If no care is given, the chance per offspring is very low, and only large numbers can compensate.
Neither strategy is better in the abstract. Each suits particular conditions. Where open water offers no way to protect eggs, producing an enormous number is the only workable approach. Where a nest or a body can shelter the young, investing in a few is far more efficient.
What is fixed is the total investment, not the number of young. An animal that produced thousands of eggs and guarded each one would need energy it does not have. So the question why only one calf? is answered not by the cow's biology alone but by the arithmetic of what a body can afford — and that is the same trade-off between quantity and quality of provision that runs through the whole of this chapter.
There is a consistent trade-off:
- Many offspring, little or no parental care. Most die, and the strategy works through sheer numbers
- Few offspring, a great deal of parental care. A high proportion survive, so fewer need be produced
Many with little care. A fish releases very large numbers of eggs into open water and swims away. A frog lays hundreds. Nothing is done to protect them, and the great majority are eaten or fail to develop. The survivors are enough to continue the species, and that is all the strategy requires.
Few with much care. A bird lays a small clutch, incubates the eggs to keep them warm, guards the nest, and then feeds the chicks until they can fly. A mammal carries one or a few young inside her, gives birth, and then feeds them on milk and protects them for weeks or months.
Everyday evidence. A stray dog has a litter of a few pups, keeps them in one place and guards them fiercely, and most of the litter survives. A fish in the same locality releases far more eggs than the dog has pups in a lifetime, and almost none of them reaches adulthood.
Survival rate is what links the two halves. If care raises the chance that each offspring survives, then fewer offspring are needed for the same result. If no care is given, the chance per offspring is very low, and only large numbers can compensate.
Neither strategy is better in the abstract. Each suits particular conditions. Where open water offers no way to protect eggs, producing an enormous number is the only workable approach. Where a nest or a body can shelter the young, investing in a few is far more efficient.
What is fixed is the total investment, not the number of young. An animal that produced thousands of eggs and guarded each one would need energy it does not have. So the question why only one calf? is answered not by the cow's biology alone but by the arithmetic of what a body can afford — and that is the same trade-off between quantity and quality of provision that runs through the whole of this chapter.
How does variation help a species survive?
Because a varied population is likely to contain individuals that can survive a change, and a uniform one may contain none.
Sexual reproduction produces offspring that differ from one another and from both parents. As the earlier parts of this chapter established, that variation comes from meiosis shuffling the chromosomes and from fertilisation combining gametes from two different parents.
Why variation is the point. Conditions change — a new disease appears, the climate shifts, a new predator arrives, a food source disappears. In a varied population, some individuals will happen to have features that let them cope: a resistance, a tolerance, a slightly different habit. Those individuals survive and reproduce, and their features become commoner in the next generation.
In a uniform population, either the change is survivable by everyone or by nobody. There is no middle outcome, and the species has no way to adapt.
Over many generations this changes the species. The individuals better suited to the conditions leave more offspring, so the population gradually comes to resemble them. That gradual change is the basis of evolution, and variation is its raw material.
Everyday evidence. A field planted entirely with one clonal variety of a crop can be destroyed completely by a single disease, because every plant has the same vulnerability. A field of a mixed traditional variety usually contains some plants that resist it, and those plants both survive and provide seed for the following season.
Variation is the advantage, not a by-product. Sexual reproduction is worse than asexual reproduction on every practical measure — it is slower, needs two parents, costs far more energy in gametes, courtship, flowers or nectar, and produces fewer offspring. It persists throughout the living world in spite of all of that, and the reason is variation.
So the two halves of this chapter answer each other. The first part showed that asexual reproduction is fast, cheap and certain, and produces clones that cannot adapt. This part shows that sexual reproduction is slow, expensive and produces variation that can. Neither is superior; each buys something the other cannot — and that is why both exist, and why many organisms use one in favourable conditions and switch to the other when conditions turn difficult.
Sexual reproduction produces offspring that differ from one another and from both parents. As the earlier parts of this chapter established, that variation comes from meiosis shuffling the chromosomes and from fertilisation combining gametes from two different parents.
Why variation is the point. Conditions change — a new disease appears, the climate shifts, a new predator arrives, a food source disappears. In a varied population, some individuals will happen to have features that let them cope: a resistance, a tolerance, a slightly different habit. Those individuals survive and reproduce, and their features become commoner in the next generation.
In a uniform population, either the change is survivable by everyone or by nobody. There is no middle outcome, and the species has no way to adapt.
Over many generations this changes the species. The individuals better suited to the conditions leave more offspring, so the population gradually comes to resemble them. That gradual change is the basis of evolution, and variation is its raw material.
Everyday evidence. A field planted entirely with one clonal variety of a crop can be destroyed completely by a single disease, because every plant has the same vulnerability. A field of a mixed traditional variety usually contains some plants that resist it, and those plants both survive and provide seed for the following season.
Variation is the advantage, not a by-product. Sexual reproduction is worse than asexual reproduction on every practical measure — it is slower, needs two parents, costs far more energy in gametes, courtship, flowers or nectar, and produces fewer offspring. It persists throughout the living world in spite of all of that, and the reason is variation.
So the two halves of this chapter answer each other. The first part showed that asexual reproduction is fast, cheap and certain, and produces clones that cannot adapt. This part shows that sexual reproduction is slow, expensive and produces variation that can. Neither is superior; each buys something the other cannot — and that is why both exist, and why many organisms use one in favourable conditions and switch to the other when conditions turn difficult.
Exam tip
Exam tip: answer fertilisation and development as two separate questions
Fertilisation is internal or external; development is oviparous or viviparous. They are separate stages, and a question giving one tells you nothing about the other.
Learn the three cases as a set: frog — external, oviparous; hen — internal, oviparous; human — internal, viviparous.
External fertilisation needs water, which is why no fully land animal uses it.
External fertilisation means very many gametes because fusion is left to chance; internal fertilisation means far fewer.
Oviparous means the embryo lives on the yolk outside the body and the young hatch; viviparous means it is nourished through the placenta and the young are born.
For the numbers question, state the trade-off explicitly: many offspring with little care, or few offspring with much care — and give one example of each.
Name the forms of care: incubating eggs, guarding a nest, feeding chicks, and feeding young on milk.
Say that neither strategy is better — each suits its conditions, and the total energy available is what is fixed.
For variation, give both sources: meiosis shuffles and fertilisation combines two parents.
And when comparing sexual with asexual reproduction, concede the disadvantages — slower, two parents, more energy, fewer offspring — and name variation as what buys them off.
Learn the three cases as a set: frog — external, oviparous; hen — internal, oviparous; human — internal, viviparous.
External fertilisation needs water, which is why no fully land animal uses it.
External fertilisation means very many gametes because fusion is left to chance; internal fertilisation means far fewer.
Oviparous means the embryo lives on the yolk outside the body and the young hatch; viviparous means it is nourished through the placenta and the young are born.
For the numbers question, state the trade-off explicitly: many offspring with little care, or few offspring with much care — and give one example of each.
Name the forms of care: incubating eggs, guarding a nest, feeding chicks, and feeding young on milk.
Say that neither strategy is better — each suits its conditions, and the total energy available is what is fixed.
For variation, give both sources: meiosis shuffles and fertilisation combines two parents.
And when comparing sexual with asexual reproduction, concede the disadvantages — slower, two parents, more energy, fewer offspring — and name variation as what buys them off.
Did you know
Why a seahorse upsets every rule on this page
The categories on this page are useful and they are not laws of nature, and one familiar fish makes that plain.
In a seahorse, the female transfers her eggs to a pouch on the male's body. He carries them, they develop there, and he eventually releases the young. The animal that gives birth is the male — which no neat division into mothers who carry and fathers who do not can accommodate.
Other cases stretch the terms in other directions. Some sharks and some snakes retain their eggs inside the body until they hatch, so the young emerge alive although they were nourished by yolk rather than by a placenta. That is neither straightforwardly oviparous nor straightforwardly viviparous, and biologists give it a third name of its own.
And parental care is not confined to birds and mammals. Some fish guard their eggs and fan fresh water over them, some frogs carry tadpoles on their backs, and a few insects guard their eggs until they hatch — all of them animals the simple rule would place firmly in the many offspring, no care group.
None of that makes the categories wrong. They describe the common pattern accurately, and they explain a great deal — the link between external fertilisation and water, between care and small numbers, between protection and cost. What the exceptions show is that the pattern is a tendency rather than a rule, and the reason is that each species faces its own conditions and settles on its own compromise.
That is the same lesson the opening chapter of this course drew about models. A category that covers most cases and is honest about its limits is a useful description. One that pretends to cover everything is just wrong in places nobody has looked.
In a seahorse, the female transfers her eggs to a pouch on the male's body. He carries them, they develop there, and he eventually releases the young. The animal that gives birth is the male — which no neat division into mothers who carry and fathers who do not can accommodate.
Other cases stretch the terms in other directions. Some sharks and some snakes retain their eggs inside the body until they hatch, so the young emerge alive although they were nourished by yolk rather than by a placenta. That is neither straightforwardly oviparous nor straightforwardly viviparous, and biologists give it a third name of its own.
And parental care is not confined to birds and mammals. Some fish guard their eggs and fan fresh water over them, some frogs carry tadpoles on their backs, and a few insects guard their eggs until they hatch — all of them animals the simple rule would place firmly in the many offspring, no care group.
None of that makes the categories wrong. They describe the common pattern accurately, and they explain a great deal — the link between external fertilisation and water, between care and small numbers, between protection and cost. What the exceptions show is that the pattern is a tendency rather than a rule, and the reason is that each species faces its own conditions and settles on its own compromise.
That is the same lesson the opening chapter of this course drew about models. A category that covers most cases and is honest about its limits is a useful description. One that pretends to cover everything is just wrong in places nobody has looked.
Exam relevance
Why does NEET keep coming back to reproductive strategies?
Because the paired terms on this page are exactly the kind of clean factual distinction NEET tests, and two Class 12 chapters assume them.
This is the foundation for the Class 12 Biology chapter Reproduction in Organisms, a standing part of the NEET syllabus. That chapter uses oviparous and viviparous with the same meanings, adds the distinction between oestrous and menstrual cycles in mammals, and formalises external and internal fertilisation with the same examples. Nothing on this page is revised later; it is simply used.
The number-and-care trade-off feeds into Class 12 Evolution and into ecology, where it becomes the formal contrast between species that produce many poorly provisioned young and those that produce few well provisioned ones. NEET questions on population growth and survivorship rest on it.
The variation argument is the direct foundation of Class 12 Evolution. Variation as the raw material of natural selection, and the consequence that a uniform population cannot adapt, are the first two ideas that chapter needs. It is also the basis of Class 12 Principles of Inheritance and Variation, where the shuffling in meiosis becomes recombination.
Internal fertilisation freeing animals from water connects to Class 11 Animal Kingdom, where the amniotic egg and internal fertilisation are used to characterise the reptiles, birds and mammals.
What the questions look like. Match-the-column items pairing an animal with its mode of fertilisation or development are the commonest form, and the pair most often got wrong is the hen — internal fertilisation with external development. Assertion-reason questions favour the statements that external fertilisation requires water, and that sexual reproduction persists because of variation despite its costs. Statement-count questions of the how many of the following are viviparous kind suit the topic, because so many familiar animals must be classified correctly.
How board and competitive emphasis differ. A board paper asks you to give two differences between external and internal fertilisation, define oviparous and viviparous with two examples each, and explain how variation helps survival. A NEET item gives an animal and asks for both classifications at once, or lists four animals and asks which share a mode — so the examples matter far more than the definitions, and the hen must be right.
The single trap that costs the most marks. Assuming internal fertilisation implies viviparous. Birds and reptiles fertilise internally and lay eggs, so they are oviparous. Learn the hen as the deliberate counter-example and the trap stops working.
A second trap worth naming. Answering sexual reproduction is better than asexual without qualification. Sexual reproduction is slower, costlier and produces fewer offspring; its single advantage is variation, and that advantage only matters when conditions change. A question asking which is better is testing whether you can state the trade-off rather than pick a side.
This is the foundation for the Class 12 Biology chapter Reproduction in Organisms, a standing part of the NEET syllabus. That chapter uses oviparous and viviparous with the same meanings, adds the distinction between oestrous and menstrual cycles in mammals, and formalises external and internal fertilisation with the same examples. Nothing on this page is revised later; it is simply used.
The number-and-care trade-off feeds into Class 12 Evolution and into ecology, where it becomes the formal contrast between species that produce many poorly provisioned young and those that produce few well provisioned ones. NEET questions on population growth and survivorship rest on it.
The variation argument is the direct foundation of Class 12 Evolution. Variation as the raw material of natural selection, and the consequence that a uniform population cannot adapt, are the first two ideas that chapter needs. It is also the basis of Class 12 Principles of Inheritance and Variation, where the shuffling in meiosis becomes recombination.
Internal fertilisation freeing animals from water connects to Class 11 Animal Kingdom, where the amniotic egg and internal fertilisation are used to characterise the reptiles, birds and mammals.
What the questions look like. Match-the-column items pairing an animal with its mode of fertilisation or development are the commonest form, and the pair most often got wrong is the hen — internal fertilisation with external development. Assertion-reason questions favour the statements that external fertilisation requires water, and that sexual reproduction persists because of variation despite its costs. Statement-count questions of the how many of the following are viviparous kind suit the topic, because so many familiar animals must be classified correctly.
How board and competitive emphasis differ. A board paper asks you to give two differences between external and internal fertilisation, define oviparous and viviparous with two examples each, and explain how variation helps survival. A NEET item gives an animal and asks for both classifications at once, or lists four animals and asks which share a mode — so the examples matter far more than the definitions, and the hen must be right.
The single trap that costs the most marks. Assuming internal fertilisation implies viviparous. Birds and reptiles fertilise internally and lay eggs, so they are oviparous. Learn the hen as the deliberate counter-example and the trap stops working.
A second trap worth naming. Answering sexual reproduction is better than asexual without qualification. Sexual reproduction is slower, costlier and produces fewer offspring; its single advantage is variation, and that advantage only matters when conditions change. A question asking which is better is testing whether you can state the trade-off rather than pick a side.
Key takeaways
Fertilisation, development and variation in animals: quick revision
- Fertilisation is internal or external; development is oviparous or viviparous. Two separate questions about two separate stages.
- External fertilisation: gametes fuse outside the body, in water. Fish, frogs and amphibians. Requires water, and needs very large numbers of gametes.
- Internal fertilisation: gametes fuse inside the female. Birds, reptiles, mammals and some fish. Needs no external water, and produces far fewer eggs.
- Internal fertilisation is what allowed animals to reproduce away from water — no fully land animal fertilises externally.
- Oviparous: the embryo develops outside, on the yolk, and the young hatch. Birds, most reptiles, fish, amphibians, insects.
- Viviparous: the embryo develops inside, nourished by the placenta, and the young are born. Most mammals.
- The three cases: frog external and oviparous; hen internal and oviparous; human internal and viviparous.
- Internal fertilisation does not mean viviparous — the hen proves it.
- A hen incubates her eggs because the embryos need warmth from outside; a cow does not, because the calf is inside her.
- The trade-off: many offspring with little care, or few offspring with much care. An animal cannot afford both.
- Fish and frogs release very many eggs and give no care; birds incubate and feed chicks; mammals gestate and feed on milk.
- Survival rate links the two: more care per offspring means fewer offspring are needed.
- Neither strategy is better — each suits its conditions, and the total energy is what is fixed.
- Variation comes from meiosis shuffling chromosomes and fertilisation combining two different parents.
- A varied population is likely to contain individuals that survive a change; a uniform one may contain none — which is why a single clonal crop can be lost entirely to one disease.
- Over generations this gradual change is the basis of evolution.
- Sexual reproduction is slower, needs two parents, costs more energy and gives fewer offspring — and it persists because of variation.
Take six animals and classify each twice, by fertilisation and by development — the ones that take a different answer to each question are the ones examiners choose.
- External fertilisation: gametes fuse outside the body, in water. Fish, frogs and amphibians. Requires water, and needs very large numbers of gametes.
- Internal fertilisation: gametes fuse inside the female. Birds, reptiles, mammals and some fish. Needs no external water, and produces far fewer eggs.
- Internal fertilisation is what allowed animals to reproduce away from water — no fully land animal fertilises externally.
- Oviparous: the embryo develops outside, on the yolk, and the young hatch. Birds, most reptiles, fish, amphibians, insects.
- Viviparous: the embryo develops inside, nourished by the placenta, and the young are born. Most mammals.
- The three cases: frog external and oviparous; hen internal and oviparous; human internal and viviparous.
- Internal fertilisation does not mean viviparous — the hen proves it.
- A hen incubates her eggs because the embryos need warmth from outside; a cow does not, because the calf is inside her.
- The trade-off: many offspring with little care, or few offspring with much care. An animal cannot afford both.
- Fish and frogs release very many eggs and give no care; birds incubate and feed chicks; mammals gestate and feed on milk.
- Survival rate links the two: more care per offspring means fewer offspring are needed.
- Neither strategy is better — each suits its conditions, and the total energy is what is fixed.
- Variation comes from meiosis shuffling chromosomes and fertilisation combining two different parents.
- A varied population is likely to contain individuals that survive a change; a uniform one may contain none — which is why a single clonal crop can be lost entirely to one disease.
- Over generations this gradual change is the basis of evolution.
- Sexual reproduction is slower, needs two parents, costs more energy and gives fewer offspring — and it persists because of variation.
Take six animals and classify each twice, by fertilisation and by development — the ones that take a different answer to each question are the ones examiners choose.