Stage 1 of 4
Egg
Laid on a leaf, with only enough yolk to make a small larva.
Class 9 BiologyChapter 11NCERT 11.3 and 11.4
Sexual reproduction in animals
A frog lays thousands of eggs, a bird lays a handful. Both strategies work.
Think it over
- A frog lays thousands of eggs in a pond, yet ponds are not overflowing with frogs. What happens to most of the eggs?
- A hen lays one egg at a time. How can laying so few still keep the species going?
- A caterpillar and a butterfly look nothing alike. Why would one animal have two such different bodies in its life?
In one minute
- Animals reproduce asexually (budding in hydra) and/or sexually. Sexual reproduction must get the gametes together and keep the young alive.
- External fertilisation: frogs and most fish, in water. Many eggs, many lost, low survival.
- Internal fertilisation, in reptiles, birds and mammals: fewer eggs, better protected, higher survival.
- Insects, fish and amphibians lay many eggs with just enough yolk to make a larva, a feeding stage that later transforms into the adult: egg, larva, pupa, adult in a butterfly.
- Reptile and bird eggs hold enough yolk for the young to hatch; in mammals the young grow inside the mother and are fed on milk after birth.
Outside or inside the body?
Animals reproduce asexually, sexually, or both. You have already met budding in hydra. In sexual reproduction the key question is where the sperm meets the egg. Switch between the two below.
External
Frogs and most fish: eggs and sperm are both shed into the water, sperm over the eggs, so fertilisation happens outside the body. Eggs come by the thousand, since currents and hungry animals take so many.
- 1External
Frogs and most fish: eggs and sperm are both shed into the water, sperm over the eggs, so fertilisation happens outside the body. Eggs come by the thousand, since currents and hungry animals take so many.
- 2Internal
Reptiles, birds, mammals: here sperm and egg meet inside the mother. Far fewer eggs are made, but each is sheltered, so more of the young usually survive.
Frogs and most fish breed in water. Eggs and sperm are shed straight into it: first the eggs, then sperm over them, so the meeting happens outside any body. This is external fertilisation. The water is a dangerous place for eggs. Currents sweep many away and other animals eat many more, so these animals lay them in huge numbers.
Turn to reptiles, birds and mammals and the picture flips. Sperm reach the egg inside the female's body, which is internal fertilisation. Sheltered there, the fertilised egg or embryo is safer, and more of the young usually survive.
Quick check
Which of these animals fertilises its eggs inside the female's body?
Show the answer
C. A pigeon Birds, like reptiles and mammals, use internal fertilisation. Frogs and most fish use external fertilisation, in water.
Many eggs or few: Table 11.4 as a chart
Two problems face every animal that reproduces sexually, whichever method it uses: getting sperm and egg together, and keeping the young alive until they are old enough to breed. Four animals from Table 11.4 show two very different answers.
- FishWater, external fertilisationHundreds to thousands eggs at a timeLow
- FrogWater and land, external fertilisation5,000 to 50,000 eggs at a timeLow
- LizardLand, internal fertilisation2 to 20 eggs at a timeModerate
- BirdWater and land, internal fertilisation1 to 15 eggs at a timeModerate to high
Eggs laid at a time, on a log scale where each step is ten times more. Survival of the young, from Table 11.4.
The pattern runs both ways. Animals that fertilise externally make eggs in the hundreds or thousands and few survive. Animals that fertilise internally make a handful and more of them make it.
Quick check
According to Table 11.4, which animal produces 2 to 20 eggs at a time with moderate survival?
Show the answer
C. Lizard The lizard lives on land, fertilises internally, lays 2 to 20 eggs at a time, and its young have moderate survival.
Yolk, larvae and a change of body
An egg has to feed the embryo growing inside it, and it does so with yolk, a store of nutrients. Fish, amphibians and insects lay several hundred to several thousand eggs at a time. A mother cannot pack that many eggs with large amounts of yolk.
So their eggs carry only enough yolk to make a larva. The larva hatches and feeds on whatever it finds, such as rotting food or manure. This is an in-between stage of life whose main job is eating. Once it has stored enough nutrition, its body is rebuilt into the adult form. You can see this in butterflies and in frogs. Follow a butterfly round its life cycle (Fig. 11.17).
Stage 2 of 4
Larva
The caterpillar hatches and eats and eats. This is the feeding stage, storing up nutrition.
Stage 3 of 4
Pupa
Inside a case it hardly moves, while its body is rebuilt into the adult form.
Stage 4 of 4
Adult
The butterfly emerges, feeds on nectar, mates and lays eggs, and the cycle begins again.
Reptiles and birds solve the feeding problem differently: each of their eggs packs enough yolk to carry the embryo right through to hatching as a young one. A mammal mother keeps the zygote in her own body, and it grows there.
- Yolk
- The nutrient store in an egg that feeds the developing embryo.
- Larva
- The young stage that hatches from the egg and mainly feeds; for example a caterpillar.
- Pupa
- The stage in which the larva's body is changed into the adult body.
- Metamorphosis
- You may also meet: the name for this big change of body form from larva to adult.
Quick check
Why do many insects and amphibians have a larval stage?
Show the answer
A. Their eggs carry only enough yolk to make a larva, which then feeds itself With so many eggs, each gets little yolk. The larva hatches early and feeds itself until it can change into the adult.
Ready at birth, or cared for?
Some young feed themselves from the moment they hatch or are born. Others need a long stretch of feeding and care first. A good guide is how much developing the embryo did before it came out, inside the egg or inside its mother.
For a while after birth, a mammal mother nurses her young on breast milk.
Quick check
After birth, what does a mammal mother feed her young on?
Show the answer
B. Milk Nursing on breast milk for a while after birth is typical of mammals.