Why reproduction matters
Every animal eventually dies, so a species survives only if its members reproduce. Reproduction passes genes to the next generation and keeps the population going. A life cycle is the series of stages an animal passes through from a fertilised egg (or a bud) to an adult that can reproduce itself.
Sexual and asexual reproduction
In asexual reproduction one parent produces offspring genetically identical to itself. Hydra grow buds that break off; flatworms split in two; some sea stars regrow a whole body from a lost arm; and inparthenogenesis females produce young from unfertilised eggs, as aphids do all summer and some lizards always do. Asexual reproduction is fast and needs no mate. In sexual reproduction, two parents each contribute a gamete (sperm or egg), and their genes are mixed, so every offspring is different. Half the population (the males) does not give birth, which is costly, but the variety helps populations survive diseases and changing environments. Many animals, like aphids, switch between the two.
Fertilisation and early development
Fertilisation is the fusion of a sperm and an egg. Each gamete carries one set of chromosomes, made by meiosis, so the fertilised egg, the zygote, has two sets. Many aquatic animals, such as frogs and most fish, use external fertilisation, releasing eggs and sperm into the water. Land animals mostly useinternal fertilisation, which protects the gametes from drying out. The zygote then divides rapidly (cleavage) into a ball of cells, which becomes hollow (the blastula). Duringgastrulation cells move inwards to form three germ layers, ectoderm, mesoderm and endoderm, which give rise to all the tissues and organs.
Metamorphosis in insects
| Type | Stages | Examples |
|---|---|---|
| Complete | Egg → larva → pupa → adult | Butterflies, beetles, flies, bees |
| Incomplete | Egg → nymph → adult | Grasshoppers, dragonflies, cockroaches |
Insects have a hard exoskeleton that cannot stretch, so they grow by moulting. In complete metamorphosis the larva is a feeding stage that looks nothing like the adult; inside the pupa, its body is largely broken down and rebuilt. Larvae and adults often eat different food, so they do not compete. Most insect species develop this way. In incomplete metamorphosis, nymphs resemble small adults and change gradually.
The frog’s double life
Frogs lay jelly-coated eggs in water, fertilised externally. The tadpole lives like a fish, with gills and a tail, and mostly eats algae. During metamorphosis, controlled by thyroid hormones, it grows legs, develops lungs, absorbs its tail, and its gut changes for a diet of insects. The adult lives on land but must return to water to breed. The name amphibian means “double life”.
Birds and the shelled egg
Reptiles, birds and mammals have an amniotic egg: membranes enclose the embryo in fluid, store waste and exchange gases. In birds and most reptiles, a shell protects the egg and a yolk feeds the embryo, so eggs can be laid on dry land. A chicken embryo develops in 21 days and hatches as a small version of the adult: this isdirect development, with no larva.
Eggs or live birth?
- Oviparous animals lay eggs that develop outside the mother: most insects, fish, amphibians, reptiles, all birds, and the egg-laying mammals (platypus and echidnas).
- Ovoviviparous animals keep eggs inside the body until they hatch, so the young are born live, but are fed mainly by yolk: some sharks, many vipers and garter snakes.
- Viviparous animals nourish the embryo inside the mother, in most mammals through a placenta: most mammals, and some sharks, lizards and fish.
Biologists now often describe these as points on a range, because many species fall between the categories. In seahorses it is even the male that carries the developing young.
Many young or few?
A frog may lay 2,000 eggs and give no care; a wild hen lays a handful and guards her chicks; an elephant has one calf every few years and raises it for years. Producing many cheap offspring and producing a few well-protected ones are both successful strategies. Because most young die, a population stays stable when, on average, each pair leaves about two offspring that survive to breed.
Common misconceptions
- “A pupa is resting.” It is the stage of greatest change.
- “All insects have a caterpillar stage.” Only insects with complete metamorphosis have larvae; grasshoppers have nymphs.
- “Every egg a hen lays could become a chick.” Only fertilised eggs that are incubated develop.
- “Asexual reproduction only happens in plants and bacteria.” Many animals reproduce asexually.
- “All mammals give birth to live young.” The platypus and echidnas lay eggs.
What the model simplifies
The drawings are simplified and not to scale, and each animal represents a large group. Durations are typical ranges for one example species and depend on temperature and food. The survival numbers are illustrative orders of magnitude, not measurements. The development steps are generalised: frogs and chickens differ in how their eggs divide, because of their very different amounts of yolk. The population model uses two genes with three variants each, equal numbers of young per female, a fixed space limit, and a disease that targets the most common type; real populations, mutation rates and diseases are far more complex.
Key takeaways
- Reproduction keeps species going; life cycles describe the stages from egg to breeding adult.
- Asexual reproduction is fast but makes identical offspring; sexual reproduction makes varied offspring.
- Fertilisation joins two haploid gametes into a diploid zygote, which divides and forms germ layers and organs.
- Complete metamorphosis has a pupa; incomplete metamorphosis has nymphs; frogs change from aquatic tadpoles to land-living adults.
- Animals may lay eggs (oviparous), keep eggs inside until they hatch (ovoviviparous) or nourish embryos inside the mother (viviparous).