Natural sciences Year 5: Eggs, birth and metamorphosis
Natural sciences, Year 5 (Portugal): Animals can be oviparous, ovoviviparous or viviparous depending on where the embryo grows and what feeds it; some, like butterflies, change form completely as they develop.
Three ways to be born
Animals differ in where the embryo grows and what feeds it. Oviparous: it grows in an egg outside the mother and feeds on the yolk (hen, fish, frog, turtle). Ovoviviparous: the eggs stay inside the mother, the embryo feeds on the yolk and the young are born already hatched (the viper, some sharks). Viviparous: the embryo grows inside the mother and is fed by her, through the placenta (dog, horse, humans).
No way is «the best»
Each way is a way of getting young to survive in a given environment. Many oviparous animals, such as fish and frogs, lay a large number of eggs and give little care, so some young survive. Viviparous animals have few young, protect them inside the body and often look after them afterwards. Ovoviviparous animals protect the eggs inside the mother. None is better in general: each suits the life of the species.
Reading a butterfly's life cycle
Example table (illustrative days): egg 5, larva (caterpillar) 15, pupa (chrysalis) 10, adult 14. Step 1: the order is egg → larva → pupa → adult. Step 2: the larva is very different from the adult: it chews leaves, while the adult has wings and drinks nectar. Step 3: the total is 5 + 15 + 10 + 14 = 44 days. Step 4: the longest stage is the larva, when it eats and grows. That is complete metamorphosis: four very different stages.
Born alive does not mean viviparous
A common idea is «if the young are born alive, the animal is viviparous». The viper gives birth to live young, yet it is ovoviviparous: the embryo grew in an egg inside her and fed on the yolk, not on the mother. The test is where the embryo grows and what feeds it. Another false idea: «all mammals are viviparous». The platypus, an Australian mammal, lays eggs.
Metamorphosis around you
The silkworm is a caterpillar that spins a cocoon, from which silk is obtained. In a beehive, the queen lays eggs and the larvae grow in the cells of the comb. Mosquitoes lay eggs in still water, where the larvae live, so emptying water that stays in pots and buckets helps to reduce them. Spot a caterpillar or a butterfly in a garden and think: which stage of its life cycle is it in?
Test what you learned
A hen's embryo grows in an egg outside the mother and feeds on the yolk. What is a hen?
- a) Oviparous
- b) Ovoviviparous
- c) Viviparous
Correct answer: a) Oviparous — Right: the embryo grows in an egg outside the mother's body.
Example table for a butterfly, in days: egg 5, larva 15, pupa 10, adult 14. How long is one full cycle, and which stage is the longest?
- a) 34 days; the larva
- b) 44 days; the adult
- c) 30 days; the larva
- d) 44 days; the larva
Correct answer: d) 44 days; the larva — Right: 5 + 15 + 10 + 14 = 44, and the larva, at 15 days, is the longest stage.
The viper gives birth to live young. Why is it ovoviviparous and not viviparous?
- a) Because it lays its eggs in a nest
- b) Because live birth always means viviparous, so it is viviparous
- c) Because the embryo grows in an egg inside her and feeds on the yolk, not on the mother
Correct answer: c) Because the embryo grows in an egg inside her and feeds on the yolk, not on the mother — Right: that is the ovoviviparous pattern.
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