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School β€Ί Animal life cycles β€Ί Ages 15–16

Metamorphosis, hormones and survival

How hormones control metamorphosis in insects and amphibians, what happens inside a pupa, the timeline of chick embryo development, and why many species produce huge numbers of young. With a survival-model investigation and calculations.

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Insects: brain hormone triggers the prothoracic gland to release ecdysone, the moulting hormone, which causes a new cuticle to form and the old one to be shed. Juvenile hormone keeps the larval form while it circulates; when its level falls, the larva moults into a pupa and then an adult.[5]

Inside the pupa the larval body breaks down, and imaginal discs (clusters of cells formed in the embryo) grow into adult legs, wings, antennae and mouthparts. The new adult pumps air and fluid into its wings to unfurl them before they harden.[4]

Amphibians: thyrotropic hormone from the pituitary activates the thyroid gland to make thyroxine, which drives metamorphosis. The tadpole loses its gills and larval mouthparts; the tail muscles degenerate and the tail cells are destroyed and eaten by phagocytic cells.[5]

The environment matters: in common frogs, cold water or abundant food can delay metamorphosis by several months, while predators such as fish can speed it up.[2]

Chick embryo: the eye begins at about 24 hours, the heart starts beating at about 42–44 hours, wing and leg buds are visible at the end of day 3, digits appear by day 7, feathers after day 10, and the chick hatches on day 21.[6]

Many young, few survivors: frogs lay large numbers of eggs, which increases the chance that some survive to adulthood; spawn can be killed by frost and every stage has predators. Mammals, in contrast, feed their young on milk from mammary glands.[2][7]

Try it: Modelling survival through a life cycle

⏱ 45 minutes

You need

  • 100 counters, beans or paper squares per group
  • A dice per group (or a random-number generator)
  • Record table and graph paper or a spreadsheet

Safety

No live animals are used. Keep counters away from very young children (choking hazard) if the room is shared.

What to do

  1. Start with 100 counters: these are eggs. The survival rates in this model are made up to show the idea; they are not real data.
  2. Egg β†’ tadpole: for each counter roll the dice; a 1, 2 or 3 means it does not survive (50% survive). Count the survivors.
  3. Tadpole β†’ froglet: a 1 to 4 means it does not survive (about 33% survive). Count again.
  4. Froglet β†’ breeding adult: a 1 to 5 means it does not survive (about 17% survive). Count again.
  5. Plot survivors (y) against stage (x). Calculate the overall survival as a percentage of the eggs and compare with the expected value 100 Γ— 1/2 Γ— 1/3 Γ— 1/6.
  6. Pool the class results and compare the spread between groups with the class mean. Why do small samples vary so much?
What happens, and why?

The expected number of breeding adults from 100 eggs is 100 Γ— 1/2 Γ— 1/3 Γ— 1/6 β‰ˆ 2.8, so less than 3% survive in this model. Individual groups get different numbers because each roll is random; pooling many groups gives a mean closer to the expected value. Producing many eggs means that some survive even when most die, which is the idea the NHM describes for frogs.[2]

Play: Insect or amphibian?

Each statement is about how metamorphosis is controlled or what happens during it. Is it about insects or about amphibians (frogs)?

Your best score is kept only on this device. Nothing is sent anywhere.

Quiz

Other ages: Ages 6–7 Β· Ages 10–11