Two lives, one animal
Why Do Frogs Start as Tadpoles?
A tadpole is built for water and a frog for land, so the animal rebuilds halfway through.
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Step 1 of 5
Eggs must be laid in water.
Frog eggs have no shell — only jelly — so they dry out in air. Water is not a preference, it is a requirement.
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Step 2 of 5
A tadpole is a water animal.
It has gills for underwater breathing, a tail for swimming, and a long gut suited to grinding through algae.
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Step 3 of 5
Then the body is rebuilt.
Legs grow, lungs develop, the gut shortens for a meat diet, and the tail is absorbed for its nutrients rather than being shed.
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Step 4 of 5
Two lives means two food supplies.
A pond can therefore feed far more tadpoles than it could ever feed frogs. Tadpoles eat algae; adults eat insects. Young and adults never compete for the same food, which lets far more of them survive.
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Step 5 of 5
Adults still need damp skin.
Frogs take in as much as half their oxygen through their skin, which only works while wet. That is why they stay near water for life.
Some frogs skip the tadpole stage entirely, laying eggs that hatch as tiny fully formed frogs.
The short version
Frogs live two lives — a gilled water tadpole and a lunged land adult — so young and adults never compete.
Try it yourself
Watch a pond through spring. Back legs appear first, then front legs, then the tail shrinks over a few days.