The Amazing Life Cycle of a Frog
A tadpole and an adult frog look so unalike that you would never guess they are the same animal. One is a swimmer with a tail and no legs. The other sits on a bank with four legs and no tail. Getting from one to the other is called metamorphosis, and it means taking a body apart and rebuilding it while still using it.
From egg to frog, one stage at a time
Step through the four stages and watch what changes at each one.
- A female frog lays her eggs in water. Depending on the species there may be a few hundred or a few thousand, wrapped in clear jelly that keeps them damp and makes them slippery and hard to swallow.
- A tadpole hatches. It has a long tail for swimming, gills for taking oxygen out of the water, and no legs at all. It spends its days grazing on algae.
- The back legs push out first. The front legs come later, growing under the skin before they break through. The tail is already getting shorter.
- The tail is gone, the lungs work, and the young frog can leave the water. It will come back to lay its own eggs.
The tail is not lost, it is eaten
This is the part most people get wrong. A tadpole's tail does not drop off, and nothing bites it off. The frog absorbs it. The tail is made of muscle and other useful material, so the body breaks it down from the inside and uses it as food.
Breathing changes twice
A tadpole uses gills, which pull oxygen out of water. An adult frog uses lungs, which pull it out of air. In between, both are working at once for a while.
Adult frogs also breathe through their skin. Oxygen passes straight through it into the blood, but only while the skin stays wet. That is the real reason a frog needs damp ground and cannot dry out in the sun: a dry frog is a frog that has switched off part of its breathing.
A tadpole is losing its gills and its lungs are not finished yet. How does it manage?
- It holds its breath until the lungs work
- Gills and lungs both work for a while, so the change overlaps
- It stops needing oxygen while it changes
What to look for in a real pond
In cool countries this all happens in spring and early summer, so a pond in March or April is the place to look.
- Clumps of jelly with black dots are frogspawn. Long strings instead of clumps usually mean toads.
- A tadpole with back legs only is partway through — front legs are on the way.
- A tiny frog the size of your fingernail at the water's edge in late summer has just finished the whole journey.
Look, and leave them where they are. Spawn taken home in a jar rarely survives, and a pond needs its frogs more than a jar does.