B. Life Systems · Growth and Changes in Animals

Amphibian Life Cycles: From Egg to Frog

B2.3 B2.4 A1.1 A1.4About 35 minutes

Learning goals

I can name and order the stages of a frog's life cycle.
I can describe how a frog's body and behaviour change from tadpole to adult.
I can explore a pond safely and respectfully.

Words to know

tadpole
A young frog or toad. It lives in water, has a tail and breathes with gills.

gills
Body parts that let an animal take in air from the water. Fish and tadpoles have gills.

lungs
Body parts for breathing air. Adult frogs and people have lungs.

froglet
A young frog that has legs but still has a short tail.

larva
The young stage of an amphibian, such as a salamander larva or a tadpole.

habitat
The natural home of an animal, where it finds food, water and shelter.

Peep! Peep! Peep!

On a warm spring evening in Ontario, you might hear loud peeping coming from a pond. It sounds like hundreds of tiny whistles. It is spring peepers, tiny frogs no bigger than your thumb. The males are calling to find a mate.

Soon there will be frog eggs in the pond. Let's follow one frog's whole life cycle.

Remember metamorphosis?

A monarch butterfly goes through metamorphosis, a big change in body shape. Frogs do too! A frog starts life as a swimming tadpole with a tail, and it ends up as a hopping frog with no tail.

Several tadpoles with growing legs and long tails swimming in shallow pond water. — These tadpoles are part way through metamorphosis. They have grown legs, but they still have tails. AI-generated image

The stages of a frog's life cycle.

Stage	What it looks like	How it lives
1. eggs	small black dots in clear jelly, floating in a clump in a pond	The jelly protects the eggs. After a week or two, they hatch.
2. tadpole	a round body and a long tail, no legs	lives only in water, swims with its tail, breathes with gills, eats tiny plants called algae
3. tadpole with legs	back legs grow first, then front legs	still swims; starts to grow lungs
4. froglet	4 legs and a short tail	comes up to breathe air; the tail gets shorter and shorter as the body uses it up
5. adult frog	no tail, 4 legs, long back legs	hops on land and swims; breathes with lungs; catches insects and worms with a sticky tongue; lays eggs in water

How a frog changes as it grows

Watch the tail and the legs: the back legs grow first, then the front legs, and the tail shrinks away until the adult frog has none.

Different frogs, different timing

A wood frog goes from egg to froglet in about 2 months, in spring and early summer.
A bullfrog tadpole is much slower. It can stay a tadpole for 2 or 3 years before it becomes a frog!
An American toad lays its eggs in long strings, like a necklace, instead of a clump.

A frog that freezes!

In winter, a wood frog hides under the leaves on the forest floor. It can freeze almost solid, and its heart stops beating. In spring, it thaws out and hops away to a pond to lay eggs! This is a real Canadian frog.

Salamanders are amphibians too

A spotted salamander (black with yellow spots) also lays its eggs in ponds in spring. Its young larva lives in water and has feathery gills on the sides of its head. As it grows, it loses its gills, grows lungs and moves onto land. It lives under logs and leaves in the forest.

Unlike a frog, a salamander keeps its tail its whole life.

Worked example

Let's describe the changes

How does a frog's appearance and behaviour change from tadpole to adult?

Body shape: Tadpole: round body, long tail, no legs. Adult: no tail, 4 legs.
Breathing: Tadpole: gills for breathing under water. Adult: lungs for breathing air.
Moving: Tadpole: swims with its tail. Adult: hops on land and swims with its webbed back feet.
Eating: Tadpole: eats algae (tiny plants). Adult: catches insects with its sticky tongue.
Where it lives: Tadpole: only in water. Adult: on land and in water.

Almost everything changes: shape, breathing, moving, eating and where it lives. That is metamorphosis!

Just for fun

Why are frogs always so cheerful?

They eat whatever bugs them!

Worked example

Let's order the mixed-up stages

Put these in order: froglet, eggs, adult frog, tadpole, tadpole with back legs.

First come the eggs in the jelly.
A tadpole hatches. It has no legs.
Next, it grows back legs (then front legs).
Then it is a froglet, with 4 legs and a short tail.
Last, the tail is gone. It is an adult frog.

eggs → tadpole → tadpole with back legs → froglet → adult frog

Hands-on activity

Pond explorers (look, listen, leave)

You need: a grown-up, rubber boots, a notebook and pencil. (No pond nearby? Do the "play dough life cycle" below instead.)

Safety rules first:

Always have a grown-up with you near water. Stay on the shore.
Look, but do not take frogs, eggs or tadpoles home. The pond is their habitat.
If you touch pond water or an animal, keep your hands away from your face and wash your hands with soap afterwards.
Do not touch frogs if you have sunscreen or bug spray on your hands. Frogs take things in through their skin.

What to do: In April or May, visit a pond or wetland. Listen for frog calls. Look for egg clumps or tadpoles near the edge. Draw what you see and write the date. Visit again in 2 or 3 weeks. What changed?

Play dough life cycle: Make each stage from play dough on a paper plate: eggs, tadpole, tadpole with legs, froglet, frog. Draw arrows between them. Tell the story to a family member.

Your turn

What is a baby frog called?

How does a tadpole breathe?

How does an adult frog breathe?

Which legs grow first on a tadpole: the front legs or the back legs?

What happens to the tadpole's tail?

What does a tadpole eat? What does an adult frog eat?

Why should you look but not take frog eggs from a pond?

More practice: warm-up

Where do frogs lay their eggs?

Does a tadpole have legs when it hatches?

What do you call a young frog with legs and a short tail?

Name a frog that lives in Ontario.

What should you do after touching pond water?

More practice: on your own

Give 2 ways a frog's behaviour changes as it grows.

How are toad eggs different from wood frog eggs?

How is a frog's life cycle like a monarch's?

How is a salamander different from a frog when it is an adult?

Why would a dried-up pond be a problem for frogs?

Challenge

You find a tadpole in a pond in August. It is big and has no legs yet. What kind of frog might it be? Explain.

Why does a tadpole need gills, but an adult frog needs lungs?

A wood frog freezes in winter and thaws in spring. Why do you think hiding under leaves helps it?

Check your understanding

Quiz

What comes right after the egg in a frog's life cycle?

How does a tadpole breathe?

Which legs grow first on a tadpole?

What does an adult frog eat?

What is special about a wood frog in winter?

What is the safe and respectful way to explore a pond?

For parents and teachers

Big idea: Amphibians (frogs, toads, salamanders) go through metamorphosis: eggs in water, a gilled larva (tadpole), then an adult that breathes air with lungs (B2.3). Body shape, breathing, movement, food and habitat all change (B2.4).

Safety (A1.4): Supervise closely near any water. Amphibians absorb substances through their skin, so avoid handling them with sunscreen, lotion or bug spray on your hands, and wash hands after touching pond water. Many Ontario amphibians are protected, and moving eggs or tadpoles between ponds can spread disease, so "look and leave" is the rule.

Accuracy notes: Tadpoles mostly eat algae. Timing varies by species and temperature (wood frogs are fast; bullfrog tadpoles take 2 to 3 years in Ontario).

Common difficulties:

Thinking the tail falls off. It is absorbed by the body.
Thinking tadpoles are fish. They have gills and tails, but they become frogs.

How to help: Listen for spring peepers and wood frogs on spring evenings. Many conservation areas have spring frog-listening walks.