"Double life"
The word amphibian comes from Greek words meaning "double life." Most amphibians start life in water and move onto land as adults. There are more than 8000 species of amphibians.
Amphibian features
| Feature | Description |
|---|---|
| Moist skin | Thin, smooth skin without scales; it must stay damp |
| Cold-blooded | Body temperature changes with surroundings |
| Backbone | Vertebrates |
| Two stages of life | Usually a water stage and a land stage |
| Breathing | Gills as larvae; lungs and skin as adults |
| Jelly-like eggs | Usually laid in water, with no shell |
Many amphibians can breathe through their skin. Oxygen passes through the moist skin into blood vessels just underneath. This is why their skin must stay wet, and why they live near water or in damp places.
Three groups
| Group | Features | Examples |
|---|---|---|
| Frogs and toads | No tail as adults; long back legs for jumping | Spring peeper, wood frog, American toad |
| Salamanders and newts | Long body, tail, four legs | Spotted salamander, red-spotted newt |
| Caecilians | No legs; worm-like; live underground in the tropics | Caecilians |
Frogs vs. toads
| Frogs | Toads |
|---|---|
| Smooth, moist skin | Dry, bumpy skin |
| Long legs; hop high | Shorter legs; walk or make short hops |
| Live near water | Can live farther from water |
| Eggs in clusters | Eggs in long strings |
All toads are actually a kind of frog.
The frog life cycle
Frogs go through metamorphosis, a dramatic change in body form.
| Stage | Description |
|---|---|
| Eggs | Laid in water in jelly-covered clumps |
| Tadpole | Hatches with gills and a tail; eats algae |
| Tadpole with legs | Back legs grow first, then front legs |
| Froglet | Lungs develop; tail shrinks; it starts to breathe air |
| Adult frog | No tail; lives on land and in water; eats insects |
This can take a few weeks to several years, depending on the species. Bullfrog tadpoles can take two years or more to change.
Surviving winter
In Canada, amphibians must survive freezing winters. Many hibernate in mud at the bottom of ponds. The wood frog does something amazing: it can freeze almost solid! Its heart stops and ice forms in its body, but sugar in its cells acts like antifreeze. In spring it thaws and hops away.
Amphibian defences
- Camouflage: tree frogs blend in with bark and leaves.
- Poison: some skin glands make toxins. Brightly coloured poison dart frogs in South America warn predators to stay away.
- Puffing up: toads inflate their bodies to look bigger.
- Losing tails: some salamanders drop their tails to escape.
Why amphibians matter
- Adults eat huge numbers of insects, including mosquitoes.
- Tadpoles eat algae, helping keep ponds clean.
- They are food for birds, fish, snakes and mammals.
- Their skin chemicals are studied for new medicines.
Indicator species
Because amphibians absorb water and chemicals through their skin and live in both water and land, they are very sensitive to pollution and habitat change. This makes them an indicator species: when amphibians decline, it warns us that the ecosystem is in trouble.
Amphibians in danger
Amphibians are the most threatened group of vertebrates. About 40% of species are at risk of extinction. Threats include:
- habitat loss, especially draining wetlands
- pollution and pesticides
- climate change
- a fungal disease called chytrid, which has wiped out many species worldwide
- roads that frogs and salamanders must cross to reach breeding ponds
You can help by protecting wetlands, avoiding pesticides, and joining frog-call surveys like FrogWatch.
Common mistakes
- Thinking amphibians are reptiles. Reptiles have dry, scaly skin and shelled eggs.
- Thinking tadpoles are fish.
- Thinking all frogs live in water all the time.
Try it yourself
- Visit a pond in spring (with an adult) and listen for frog calls. Try to identify them using a frog-call guide.
- Draw the frog life cycle as a circle with labels.
- Make a Venn diagram comparing frogs and toads.
Quick recap
- Amphibians usually live in water when young and on land as adults.
- They have moist skin and can breathe through it.
- Frogs go through metamorphosis from egg to tadpole to adult.
- Frogs, toads, salamanders and caecilians are amphibians.
- Amphibians are indicator species and are seriously threatened.