Amphibians

Amphibians are a group of cold-blooded vertebrates that live part of their lives in water and part on land. The word "amphibian" comes from Greek and means "two lives", which is exactly what most amphibians have: they start as water-breathing tadpoles and grow into air-breathing adults. There are around 8,500 known species of amphibian. They were the first vertebrates to colonise land, about 370 million years ago, and they are also the most threatened group of vertebrates today, with around 40% of species at risk of extinction.

  • Known speciesapprox. 8,500Most are frogs and toads
  • Three main groupsFrogs/toads, salamanders/newts, caecilians
  • SmallestPaedophryne amauensis7.7 mm long, world's smallest vertebrate
  • LargestChinese giant salamanderUp to 1.8 m long
  • First land vertebratesapprox. 370 million years agoCrawled out of the Devonian swamps
  • Most poisonous animalGolden poison frogSkin toxin can kill 10 adults

What makes an amphibian?

  • Cold-blooded: body temperature follows the environment.
  • Vertebrates: have a backbone.
  • Soft moist skin: usually slimy, not scaly. Amphibians breathe partly through their skin.
  • Lay eggs in water (mostly): the eggs have no shell and would dry out on land.
  • Metamorphosis: most go through a dramatic body change between an aquatic larva and an air-breathing adult.
  • Two-stage life cycle: water-breathing youth, land-breathing adult.

The three groups

  • Frogs and toads (Anura, meaning "tailless"): the biggest group, around 7,500 species. Adults have four legs, no tail and good jumping. Tadpoles are vegetarian and water-living; adults are carnivorous.
  • Salamanders and newts (Caudata, meaning "tailed"): around 800 species. Look like long lizards with smooth wet skin. Many keep their tails as adults.
  • Caecilians (Apoda, meaning "without feet"): the strangest amphibians. Look like long worms or snakes with no legs. Around 200 species, mostly burrowing in tropical soils.

The amphibian life cycle

The classic amphibian life cycle is a remarkable transformation. A frog starts life as a tiny egg laid in jelly in a pond. It hatches into a tadpole: a small water-breathing creature with gills, a tail and no legs, that eats algae. Over weeks or months it goes through metamorphosis: it grows legs, loses its tail, develops lungs, and stops eating water plants in favour of catching insects. By the end it has transformed into an adult frog ready to live on land.

Frogs vs toads

People often ask the difference between a frog and a toad. There is no strict scientific division (toads are technically just a kind of frog), but in everyday use:

  • Frogs: typically have smooth wet skin, long powerful back legs for jumping, and live close to water.
  • Toads: typically have dry warty skin, shorter legs better suited to walking and hopping, and can live further from water.

Amphibian skin: the breathing organ

Amphibian skin is one of the most remarkable in the animal kingdom. It is:

  • Thin and permeable, so it can absorb oxygen directly from water or moist air.
  • Always moist: amphibians produce a constant layer of slime to keep their skin damp, otherwise gas exchange stops.
  • Often chemically defended: many species secrete toxins through their skin to discourage predators.
  • Sheddable: amphibians regularly slough off their outer skin layer and (often) eat it.

This permeable skin is also one reason amphibians are in such trouble globally. They absorb pollution, toxins and changes in their environment directly through their skin, making them very sensitive to environmental change.

Fact The golden poison frog of Colombia is the most poisonous animal on Earth, gram for gram. A single frog the size of your thumbnail contains enough toxin in its skin to kill 10 adult humans. The toxin is so potent that local indigenous peoples used to wipe their dart-tips on the frog's skin to coat them with poison, which is how the family got its name "poison dart frogs". The toxin actually comes from the beetles the frogs eat in the wild; frogs raised in zoos eating other food are not poisonous.

Amphibians in trouble

Amphibians are the most threatened group of vertebrates on Earth. Around 40% of all amphibian species are at risk of extinction. The reasons include:

  • Habitat loss: ponds drained, wetlands filled in, forests cleared.
  • Climate change: many amphibians need very specific moisture and temperature conditions.
  • Pollution: their permeable skin makes them very sensitive to chemicals.
  • The chytrid fungus: a deadly fungal disease (called Bd) that has spread around the world since the 1980s and has driven dozens of amphibian species to extinction. It attacks their skin and is one of the worst wildlife pandemics in recorded history.
Did you know? The axolotl is an unusual salamander from Mexico City's ancient lakes. It never goes through normal amphibian metamorphosis. It stays in its larval form (with external gills) for its entire life, even when fully grown and reproducing. Axolotls also have an extraordinary ability to regenerate entire lost limbs (including bone, muscle and nerves), making them important model organisms for medical research. Sadly, the wild axolotl population is on the brink of extinction; almost all axolotls today live in captivity.
Deeper dive: when fish crawled onto land

The amphibians are descendants of one of the most important events in evolutionary history: the moment when the first vertebrates crawled out of the water onto land, around 370 million years ago.

Before that, the only vertebrates were fish. They lived in the seas, rivers and shallow swamps of the Devonian period. Land had already been colonised by plants and insects, but no animal with a backbone had ever come ashore.

Then a group of lobe-finned fish (related to modern lungfish and coelacanths) started to evolve features that worked on land. Their fleshy fins, with their internal bones, gradually evolved into limbs with toes. Their swim bladders, originally for buoyancy in water, evolved into lungs. Their bony skeletons strengthened to support their weight on land. By 365 million years ago, the first true amphibians had emerged, creatures like Acanthostega and Ichthyostega, that could walk (clumsily) on muddy shores.

One of the most famous transitional fossils ever found is Tiktaalik, a 375-million-year-old fish-like creature discovered in Arctic Canada in 2004. Tiktaalik has features halfway between fish and amphibians: fish-like scales and gills, but also primitive wrists, ankles and a moveable neck like a land animal. It is one of the clearest "missing links" ever found.

These first amphibians eventually gave rise to all four-legged land vertebrates: not just modern amphibians, but reptiles, dinosaurs, birds and mammals (including us). Every creature with four legs (or two legs and two arms) on land today is descended from those Devonian fish that crawled out of the water 370 million years ago. Including you.

For other vertebrates, see fish (the amphibians' ancestors) and reptiles (their descendants).