WHAT IS AN AMPHIBIAN?
The first amphibians evolved from fish that were able to breathe air and crept out of the water on four limb-like fins to find prey on land. But the fish had to lay their eggs in water, and their amphibian descendants still have to breed in pools or damp places. They also have thin skins that lose body moisture easily, so they must be careful not to dry out.
TYPES OF AMPHIBIAN
The most familiar amphibians are the frogs and toads, with their big heads and tailless bodies. The long-tailed salamanders and newts lead similar lives, but the wormlike tropical caecilians are secretive burrowers.
Frogs and toads
This is the largest group, with 6,641 species. There is no scientific difference between frogs and toads. They have the same basic form, but typically frogs have smoother skins.
Salamanders and newts
As with frogs and toads, these are basically different names for the same type of animal. There are 683 species; some are wholly aquatic while others spend most of their lives on land.
The 205 species of caecilians have no limbs and are almost blind. They live underground, using their reinforced heads to burrow in search of worms and insects.
There are only three types of amphibians, but they include animals with a remarkably wide variety of lifestyles and breeding systems. Despite this, most of them share certain key features. All amphibians are air-breathing, cold-blooded vertebrates, and they have thin skins that are not waterproof. Most lay eggs that must be kept moist and many spend part of their lives in water.
Like their bony fish ancestors, all amphibians have internal skeletons made of bone.
An amphibian’s body temperature is the same as that of the air or water around it.
Most lay eggs
Caecilians bear live young, but most amphibians lay eggs with no hard shells.
Typically the young hatch as aquatic tadpoles that eventually turn into adults.
An amphibian’s thin, moist skin loses water easily, but can also absorb oxygen.
Amphibians are the only vertebrates that change their form as they turn to adults—a process known as metamorphosis. A typical frog egg develops into a long-tailed tadpole that lives like a fish for several weeks. Over time it develops legs and eventually turns into a tiny airbreathing froglet. Finally, its tail shrinks away and it hops onto land to begin its adult life.
Many amphibians start life as aquatic tadpoles, with gills that absorb vital oxygen from the water. When they become adults they develop lungs and can breathe air. But their thin skins can also absorb oxygen from water or air, provided they stay moist, and this allows one group, the lungless salamanders, to survive without either lungs or gills.
The dramatic pattern of this tropical American treefrog warns birds and other enemies that it is extremely dangerous to eat. Its skin oozes poisons made from chemicals in its insect prey, and these are so powerful that these frogs’ secretions have been used to make poisoned arrows. Many other amphibians have similar toxic defenses.
The bright colors warn predators of the frog’s toxicity.
INSIDE AN AMPHIBIAN
This European common frog has the four-limbed body plan that has been inherited by all land vertebrates, although some amphibians—and reptiles—have lost their legs during the course of evolution.
A frog has excellent vision for hunting small animals by sight.
Frogs have very big mouths for gulping their prey down whole.
Keeping the skin moist allows the frog to absorb oxygen from the air.
The heart pumps blood around the frog’s body to supply its vital organs.
The stomach can expand to contain prey swallowed whole.
A frog’s skeleton is specialized for leaping, with a short back and strong limbs.
Long, webbed toes enable this partly aquatic frog to swim well.
Like most frogs, this species has long hind legs adapted for leaping.
A frog breathes by using its throat to pump air into its lungs.