There are more than 6,000 species of amphibians living today. This animal class includes toads and frogs, salamanders and newts, and caecilians.

Almost all amphibians have thin, moist skin that helps them breathe. No other group of animals has this special skin. Most amphibians undergo a unique change from larvae to adults, called metamorphosis. All amphibians are ectotherms (what used to be called "cold-blooded"), a trait they share with invertebrates, fish, and reptiles.

Thin-skinned

Most amphibians have thin skin that is very permeable (allowing liquids and gases to pass through it easily). This is important for two reasons. First, it means that their skin helps them breathe, since oxygen passes easily through it. Second, it means that amphibians lose a lot of water through their skin. This is why most amphibians are found in moist or humid environments, where they can re-load their water reserves.

Leading a Double-Double Life

The word amphibian comes from the Greek word amphibios, meaning "a being with a double life." Some say their name refers to the fact that amphibians live in two places -- on land and in water. While dual residence is the rule for most amphibians, some species are strictly aquatic (water-dwelling) and some are strictly terrestrial (land-dwelling).

More accurately, amphibians' "double life" refers to two distinct life stages -- a larval stage and an adult stage. Most amphibians lay eggs, which hatch into larvae and undergo an amazing transformation (or metamorphosis) as they move from larval to adult stages. For instance, tadpoles (the larval stage of frogs) have gills and a tail -- features that enable them to live underwater. During metamorphosis, tadpoles lose their gills and develop lungs so they can breathe out of water. At the same time, they begin to grow limbs and lose their tails. The end result: adult frogs who spend much of their time on land.

Just the Right Temperature

Amphibians, like reptiles, are ectotherms. This means that they cannot produce sufficient internal heat to maintain a constant body temperature. Instead, amphibians' body temperature varies, depending on the surrounding temperature.

So what does this mean for amphibians? It means that they're responsible for regulating their own body temperature. When it's cold outside and they need to warm up, amphibians often bask in the sun to raise their body temperature. When it's too cold to even bask, amphibians may brumate. This means they're in a hibernation-like state, but they may have periods of wakefulness and even drink when necessary.

When it's hot outside, amphibians spend much of the time burrowing during the day, becoming active only at night.

Amphibians in Danger: The Extinction Crisis

At this very moment, amphibians are facing an extinction crisis.More than 160 species of amphibians may already be extinct.More than 1,800 amphibian species are threatened with extinction – that’s 32% of all known amphibian species.And at least 43% of all species have undergone population declines, while less than one percent show population increases.

The greatest threat facing amphibians is habitat loss and degradation.Other significant threats include pollution, climate change, introduced species, and over-collection. Perhaps the most sinister threat is a newly-recognized fungal disease, which can cause rapid and severe amphibian declines. What is being done to help amphibians?As a first step, hundreds of experts have contributed to a Global Amphibian Assessment, an ongoing project that looks at the distribution and conservation status of all known species.In addition, amphibian specialists around the world are working on understanding the causes of the declines, developing long-term conservation programs, and responding to immediate crises.

Amphibians have been referred to as “canaries in the global coalmine” and “nature’s indicators” because they are some of the first living things to be affected by environmental changes.When amphibians show a decline, it serves as a warning to other species, including humans.By helping amphibians, we are helping the world.