Frogs are slimy, smooth-skinned animals that are called amphibians because most species use both land and water in their life cycles. Although frogs possess lungs, most frogs also breathe partly through their skin, which needs to be kept damp.
Most species of frogs need to be around environments with a water source to reproduce, but other than that requirement, they are found on every continent except Antarctica and in almost every kind of environment.
Most species begin life by hatching in water from large clusters of eggs called spawn to become tadpoles. At this stage they are more like fish in shape and movement, although they possess feathery external gills for respiration. However, they soon grow legs and develop lungs before climbing onto land where most will spend the majority of their adult lives.
There are numerous species of frogs worldwide that have been identified with probably more yet to be discovered. The various species have evolved a host of interesting strategies to survive in a wide range of diverse environments; some, where you might think they couldn't possibly survive.
For example, frogs are cold-blooded, meaning their body temperatures change with the temperature of their surroundings. Frogs become very sluggish or inactive in cold conditions. When temperatures drop, some species survive the cold by digging burrows underground as in the mud at the bottom of a pond. They can hibernate in this burrow until spring. When hibernating, most frogs absorb what little oxygen they need through their thin skin.
The wood frog uses another strategy to survive the cold season. They keep some 65 percent of their body frozen during hibernation. This species uses the simple sugar glucose found in its blood as a kind of antifreeze that is concentrated in its vital organs. This protects the vital organs from damage while allowing the rest of the body to freeze solid.
At the other extreme is the Australian water-holding frog that lives in the hot desert and can survive while avoiding any rain for up to seven years. It burrows underground and lives in a transparent cocoon that is constructed from its own shed skin.
Larrie Stone is a retired Dana College science professor.