Frogs bear an outer covering of skin, just as humans and many other animals do. However, because they are amphibians, the skin of frogs is very permeable to gases and liquids. This means that frogs are able to absorb water without drinking it, exude toxins from their skin, and, in some cases, breathe while under water.
Description of the Skin
Most frogs have smooth skin, but exceptions exist. Some frogs can change the texture of their skin. However, contrary to popular perception, the presence or absence of warts on the skin does not indicate whether a frog is a member of the toad genus (Bufo); not all frogs with warty skin are toads, and some toads have smooth skin.
Most frogs bear cryptic colors on their skin to help them evade the attention of predators, but a few species -- notably poison dart frogs of the genus Dendrobates -- bear aposematic, or bold, colors. Because these frogs possess strong poisons in their skin, the bold colors serve to warn away would-be predators.
Most frogs have a limited capacity to change their skin color, and some are capable of rather stark color changes. For example, barking tree frogs (Hyla gratiosa), can change their ground color from brown to green and display or hide a series of spots on their dorsal surface.
Most frog species require ready access to water to avoid dehydration; however, no frog species drinks water to quench their thirst. Instead, frogs absorb water directly through skin on their ventral surface and the skin of their inner legs. If you see your pet frog sitting in his water dish, he is likely trying to rehydrate.
In addition to their lungs and mouths, which also can accomplish the task, frogs can absorb oxygen directly through their skin. This allows the amphibians to spend prolonged periods under water. In fact, as long as they do so in water that has enough dissolved oxygen, many frogs can hibernate under water, remaining there all winter.
Most, if not all, frogs produce toxins in their skin. While most common pet species produce mild toxins that do not represent a serious health threat for humans, others, such as poison dart frogs, are capable of sickening an adult who inadvertently brushes against them. Fortunately, as poison dart frogs sequester alkaloids from their insect prey to produce their poison, they cannot produce their toxins after being in captivity for some time.
Always wash your hands before and after touching an amphibian, as they may become sick from oils and other pollutants on your hands and you may become sick from their skin secretions. Use particular care to avoid getting any of their skin secretions in your eyes, mouth or nose.
Replacement of the Skin
Like many other animals, frogs grow and suffer damage to their skin cells. To address this issue, frogs replace their outer skin covering periodically in a process called shedding. When their new layer of skin cells are ready, frogs slip out of the older, outer layer of skin. As they free themselves from their old skin, they typically push it into their mouths and eat it to avoid wasting any nutrients.