Do Frogs Drink Water? How Frogs Absorb Water Through Skin

How frogs absorb water through their skin

A frog’s skin is very different from ours. It has no fur, feathers or scales, and its outer layer is extremely thin and moist. Water can pass through it in both directions, and the direction depends on conditions. In fresh water, where the frog’s body fluids are saltier than the water around it, water moves into the body by osmosis without any effort from the frog.

This is why a frog sitting in a pond, like the common frog in the photo, is drinking in a sense, even though its mouth stays closed. Because the skin also lets the frog take in oxygen, it plays a double role: frog skin works as both a drinking surface and an extra breathing organ. Learn more about that in Can frogs breathe underwater?

The frog drinking patch

Not all of a frog’s skin absorbs water equally well. In many frogs and toads, the skin on the lower belly and the underside of the thighs, called the pelvic patch, seat patch or drinking patch, does most of the work. Here the skin is especially thin, folded and packed with tiny blood vessels.

When a frog is short of water, hormones make the cells of this patch more permeable. They open more water channels, proteins called aquaporins, which let water flood across the skin into the blood. A dehydrated frog also changes its behavior: it seeks out wet soil, moss or a puddle and presses its belly and thighs flat against it, a posture scientists call the water absorption response. Toads, which live in drier places than most frogs, often have a very well developed drinking patch.

Do frogs ever take in water through their mouths?

Some water is swallowed along with food, and frogs get moisture from the bodies of the insects, worms and slugs they eat. But frogs do not lap or sip water as a normal way of drinking. Even in the water, the skin does almost all the work.

Large aquatic frogs, such as the American bullfrog, take in so much water through their skin that their main challenge is the opposite of thirst. Their kidneys must constantly pump out large amounts of dilute urine to keep from becoming waterlogged, while holding on to precious salts.

Water storage: the frog’s bladder

Frogs have another trick for staying hydrated: their urinary bladder works as a water tank. When a frog has plenty of water, it stores dilute urine in the bladder. When it later becomes dry, it can reabsorb water from the bladder back into the body.

This is why a frog or toad that is picked up often releases a squirt of clear liquid. It is not poison but stored water, and losing it is a real cost to the animal. That is one good reason to leave wild frogs alone rather than handle them.

How frogs survive in dry places

Because water passes so easily through their skin, most frogs can dry out fast in warm, dry air. Common frogs and many other species avoid the problem by hiding in damp vegetation, under logs or in burrows by day and coming out mainly on wet nights.

Frogs that live in deserts and seasonally dry places have evolved stronger defenses:

• The water-holding frog of inland Australia (Ranoidea platycephala, formerly Cyclorana platycephala) fills its bladder after rain, then burrows underground and seals itself in a cocoon made from layers of shed skin. It can wait there for months, sometimes more than a year, until the next heavy rain.
• Spadefoot toads of the North American deserts dig backward into the soil with hard spades on their hind feet and stay buried for most of the year, absorbing moisture from the surrounding soil.
• Waxy monkey tree frogs (Phyllomedusa sauvagii) of the South American Gran Chaco wipe a waxy secretion over their skin with their legs, which greatly reduces water loss while they sit in the open.
• Reed frogs in African savannas survive the dry season partly through skin cells that reflect heat, keeping water loss down.

Frogs and salt water

Salt water is a problem for frogs, because their permeable skin would lose water to the sea rather than gain it. That is why almost no frogs live in the ocean or in salty lakes. The best-known exception is the crab-eating frog (Fejervarya cancrivora) of Southeast Asian mangroves, which survives in brackish water by raising the amount of urea in its blood so that its body fluids are about as salty as the water around it.

Why this matters for frogs today

The same skin that lets frogs drink and breathe also lets in what is dissolved in the water. Pesticides, fertilizers and other pollutants in ponds and ditches can pass through the skin more easily than through the scales of a fish or reptile. This is one reason amphibians are often described as sensitive indicators of the health of wetlands, and why clean, unpolluted ponds are so important for frog populations.