Can Frogs Breathe Underwater? Skin, Lungs and Gills Explained

Three ways a frog can breathe

Over its life, a frog uses three different breathing surfaces:

The mix changes as the frog grows. A young tadpole relies on its gills and skin. As it develops, its lungs grow and it starts gulping air at the surface. After metamorphosis the gills disappear, and the adult breathes with its lungs and its skin. The changes are described in detail in How do tadpoles turn into frogs?

How frog lungs work

Frogs have no ribs to expand the chest and no diaphragm. Instead, they use their throat as a pump. With the mouth closed, a frog lowers the floor of its mouth to suck air in through the nostrils. Then it closes the nostrils and raises the throat, pushing the air down into the lungs. This is called buccal pumping, and it is why the throat of a resting frog flutters constantly.

The lungs are simple, balloon-like sacs with far less internal surface than mammal lungs. That is fine for an animal whose body temperature, and therefore oxygen demand, rises and falls with the weather, and which has a second breathing surface on the outside of its body.

Breathing through the skin

Frog skin is thin, moist and packed with small blood vessels just below the surface. Oxygen dissolved in water or in the film of moisture on the skin passes into the blood, and carbon dioxide passes out. This is called cutaneous respiration. The skin is especially good at getting rid of carbon dioxide, and in many frogs it removes most of the carbon dioxide the body produces.

How much oxygen the skin supplies depends on the species, the temperature and how active the frog is. A warm, active frog needs far more oxygen than its skin can deliver and must breathe air. But a cold, motionless frog uses very little, and its skin alone can be enough. Because the skin must stay moist to work, frogs avoid drying out and spend dry, sunny days in damp hiding places. The same skin also takes up water, as explained in Do frogs drink water?

Can frogs stay underwater all winter?

Yes, many can. The common frog of Europe and many North American frogs, such as leopard frogs and bullfrogs, often spend the winter resting on or near the bottom of ponds, lakes and slow streams. In water just above freezing their metabolism slows dramatically, and oxygen taken in through the skin keeps them alive until spring. Frogs choose spots where the water stays oxygenated, often where there is a slight current or inflow.

This strategy has a risk. If a pond freezes over for a long time and rotting plants use up the oxygen in the water, frogs at the bottom can suffocate, a problem known as winterkill. Frogs that hibernate on land avoid this danger but face others, such as frost. Read more in Where do frogs go in winter?

Can frogs drown?

Frogs can drown, even though they are excellent swimmers. A frog that is active in warm water needs to breathe air, and if it is trapped below the surface, tangled in netting or stuck in water low in oxygen, its skin cannot keep up. Exhausted frogs can also drown in swimming pools, water tanks and other steep-sided water with no easy way out.

Frogs that rely most on their skin

A few frogs have pushed skin breathing much further than most:

• Lake Titicaca water frog (Telmatobius culeus). This large frog lives only in the Lake Titicaca basin of Peru and Bolivia, at about 3,800 m (12,500 ft) above sea level. Its lungs are only about one third the relative size of other frogs’ lungs, but it has enormous, baggy folds of skin that greatly increase the surface for absorbing oxygen. It rarely needs to surface, and when oxygen is low it bobs its body up and down to move fresh water over its skin. The IUCN lists it as Endangered.
• Hairy frog (Trichobatrachus robustus). In the breeding season, males of this Central African frog grow hair-like strands of skin on their thighs and flanks. These strands are thought to help them absorb extra oxygen while they stay in the water guarding their eggs.
• Bornean flat-headed frog (Barbourula kalimantanensis). This rare frog of cold, fast rivers in Borneo was described in 2008 as the first known lungless frog. Later scans suggest it has tiny lungs after all, but it still depends mainly on its skin for oxygen.

Myths and facts

Myth: Frogs breathe water like fish. Adult frogs do not have gills and cannot extract oxygen from water fast enough to stay active underwater. They are air-breathers with an unusually helpful skin.

Fact: Frogs can breathe with their mouths closed. In fact they nearly always do. Because frogs pump air in through their nostrils, a frog that is forced to keep its mouth open cannot breathe properly.