1. How Some Red Frogs Stay Underwater All Winter
Some North American red frogs spend the winter submerged in water. A review states that when water temperatures are 5°C (41°F) or lower, these frogs can obtain sufficient oxygen solely through gas exchange across their skin, meaning they do not need to surface to breathe [1]. It is thought that the cold water slows the frog’s body movements, reducing its oxygen needs. However, this is conditional. If the water freezes or oxygen in the water almost disappears, the frogs can die during winter, with mortality rates reaching 100% in some cases [1]. Not all frogs overwinter underwater; some burrow into the soil.
2. How Amphibians Breathe Through Their Wet Skin
Amphibian skin is always wet thanks to mucus-secreting glands. Through this thin, wet skin, they absorb oxygen and release carbon dioxide [2]. The process begins when oxygen in the environment dissolves into the water film on the skin’s surface. From there, it enters blood vessels in the skin and travels through the body via the bloodstream [3]. The wet skin acts as a "water path" for dissolved oxygen. If the skin dries out, this path closes, which is life-threatening for amphibians [3].
Frogs do not actively "inhale." Oxygen moves naturally from areas of high concentration to low concentration through diffusion. A study on lungless salamanders showed that when surrounding oxygen levels dropped, the amount of oxygen absorbed through the skin also decreased [4]. This is similar to how a drop of ink spreads in water, though oxygen is invisible and diffusion speeds differ greatly between air and water.
3. Why Low Oxygen in Water Affects Frog Movement
Water holds far less oxygen than air. While air is about 21% oxygen by volume, the dissolved oxygen in water at 20–30°C (68–86°F) is only about 1/30th to 1/40th of the amount in air [5]. Colder water holds more; at 0°C (32°F), it holds about twice as much as at 30°C. In still water, oxygen spreads very slowly. Technical documents in fisheries note that in unmoving water, it can take about 6 years for oxygen to reach a depth of 6 meters (about 20 feet) from the surface. Most oxygen in natural waters is moved by mixing [5].
Animals that rely on skin breathing need ways to refresh the water around them. The Japanese giant salamander, a special natural monument, lives in fast-flowing mountain streams. It has skin folds to increase surface area. It is described as rocking its body side-to-side to replace the oxygen-depleted water around its skin with fresh water [6]. A university column states that breathing is mainly done through the skin [7], while the Kyoto Aquarium notes that adults also use lungs, bringing just their noses above the water surface to breathe about once every 20 minutes [8]. The Lake Titicaca frog, living in cold lakes at 3,812 meters (about 12,500 feet), uses folded skin and "bobbing" movements to bring water over its skin when oxygen is low [9].
5. Lungless Salamanders and the Retracted Lungless Frog Report
Lungless amphibians are not rare. Many salamander species lack lungs. A database (AmphibiaWeb) lists 523 species in the Plethodontidae family, about 60% of all salamanders, and this group breathes primarily through skin [13]. These lungs are not absent from the start; research shows that tissue that will become lungs forms in the embryo but disappears before hatching [14].
In 2008, the Bornean flat-headed frog was reported as the world’s first lungless frog. However, in 2024, researchers used CT scans on original specimens and found a larynx and small lungs, leading to the retraction of the original paper. This happened because shrunken organs can be missed during dissection or when examining tissue slices [15]. One hypothesis for the tiny lungs is that large lungs would cause buoyancy problems in fast-flowing rivers. Researchers stated the frog gets some oxygen from skin and the rest from lungs, but further investigation is needed [16]. This story highlights the difficulty of proving something is "absent."
6. Why Wet, Thin Skin Makes Amphibians Lose Water
Wet, thin skin is good for breathing but lets water escape easily. Many amphibian skins allow gases, water, and ions to pass freely because the skin handles both respiration and body water/salt regulation [17]. Some tree frogs living in trees secrete waxy lipids from glands and wipe them over their bodies to prevent water loss. Some lose water in a way similar to desert reptiles [18].
Skin is also where frogs absorb water. It is said that frogs do not drink through their mouths but press their belly skin against wet ground to absorb water [19]. If skin function fails, it can be fatal. A 2009 study on chytridiomycosis (a disease caused by a fungus) found that infected frogs had their skin’s electrolyte transport blocked by more than half. This caused sodium and potassium levels in the body to drop, leading to heart failure and death [20].
7. How to Check Dates When Searching for "Lungless Frog"
If you search for "lungless frog," check the date of the article. Pages written before 2024 may still describe it as lungless, while newer pages mention the retraction. This shows how information changes with time. Also, try explaining in your own words why the giant salamander rocks its body, linking it to oxygen levels in the water. The finding that bullfrog tadpoles get about 60% of oxygen through skin is available in the abstract of the Burggren and West paper [10].