Amphibians
Bornean Flat-headed Frog
Barbourula kalimantanensis · also called bornean flat-headed frog, lungless frog
The Bornean flat-headed frog looks like a squashed, drab, unremarkable stream frog. Its distinction is invisible: it has no lungs. Out of roughly 7,000 kinds of frog in the world, this is the only one that breathes entirely through its skin — an adaptation to a life spent pinned to the bed of a cold, roaring forest stream.
Overview
The Bornean flat-headed frog lives only in a small area of the upper Kapuas River basin in Indonesian Borneo, in clear, cold, fast-flowing rainforest streams. It is 4–7 cm long, flattened and broad-headed, mottled to match streambed gravel, with fully webbed feet. It hunts aquatic insects and larvae.
No lungs
Imaging and dissection found that this frog has no lungs, and no cavity for them — the other organs fill the space. It is the only lungless frog known.
Why that helps
In cold, fast, oxygen-rich water, skin breathing alone is enough. Having no air-filled lungs removes buoyancy that would lift the frog off the bottom, so it can stay pressed to the streambed in the current. The extreme flattening both increases breathing surface and streamlines the frog so it can wedge under stones.
Rediscovery
Described in 1978 from a few specimens, then ignored for decades, the frog was rediscovered in 2007–08 by an expedition that confirmed the missing lungs.
Status
The Bornean flat-headed frog is Endangered. Its tiny range of clear cold streams is being silted and poisoned by gold mining and warmed by deforestation, and a frog that breathes only through its skin cannot tolerate polluted or low-oxygen water.
Bornean Flat-headed Frog: common questions
How can a frog have no lungs?
All amphibians can breathe through their skin to some degree, because their skin is thin, moist, and rich in blood vessels, and many salamanders rely on it entirely. Frogs normally use both skin and lungs, and no frog had ever been confirmed to lack lungs completely until this species was examined closely. In the Bornean flat-headed frog, imaging and dissection found no lungs and no cavity for them, and the space is filled instead by the other organs, which have shifted forward. It gets by because its habitat, cold water moving fast over rocks, holds a lot of dissolved oxygen, and because the frog is small and not very active, so its total oxygen demand is low enough for the skin to meet.
Why would evolution take away a useful organ like lungs?
Because in this particular environment the lungs were more of a liability than a benefit. Lungs are full of air, and air is buoyant, so a frog with inflated lungs tends to float and is harder to hold down against a current. A frog that lives on the bed of a torrent, needing to stay wedged among the stones, gains from being denser and less buoyant. If the water already supplies enough oxygen through the skin, then losing the lungs makes the frog better at staying put without costing it the ability to breathe. Similar lung reduction or loss has happened in a lungless salamander family and one caecilian, all of them stream animals.
Is the Bornean flat-headed frog endangered?
Yes, it is listed as Endangered. Its entire known range is a small part of one river basin in Indonesian Borneo, and it depends absolutely on streams that are clear, cold, and fast. Those streams are being harmed by widespread small-scale gold mining, which loads the water with sediment and with mercury used to extract the gold, and by the clearing of forest for timber and agriculture, which warms the water and increases run-off. A frog that breathes only through its skin is especially vulnerable to pollution and to any drop in the oxygen content of its water.
How was such an unusual animal only discovered recently?
It was not so much discovered recently as understood recently. The species was formally named in 1978, based on a few museum specimens, but almost nothing was known about how it lived, and it was not seen again for many years. In 2007 and 2008 an international team went specifically to look for it in the streams of West Kalimantan, found live animals, and studied them properly for the first time, which is when the complete absence of lungs was confirmed. Its remote habitat and secretive, bottom-dwelling habits are why it stayed a mystery for so long.