First Lungless Frog Found
for National Geographic News
|April 7, 2008|
The first recorded species of frog that breathes without lungs has been found in a clear, cold-water stream on the island of Borneo in Indonesia.
The frog, named Barbourula kalimantanensis, gets all its oxygen through its skin.
"Nobody knew about the lunglessness before we accidentally discovered it doing routine dissections," study lead author David Bickford, a biologist at the National University of Singapore, said in an email.
His colleague Djoko Iskandar at the Bandung Institute of Technology in Indonesia first described the frog in 1978 from one specimen. About 15 years later, fishermen found another individual.
"Each specimen was deemed so valuable that scientists did not want to sacrifice the animals for dissection," Bickford said.
But the biologist immediately partially dissected several frogs when he found the species on a recent expedition to Borneo.
The team describes the peculiar frog in the May 6 issue of the journal Current Biology.
(Read about other new species found in Borneo.)
Previously the only four-limbed creatures known to lack lungs were salamanders.
A species of earthwormlike, limbless amphibian called a caecilian is also lungless.
Tetrapods, or four-limbed creatures, that develop without lungs are rare evolutionary events, Bickford and colleagues write.
The trait in amphibians is likely an adaptation to life between water and land and their ability to respire through the skin.
The researchers suggest lunglessness in B. kalimantanensis may be an adaptation to the higher oxygen content in fast-flowing, cold water.
"Cold water can hold more dissolved oxygen than warm water," Bickford explained.
The frog also has a low metabolic rate, which means it needs less oxygen.
(Related news: "Penguins Safely Lower Oxygen to 'Blackout' Levels" [December 7, 2007].)
What's more, the species is severely flat compared to other frogs, which increases the surface area of the skin.
"Along with the fact that having lungs makes you more likely to be swept away in a fast-flowing stream—because you would float—this [is] a very strong context for the evolution of loss of lungs," Bickford said.
David Wake is a biologist and expert in amphibian evolution at the University of California, Berkeley.
He said the finding of a lungless frog is unsurprising since tailed frogs are already known for their greatly reduced lungs.
Wake added that for most amphibians, the majority of gas exchange happens through the skin. A low but significant amount of respiration occurs via simple, sac-like lungs.
Most species, he noted, have mating calls that require lungs.
So biologists are unsure why a few species have entirely gotten rid of the organs, Wake said.
"This species is so rare that we know next to nothing concerning its biology," he wrote in an email. "But it is aquatic and lives in cold streams and doubtless has low basal metabolic rate."
Thus loss of lungs as an adaptation to the cold, fast-flowing water "seems like a rational hypothesis to me," he said.
Further studies of the frog to test the hypothesis, however, may be hampered by the species' rarity and endangered habitat, according to Bickford and colleagues.
For instance, the frog's cold-stream habitat is being destroyed by illegal gold mining, Bickford said.
The mining activity makes the water cloudy with sediment and contaminates it with mercury.
In addition, much of the surrounding habitat is under threat from legal and illegal logging, which increases runoff into the streams.
"Most of the frog's presumably original range is now completely uninhabitable," Bickford said.
Further threats, he added, may come from changes in temperature and precipitation patterns due to climate change.
"This frog has a grim future and it is entirely our fault," Bickford said.
"It is our responsibility to try and remedy the situation."
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