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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