Biologists have found the first recorded species of frog that breathes without lungs in a clear, cold-water stream on the island of Borneo in Indonesia.
According to a report in National Geographic News, 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," said David Bickford, a biologist at the National University of Singapore.
Though two specimens of the frog were discovered in 1978 by Djoko Iskandar at the Bandung Institute of Technology in Indonesia, they were not dissected as scientists deemed them as valuable.
But Bickford immediately partially dissected several frogs when he found the species on a recent expedition to Borneo.
According to Bickford and colleagues, tetrapods, or four-limbed creatures, that develop without lungs are rare evolutionary events.
The trait in amphibians is likely an adaptation to life between water and land and their ability to respire through the skin, they added.
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.
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," said Bickford.
But, biologists are unsure why a few species have entirely gotten rid of the organs.
"This species is so rare that we know next to nothing concerning its biology. But, it is aquatic and lives in cold streams and doubtless has low basal metabolic rate," said David Wake, a biologist and expert in amphibian evolution at the University of California, Berkeley.
"Thus, loss of lungs as an adaptation to the cold, fast-flowing water seems like a rational hypothesis to me," he said.