World’s only lungless frog species actually does have lungs after all

A tiny frog from Borneo was thought to be the only known frog species with no lungs – but it turns out they do have lungs after all.

In 2008, a team led by David Bickford at the National University of Singapore dissected 11 specimens of the rare Bornean flat-headed frog (Barbourula kalimantanensis). They were surprised when they couldn’t find any of the frogs’ lungs. “Immediately upon opening up the Barbourula, something was weird,” says Bickford.

These frogs are fully aquatic and live in cold and fast-flowing streams, so after running a couple of tests, the researchers reasoned that the lunglessness was an adaptation to their habitat. While breathing through the skin doesn’t usually give amphibians enough oxygen to forgo lungs – just one caecilian and a few salamanders so far are known to be lungless – this species was thought to be the first frog getting its fix solely through the island’s oxygen-rich rivers. Large lungs filled with air would also make the frogs more buoyant and susceptible to strong currents.

But now, David Blackburn at the Florida Museum of Natural History and his colleagues have taken another look at two of Bickford’s specimens. They ran them through a high-resolution micro-CT scanner and saw a respiratory system with small, thin lungs hiding in plain sight.

“Right away, both of us were like, why is there a hole on the bottom of the mouth? That’s not supposed to be there,” says Blackburn. He had spotted the glottis, where the mouth connects with the lungs.

Bickford says he is “very happy” science has learned more about this rare, elusive species. He adds that these lungs are “very, very tiny” for the frog’s size, so he still thinks that Barbourula largely relies on oxygen taken in through its skin.

“I’m very open to the possibility of the reduced lungs having some kind of function, but I’m sceptical,” he says.

This development was not entirely unexpected, according to Rafe Brown at the University of Kansas, who was not involved in the study. After 15 years, Blackburn was armed with better technology and more extensive frog collections to compare with. Brown says many more species now need to be studied, to better understand whether all frogs that have evolved to live in streams have shrivelled lungs, and whether others have potentially lost their ability to breathe through them.