by Matthew Cobb
A year ago, a brief paper appeared in Current Biology, written by researchers from the National University of Singapore (NUS) and from Indonesia, describing the Bornean flat-headed frog, Barbourula kalimantanensis. The frog was first described in 1978, and on the International Union for the Conservation of Nature (IUCN) Red List it is described as “endangered”.
This frog, of which only a few specimens are known, is noteworthy because it has no lungs, but instead breathes through its skin. This is a trick ordinary frogs can do, too – in nearctic regions frogs may spend the winter hunkered down in the bottom of ponds, slowly respiring through their skin.
But losing your lungs is an amazingly rare event. Among all the thousands of tetrapod species – mammals, birds, reptiles, amphibians – lunglessness has evolved only in amphibians. Indeed, it has only been observed three times previously – in two families of salamander and a single species of caecilian. (Caecilians are legless amphibians that are not snakes; most of them live underground.)
Why has this species of frog lost its lungs? The authors of the paper – David Bickford, Djoko Iskandar (who first described the frog, but did not realize its lunglessness) and Anggraini Barlian – point to how the frog manages without lungs (it lives in oxygen-rich fast-flowing water, presumably has a low metabolic rate (the water is very cold), its flat shape increases the surface area for oxygen absorption), but beyond the only positive reason they can come up with is that being lungless would decrease the frog’s buoyancy . Furthermore, the closely related species Barbourula busuangensis has lungs, just like all other known frogs.
What progress has been made over the last year? Had to say – a quick scouring of the scientific literature brings up no more publications, and there’s no indication on David Bickford’s NUS website that there’s been any further discoveries. The evolution of lunglessness in this odd frog may end up in the list of adaptations we don’t fully understand, such as the zebra’s stripes (discussed here) or the stegosaur’s plates (we’ll deal with that one tomorrow, so don’t chip in until you’ve read the blog!).
In a blog discussion last year over at nusbiodiversity, one of the readers made an astute comment. “JD” wrote “Why would a lungless frog have nostrils? Where do they lead to?” Presumably it retains the nostrils for its sense of smell – which will also work under the water. Ordinary frogs breathe by swallowing (they have no diaphragm), so my bet would be that this lungless variant still gulps to get air flowing over its olfactory receptors.
The photo is taken from the Current Biology article, which can be found here.