Male Gracixalus treefrogs woo females with their original compositions (Image: Jodi Rowley/Australian Museum)

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: Gracixalus treefrogs

Habitat: Evergreen forests in the mountains of north and central Vietnam


Deep in the evergreen forests of Vietnam, curious little green-blooded frogs spend monsoon nights performing vocals, improvising new melodies each time they sing.

Known popularly as “frogs that sing like birds”, male Gracixalus treefrogs perform to attract females and to ward off other males.

But these are not your average frogs, croaking out the same old tunes. Gracixalus frogs shuffle notes to compose a new melody every single time they sing (listen to them in the sound clips below).

To human ears, songs of the three related species – G. quangi, G. supercornutus and G. gracilipes – sound like birds chirping.

They randomly mix high-pitched, long notes called “whistles” with short, sharp “clicks” to compose new tunes.

Each song is unique in its complexity, duration, amplitude, frequency and structure, as opposed to being specific to an individual or a species as it is in most frogs.

“We don’t know why they have such complex calls,” says Jodi Rowley from the Australian Museum Research Institute, whose team discovered G. quangi in Vietnam in 2010. “For some reason, they are saying more than your average frog.”

In subsequent expeditions, Rowley recorded and analysed the calls of three G. quangi males, four spiny G. supercornutus males and five G. gracilipes males.

The clicks and whistles were combined in various ways, always into unique songs. She also found that the calls had more territorial components or clicks when several males were around.

“One of the functions of a frog’s call, just like a bird’s call, is to attract the opposite sex,” says Rowley. “We imagine that part of it – the clicks – is territoriality and the other part – the whistles – is to attract females.”

Gracixalus treefrogs are unusual in other ways, too: they have pointed snouts, turquoise bones, translucent green skin and green blood.

The colour of their blood and skin is green probably due to a bile pigment called biliverdin.

“A few reptiles and frogs have this in their blood and there are suggestions that the pigment might deter infection by Plasmodium malaria parasites, although this has not been proven,” Rowley says.

The pigment may also help the translucent frogs camouflage themselves better among the vegetation and may make them unpalatable to predators because biliverdin is a metabolic waste product.

Journal reference: Amphibia-Reptilia, DOI: 10.1163/15685381-00003007