Five frog species from Papua New Guinea which exhibit characteristics rarely witnessed in other tree frogs have been described by scientists for the first time.
While tree frogs typically lay their eggs directly into water, the newly described species of the genus Litoria might all use structures like trees and rockfaces to deposit their offspring.
Such a mechanism might have evolved to protect tadpoles from predators in water bodies below.
The slender spotted treefrog (Litoria gracilis) and the Crater Mountain treehole frog (Litoria naispela) were the only species where arboreal laying was confirmed, though the researchers suspect the other three – the Darai Plateau treefrog (L. daraiensis), red-bellied treefrog (L. haematogaster) and Lisa’s treefrog (L. lisae) – do too.
The Crater Mountain treehole frog appears to also share unique reproductive strategies with other, less-closely related frog species that ‘glue’ their eggs in sacs above treeholes.
When the eggs hatch, the tadpoles drop into water pooled in the treehole below to continue their development.
“[This] really suggests that it’s a very good strategy to keep your eggs and tadpoles out of the water for as long as possible,” says lead researcher Dr Steve Richards, from the South Australian Museum.
“The rationale for that is there’s a lot of predators in water, so the longer you can keep your offspring safely out of the water, the better.”
Others, like Lisa’s treefrog, might attach their egg sacs to the walls of limestone sinkholes.
While the researchers weren’t able to document this taking place, they did hear the calls of these frogs from within these structures.
“There is a suspicion that they certainly have an unusual reproductive behaviour,” says Richards.
“They call from down in these limestone crevices and sinkholes… we don’t know, but it’s certainly unlikely to be on trees or leaves like the other ones, but maybe some other form of arboreal egg laying where they are putting them outside the water.”
Sound, sight and specimens
Like many organisms, physical characteristics are used to distinguish species from one another. In the field, Richards records frog calls and appearances of frog species to mount the case for describing a new species.
In the lab, his colleague from Queensland Museum Dr Paul Oliver, analysed the genetic profiles of the specimens, seeking out evidence of genetic divergence to effectively rubber stamp the definition of new species.
“What we’re looking for is levels of genetic diversity or divergence,” Oliver says.
“Basically, as levels of genetic divergence goes up, the probability that you’re dealing with something that’s a separate species or what you might say is on a separate evolutionary trajectory. As the genetic divergence values go up, the probability that you’re dealing with separate species also goes up.”
New Guinea is home to one of the most diverse concentrations of frog species in the world. Thanks to its geographic isolation, New Guinea is home to seven percent of the world’s frog species in just 0.7% of its land area.
With this in mind, it’s likely more species will continue to be discovered.
“Our estimate, at the moment is of 700 species [in New Guinea],” Oliver says.
“By comparison Australia’s got about 250. But what’s cool is that most of those species in New Guinea, we just know nothing about them.
“It’s only just now that we’re starting to document these behaviours and it emphasises just how comparatively poorly known, this incredibly, world-beatingly diverse frog fauna in New Guinea is.”
Richards and Oliver also noted the appearance of species like the Crater Mountain treefrog – one which makes the animal appear to have a “birdpoo” pattern – have evolved as a mimicry defensive mechanism to avoid predators.