Funnier than the original: Laughing frog found to be two separate species

By Dr Jodi Rowley, Professor Steve Donnellan, and Stephen Richards 12 October 2023
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Citizen scientists across Australia have helped uncover a frog species new to science, which has a longer ‘laugh’ than the original tree frog species.

Roth’s tree frog, or the northern laughing tree frog (Litoria rothii) is a large, charismatic tree frog from northern Australia and southern New Guinea. The species is generally cream or brown in colour and it has a distinctive bright red upper iris. The frog is commonly encountered throughout its range, and its call is hard to miss – a loud cackle resembling a laugh.

For some time there have been clues that Roth’s tree frog may be more than one species. Firstly, the species has a huge distribution, spanning over 1.5 million square kilometers across Western Australia, the Northern Territory and Queensland, and crossing many different biogeographic barriers, which is rather rare for frog species. Secondly, the call of the species in the western part of its range sounded noticeably different to the call of the species in the east. This became even more obvious as thousands of calls of the species were submitted to the Australian Museum’s FrogID project.

Because of these clues, we decided to determine once and for all if Roth’s tree frog really was a single species. To do so, we examined the genetics, appearance, and call of the species from across its range. Before the FrogID project, we wouldn’t have had enough recordings of the species to understand how its call varied across its range, but thanks to people across Australia using the FrogID app, we were able to harness the enormous database of over 8,000 recordings of Roth’s tree frog to find calls from across the range of the species to analyse. The natural history collections held in Australian museums were vital in determining their genetic relationships.

Related: Two new ‘loud’ frog species have been found along the east coast of Australia

After analyzing all of these data, we concluded that Roth’s tree frog was not one, but two species! Roth’s tree frog is actually restricted to Queensland, the eastern Northern Territory in Australia and southern Papua New Guinea, and another species, until now scientifically unnamed, occurs in western Queensland, the Northern Territory and Western Australia. We scientifically named the second species the western laughing tree frog (Litoria ridibunda). Unexpectedly, we found that these two species weren’t even each other’s closest relative, with Roth’s tree frog being more closely related to Everett’s tree frog (Litoria everetti) from Indonesia than it was to the new species!

With the help of the FrogID project, we confirmed that the western laughing tree frog had a longer ‘laugh’ (9-18 notes) compared to Roth’s tree frog (5-10 notes). Interestingly, this difference was most obvious in western Queensland where the two species overlap – possibly a strategy to help ensure they don’t get each other confused when it comes to finding a mate.

Roth’s tree frog (Litoria rothii). Recording credit: Dane Trembath
Western laughing tree frog (Litoria ridibunda). Recording credit: Dane Trembath

In appearance, the two species are very similar. However, we found that the colour and pattern on the back of the thigh was the best way to tell the species apart. In Roth’s tree frog, the thighs are more boldly patterned with glossy black and yellow, while the new species sports a less distinct dull black and yellowish patterning on the back of the thighs.

FrogID submissions helped reveal the breeding biology of the western laughing frog. The new species is most commonly recorded in suburban backyards and rural areas (>60% of FrogID records of the species) and most records of the species were from ponds and flooded areas rather than streams or creeks. The peak calling season for the species is the summer wet season from October to February (>84% of all FrogID records of the species).

The distribution and calls of the laughing frogs in northern Australia. The western laughing frog (Litoria ridibunda) typically has a greater number of notes per call than the Roth’s tree frog (Litoria rothii). Dots on the map represent the average number of notes per call for an individual recorded at that location. Distribution map from the Australian Frog Altas (Cutajar et al. 2022). Image credit: Jodi Rowley/Australian Museum

The western laughing frog is the 248th species of native frog known from Australia. It may seem surprising that a large, commonly encountered frog species can be “hidden” in plain sight for so long, but it just goes to show how much we still have to learn about frogs. Our research also demonstrates the power of citizen science. Recordings from the FrogID project were important in distinguishing this species and learning more about its biology, and future recordings submitted to the FrogID project will continue to contribute to our understanding of Australia’s frogs, including the western laughing frog.

This article was originally published by the Australian Museum and reproduced with permission.

Related: Citizen scientists huge help in creating Australian Frog Atlas: reveals true distributions of our frogs