How do you know a frog is ready for love? It’s in their croak.
It appears Aussie frogs belt out their own croaky tunes when they are enjoying their waterway and looking to attract a mate. And when there’s plenty of water, that means good conditions for frogs to lay eggs and reproduce.
A recent study by our bioacoustics expert Dr Simon Linke, in conjunction with Griffith University (Griffith) and the Forestry Corporation of NSW, recorded the sounds of frogs near the riverbanks of the Murray-Darling Basin. The aim was to create and test call recognisers – a software that automatically listens to and counts frogs to clearly detect a range of different species. And it worked!
A Pobblebonk love song
The research was led by Simon, along with colleagues Daniella Teixeira from the Queensland University of Technology (QUT) and Katie Turlington (Griffith). They detected eight species including:
- Common eastern froglet
- Spotted grass frog
- Long-thumbed frog
- Peron's tree frog
- Pobblebonk frog
- Eastern sign-bearing froglet
- Painted burrowing frog
- Growling grass frog.
"Some of the frogs make high frequency chirps, some 'bark', but our favourite frog – the aptly named Pobblebonk – makes a characteristic ‘boing’ that sounds like plucking a banjo," Simon said.
And what amphibian mate could resist a froggy banjo serenade?
Call recognisers capture amphibian audio
The researchers recorded frog sounds across 20 sites nested in the Koondrook State Forest along the Murray River in NSW. The remote sites helped them to detect clear sounds. Although locations did vary slightly from open woodland to flooded forest, which can impact on how a frog croak sounds.
“It's partly the individual frog that makes slightly different sounds, but also the landscape that attenuates certain sounds or parts of sounds. We deal with variation by building different 'recognisers' for the different locations,” Simon said.
Researchers were able to perfect the recognisers to the point where they can tell if individual frogs of the same species are making slightly different sounds.
The call recognisers are a 'machine learning' computer program. Our partners from NSW Forestry hung autonomous recorders in trees and sent us about a terabyte of recordings from the 20 sites on a hard drive. Researchers then ran these through the machine learning software. Call recognisers are usually used to detect single species with bioacoustics often used to detect rare animals.
However, as the use of acoustics in environmental monitoring increases, multi‐species recognisers are likely to become more important.
Our ribbeting research supports monitoring
Monitoring the effect of ecosystem restoration can be difficult and time‐consuming.
However, frog sounds are a key indicator for the success of water delivery. Autonomous sensors, such as acoustic recorders, can aid monitoring across long time scales. While the first stage of the project focussed on mastering the call registers and recognising sounds, the next stage of the project compares frog sounds from sites with plenty of water to those with low water flow to understand the differences.
The Murray-Darling Basin is a national icon under stress in a changing climate. Ensuring this important waterway remains healthy and productive is vital for the communities, environment and industries relying on it and of course for the animals that inhabit it. Part of our ongoing work in the Basin, is to monitor water flows and measure the benefits of environmental water releases. This is not the first time Simon has used ecoacoustics as an analytic tool to monitor populations after restorative water returns to wetlands.
However, traditionally researchers have manually listened to recordings of frog and bird calls. As a result, the sounds can be much weaker, and in some cases, almost insignificant, partially obscured by ambient noises, and subject to high diurnal variation.
This led Simon and his fellow researchers to conclude that the logical next step was to trial technology-driven multi‐species call recognisers.
Hopping into the future of call recognisers
The research paper for the frog project, Evaluating and optimising performance of multi‐species call recognisers for ecoacoustic restoration monitoring, has been published in the Ecology and Evolution journal. It outlines how the team successfully developed, tested and implemented call recognisers for eight species of frog in the Murray–Darling Basin.
In addition to this research, Simon and his crew are also using species recognisers in other fields – like invasive species and endangered birds. Now the recognisers have been perfected and are capturing a range of clear frog sounds, researchers are keen to analyse further how frogs respond to environmental watering. They also want to add more frogs into the research area and continue to improve the technology as well as design small devices that process the data while recording.
So, hang on to your croak frogs! It appears these little Basin friends are about to get digitally mastered!
Eavesdrop on frogs in love
You can listen to the sounds we captured in the Murray-Darling Basin MP3 (1 MB). The first 'cackle' sound is the Peron's tree frog. Then a 'click-click' of the spotted grass frog. Finally, a 'bonk' from the Pobblebonk/banjo frog.