What’s in a whistle? Your go-to guide for telling frog whistles apart

Citizen science data from the FrogID project helped document the distribution and advertisement call variability in five species of tree frog.

How do we tell frog species apart? Firstly, there’s appearance. Assuming you can see the frog, that usually that does the trick. Some frog species, however, are very secretive, and others can look almost identical.

Thankfully, there’s the male advertisement call (the sounds frogs make when they’re yelling out for a mate, of the romantic variety), which is often the only way you can tell frogs are around. Some frog species, however, sound incredibly similar, which can make identifying the frogs around you quite difficult.

The study

In a recent study, the advertisement calls of five species of whistling frogs found in Southeastern Australia were examined. These species look and sound very similar and, to complicate things, three have overlapping ranges – the Brown Tree Frog (Litoria ewingii), the Whistling Tree Frog (Litoria verreauxii) and the Victorian Tree Frog (Litoria paraewingi). Two are brand new to science – the South Australian Tree Frog (Litoria calliscelis) and the Kangaroo Island Tree Frog (Litoria sibilus) – but identifying these ‘new kids on the block’ is straight-forward as they’re geographically separated from everyone else. So, despite sounding similar to the other species, location gives them away.

We were looking for a way to tell these little whistlers apart from each other and to design a handy little guide for people to use. And while we were at it, we used advertisement calls from all over the species ranges so we could look at their geographic variation.

Citizen science

This study used data from the Australian Museum’s FrogID app. If you haven’t heard the scoop, FrogID is a citizen science app that allows anyone to capture and submit recordings of frog calls, anywhere, anytime. For this study alone, researchers combed through roughly 20,000 recordings! All of which would not have been possible without the help of citizen scientists across Australia using the FrogID app to record frogs.

So, what did we find?

We found the most variation was in the number of notes in a call, the rise and fall of how loud each note was (which we can visualise looking at amplitude over time) and the position of the longest and shortest notes in the call. It helps to think of calls and notes like sentences and words. A note can be thought of as a word and several notes (words) make up their call (like a sentence). Therefore, the number of words, the loudness of the words and where the longest and shortest words are in the sentence is how we tell these species of whistling frogs apart.

Brown Tree Frog (Litoria ewingii) recording submitted to FrogID

^{Audio & Copyright: Darren Roache}

Whistling Tree Frog (Litoria verreauxii) recording submitted to FrogID

^{Audio & Copyright: Airlie Worrall}

Like previous studies using FrogID data, we also found that there is geographic variation present in the calls of Brown Tree Frogs and the Whistling Tree Frogs, the species with the largest ranges. Examining the geographic variation is interesting as it can show us if certain populations sound a bit different to others, much like people’s accents. This is something we’re starting to understand is much more common than previously thought.

Research like this is the groundwork for frog conservation. It enables us to understand more thoroughly how to accurately identify species, and therefore understand any changes in their distribution or abundance. This large-scale research is only possible due to projects like FrogID. Citizen science is an absolute game changer for frog research and its continued future means the continued survival of frog species in Australia.

Jessica Elliot-Tate, Former Honours student, Australian Museum Research Institute & UNSW Sydney