Danielle Wallace, PhD candidate at the University of Melbourne.

It’s a mild night amongst the sandstone plateaus of Dharawal country, north of Wollongong. I’ve dragged my sister into showing me the New South Wales wildlife, while I’m visiting from Victoria for the ESA2022 conference. As we put on our headtorches and lace up our hiking boots, a soft, warm breeze carries mingled notes of frog calls. Reedy songs of Whistling Tree frogs, “reeeeeet, reet, reet, reet”. Low, infrequent tones of the Pobblebonk, “bonk…..bonk”.  And fainter, purring calls of a stream-dwelling frog, “Burr, burr, burrrrr.” All music to the ears of a frog researcher, or herpetologist.

We begin our trek down the sandy slopes and the “burrrr, burr, burrs” become louder, mixed in with the sounds of water trickling over rocks. The dry woodland opens up at last to a view of a grand slab of orange sandstone that is pitted with deep, clear stream pools. To the right of us, the sandstone slab drops off sharply where the water cascades off the edge into the valley below. As we shine our torches across the creek, pairs of shiny orbs flash back in the dark. Frog eyeshine!

Sitting politely along the stream are multitudes of Stony Creek frogs – the culprits of the soft, purring, “burr, burr, burrrrr”. The males turn bright yellow during the breeding season to attract a mate, and the length of stream is dotted with lemon-yellow blobs. But tonight we’re on the hunt for a different frog. The charismatic Blue Mountains Tree frog. Frustratingly, parts of their call can be quite similar to the “burrs” of the Stony Creek frog.

After listening, creeping along the creek, and listening intently again, we manage to spot a Blue Mountains Tree frog up in the scrub. These big frogs have beautiful green sides and are mottled brown on top. But despite his loud calling performance, this individual is not healthy. He is emaciated and lethargic. As I handle him in a gloved hand, dry pieces of skin come away from his body and stick to my glove. On closer inspection, his underbelly and thighs are red and inflamed. All of these signs point to a sinister culprit – the infectious disease, amphibian chytrid fungus.

Despite its introduction to Australia almost 40 years ago, the amphibian chytrid fungus is still causing deaths and declines in our frogs. Chytrid fungus is caused by the fungal parasite Batrachochytrium dendrobatidis, or ‘Bd’ for short, as it’s not-so-affectionately known. Chytrid fungus attacks the cells in a frog’s skin, which are very sensitive and used in water uptake and ion exchange. Chytrid fungus infection can ultimately lead to cardiac arrest and frog death. For those of you watching or playing ‘The Last of Us’, the storyline of a fungal parasite taking over the world hits a bit too close to home. Because the worldwide fungal pandemic started decades ago for frogs.

Chytrid fungus has sadly driven over 90 species to extinction, and others inhabit just a tiny fraction of their former distribution before chytrid was introduced. Fortunately, it is not all bad news for frogs. Some very special frogs have developed a way to survive with chytrid fungus, and ensure that their offspring live on.

Enter the rotund alpine specialist – the Alpine Tree frog! These cute little guys are the high- altitude cousins to the more common, Whistling Tree frogs. When chytrid arrived, the disease hit Alpine Tree frogs hard. The species declined from over 80% of its former range. In fact, chytrid fungus was so good at killing Alpine Tree frogs that almost all mature frogs die after their breeding season.

But all is not lost! Alpine Tree frogs have a secret weapon to combat chytrid fungus. My lab’s research has shown that Alpine Tree frogs actually increase their reproductive output when they are infected with chytrid fungus. Infected males have higher sperm counts, and infected females produce more eggs. But chytrid fungus is not just changing Alpine Tree frog biology. Part of my PhD research has found that chytrid fungus is changing the breeding displays of males. I found that both in the lab and in the wild, infected males were more colourful than healthy males. So chytrid fungus could be making Alpine Tree frog sexier!

Unexpectedly, Alpine Tree frogs have found a very unorthodox way of surviving with chytrid fungus. I am now expanding my research to test whether other related species are changing their breeding displays after infection with chytrid. It is crucial that we understand the sublethal effect of chytrid fungus and how infection impacts reproduction in frogs, so that we can effectively manage wild populations and try to mitigate disease impacts.  While we may not have a cure for chytrid fungus (yet!), I have hope that our frogs will find ways to survive with this disease.

Danielle Wallace was the recipient of the 2022 Jill Landsberg Trust Fund Scholarship