Southern Brown Tree Frog: Calls, Habitat, and Ice Survival

The southern brown tree frog (Litoria ewingii) is a small, surprisingly tough Australian frog with a claim to fame that sets it apart from every other vertebrate below the equator: it can survive being frozen solid. Found across southeastern Australia and Tasmania, where its high-pitched whistling call is one of the most familiar frog sounds in cooler regions, L. ewingii occupies a range spanning more than 350,000 square kilometres of moist, temperate habitat.¹ That range, its unusual physiology, and recent upheaval in how scientists classify it make the southern brown tree frog far more interesting than its modest appearance might suggest.

What It Looks and Sounds Like

Adults are small frogs, typically around three to four centimetres from snout to vent, with smooth skin in shades of brown, olive, or grey-brown. A pale stripe usually runs along the upper lip, and the backs of the thighs often show a wash of orange or yellow that flashes when the frog jumps. The belly is pale, sometimes with a faint mottling. Males are slightly smaller than females and develop a yellowish vocal sac during the breeding season. None of this screams “remarkable animal,” and people often walk right past them. What gives the species away is the call: a repeated, piercing whistle, sometimes described as a series of short, high notes that carry well across wetlands on cool evenings. It is one of the earliest calls heard in winter and spring in southeastern Australia, when many other frogs are still dormant.

Where the Southern Brown Tree Frog Lives

The species occupies a wide band of southeastern Australia, from southern New South Wales through Victoria and into Tasmania and southeastern South Australia.² It favours moist, cool environments: creek margins, farm dams, suburban garden ponds, drainage ditches, and the edges of forests. It is one of the few Australian frogs genuinely comfortable in urban and peri-urban settings, turning up in backyard water features and even downpipes.

The distribution is fragmented by well-known biogeographic barriers, including dry lowlands between mountain ranges and sea gaps separating Tasmania from the mainland. These barriers have, over long periods, isolated populations from one another and driven the kind of genetic divergence that recently led to a taxonomic revision of the species group.

A Species That Turned Out to Be Three

For decades, every population of whistling brown tree frog across southeastern Australia was lumped under the single name Litoria ewingii. A detailed genetic and morphological reassessment published in 2024 upended that view. Researchers found two additional, deeply divergent lineages hiding within what had been treated as one species. The study re-described Litoria ewingii in the strict sense as the population occurring in Tasmania, southern New South Wales, Victoria, and southeastern South Australia. A second species, Litoria calliscelis, was resurrected from historical synonymy for frogs living in the Mount Lofty Ranges and Fleurieu Peninsula of South Australia. A third, Litoria sibilus, was described as entirely new to science and is endemic to Kangaroo Island.³

That matters beyond academic naming conventions. Both of the newly recognised South Australian species have relatively small ranges compared to L. ewingii proper. Kangaroo Island in particular is a confined landmass with limited habitat, and species endemic to islands face amplified risks from habitat loss, introduced predators, and disease. Before this revision, the conservation status of those populations could be overlooked because they were assumed to be part of a widespread, common species. Now that they carry their own names, they can be assessed independently, and their restricted ranges are likely to put them in a higher risk category.

Surviving the Freeze

The headline feature of the southern brown tree frog’s biology is its ability to survive whole-body freezing. Among vertebrates, only a handful of frogs and one species of turtle are known to tolerate ice forming inside their bodies, and all the other examples live in the Northern Hemisphere. L. ewingii is the sole Southern Hemisphere vertebrate documented to pull off this trick.⁴

Field temperature recordings show that the microhabitats where the frogs shelter regularly drop below minus one degree Celsius, the point at which the frog’s body fluids begin to freeze.⁵ When that happens, ice crystals form in the spaces between cells, the heart stops, breathing ceases, and the frog appears to be dead. Yet when temperatures climb back above freezing, the frog thaws and resumes normal activity. Researchers have confirmed this recovery in wild settings, not just laboratory freezers.⁶

The microhabitats themselves play a protective role. Spots under leaf litter, logs, and bark cool down much more slowly than the surrounding air temperature. That slower cooling gives the frog’s body time to activate its protective chemistry and allows ice to form gradually rather than in a sudden, destructive rush.

How the Frog’s Body Manages Ice

Two main mechanisms help L. ewingii survive freezing. The first involves cryoprotectants, substances that reduce the damage ice crystals can inflict on cells. When L. ewingii freezes, glycerol levels in the blood rise, then fall again once the frog thaws. Glucose levels, by contrast, do not increase during freezing itself but spike on thawing.⁷ The glycerol response suggests the frog is using it as a cryoprotectant, although the concentrations measured are low compared to those found in some Northern Hemisphere freeze-tolerant frogs like the North American wood frog. Whether the modest glycerol levels are sufficient on their own or whether additional unknown protectants are at work is still an open question.

The second mechanism involves ice nucleating agents, or INAs. These are substances that seed the formation of ice crystals in a controlled way, encouraging ice to form early, at relatively warm sub-zero temperatures, and in the fluid outside of cells rather than inside them. The frog’s skin secretions contain active INAs, and so does the substrate of the microhabitat the frog shelters in. INA activity in the skin shows a strong seasonal pattern, with greater activity in winter than in summer, suggesting the frog ramps up its preparedness as cold weather approaches.⁸ This was the first demonstration that a frog’s own skin secretions contribute ice nucleating agents, not just the environment the frog sits in.

Together, the cryoprotectant and the controlled ice nucleation form a system designed to prevent the worst-case scenario: ice forming rapidly inside cells, puncturing membranes and killing tissue. The frog does not avoid ice. It manages where and how ice grows.

The Hybrid Zone with Its Near-Twin

The southern brown tree frog has a close relative, Litoria paraewingi, sometimes called the plains brown tree frog. The two species look nearly identical. They overlap in parts of southeastern Australia, and where their ranges meet, they form a narrow hybrid zone that has been studied for decades as a textbook example of how closely related species stay separate.

The primary barrier between them is their call. Males of L. ewingii produce a higher-pitched, more rapidly repeated whistle, while L. paraewingi males give a slower, lower-pitched call. Females preferentially approach the call of their own species, which limits interbreeding. Genetic analysis of populations across the contact zone confirms that the two species are genetically distinct and that hybridisation is rare. Most of the admixed individuals detected are later-generation hybrids rather than first-generation crosses, meaning that while occasional mating between the two does happen, hybrids do not seem to reproduce as successfully as pure-species individuals.⁹

The hybrid zone is best characterised as a tension zone: the boundary is narrow, allele frequencies shift sharply across it, and genetic markers from both mitochondrial DNA and nuclear markers show concordant cline centres.¹⁰ In plain language, the zone is maintained by selection against hybrids rather than by a physical barrier like a mountain range. Hybrids are at a disadvantage, possibly because their calls fall between the two species’ types and fail to attract females of either species effectively, or because of subtler genetic incompatibilities. The zone does not appear to be moving or collapsing, which tells researchers that the two species have reached a stable standoff.

For anyone who encounters these frogs in the field, the practical upshot is that the call is the easiest way to tell them apart. Visual identification alone is unreliable.

Antimicrobial Skin Chemistry

Frog skin is not just a passive barrier. Many species produce a cocktail of peptides, some of which have antimicrobial properties. In L. ewingii, skin secretions from adults and newly metamorphosed froglets contain a peptide called uperin 7.1 that inhibits the growth of E. coli in laboratory assays.¹¹ The presence of the peptide at the metamorph stage is interesting because newly emerged froglets face an immediate onslaught of environmental pathogens as they move from water to land. Having antimicrobial defences ready at that vulnerable moment is a meaningful advantage.

Amphibian skin peptides have attracted attention from biomedical researchers searching for new antibiotic leads. The peptides tend to work by disrupting microbial membranes in a way that is harder for bacteria to evolve resistance against compared to conventional antibiotics. Whether uperin 7.1 has practical pharmaceutical potential is another question, but its existence underlines how chemically sophisticated frog skin can be, even in a small, common species that most people would not look at twice.

Why a Common Frog Still Deserves Attention

The southern brown tree frog is not endangered across most of its range. It turns up readily in suburban gardens, farm dams, and disturbed habitats. That very commonness, though, can breed complacency. The 2024 taxonomic revision showed that what everyone assumed was one widespread species actually includes at least two island and peninsula endemics with far smaller ranges. Those populations were invisible to conservation planning until someone looked closely at the genetics.

The same story plays out across amphibian biology worldwide: species that seem widespread and secure turn out, on closer genetic examination, to be complexes of distinct lineages with different vulnerability profiles. For L. ewingii specifically, the newly described Kangaroo Island species L. sibilus has nowhere else to go if its habitat is compromised. Its entire known range is a single island.¹²

Meanwhile, the chytrid fungus Batrachochytrium dendrobatidis, which has devastated amphibian populations on every continent where frogs live, is present across much of southeastern Australia. L. ewingii populations appear to persist in areas where chytrid is circulating, which has made the species a useful model for studying how some frogs coexist with the pathogen while others are driven to extinction. The frog’s antimicrobial skin peptides, its cool-climate physiology, and potentially its immune response all figure into that resilience, though the precise mechanisms are still being untangled.

Backyard Encounters and Breeding Behaviour

If you live in southeastern Australia, particularly in Victoria or Tasmania, and you have a pond or even a persistently damp corner of the garden, there is a good chance you have heard L. ewingii calling. Males call from the water’s edge and from low vegetation near water, often starting in late autumn and continuing through winter and into spring, a breeding season that coincides with some of the coldest months. That timing fits the species’ cold-tolerance physiology: while many Australian frogs breed in warmer months, the southern brown tree frog is active when few competitors are calling.

Females lay eggs in loose jelly masses attached to submerged vegetation. The tadpoles develop slowly in cold water, sometimes taking several months to reach metamorphosis depending on temperature and food availability. Newly metamorphosed froglets are tiny, just over a centimetre long, and face high predation from birds, fish, and invertebrates. The extended larval period in cold conditions is another aspect of the species’ life history shaped by its cool-climate niche.

People sometimes worry when they find these frogs wedged into tight crevices behind shutters, inside meter boxes, or under bark during winter. The frogs are simply using sheltered microhabitats that buffer them against the worst of the cold and, if temperatures drop far enough, give them the slow cooling rate they need to freeze safely rather than catastrophically. Leaving the frog undisturbed is the best course of action. The same goes for garden ponds: if you hear calling in your pond during winter, the simplest contribution to the species is to leave some vegetation in and around the water and avoid introducing fish that eat tadpoles.