Koalas & Water
(what current science tells us)
(what current science tells us)
Koalas are often described as “leaf-drinkers” that seldom need a sip of free water. That simple story is fading.
Over the past decade researchers have used camera traps, water chemistry tests and tiny body-temperature loggers to watch koalas more closely than ever. These studies show when and why koalas drink, how their natural “tree tap” system works, what happens during droughts and heatwaves, and how nearby water can tip the balance between life and death for this treasured animal. (Gardiner et al. 2023).
1. From myth to measurement
For most of the twentieth century textbooks claimed that healthy koalas rarely drink because eucalypt leaves hold about fifty to sixty percent water and the animals recycle much of the nitrogen that other mammals lose in urine but a review of field notes and camera-trap footage collected between 2006 and 2019 overturned that view. We are learning more as the years go on.
Forty-six separate events showed wild koalas licking thin streams of rainwater that trickled down trunks and branches after showers. The behaviour occurred on winter nights near six degrees Celsius as well as on humid summer evenings above thirty. So it wasn't just weather-dependent.
The researchers have concluded that the long-held “no-drink” idea persisted only because koalas are nocturnal, high in the canopy and hard to watch, not because they are independent of free water (Mella et al. 2020; Mella et al. 2019).
2. Tree-trunk tap rooms and the science of stemflow
Stemflow begins the moment raindrops strike the leafy crown of a eucalypt. Part of the water is caught on the waxy surface of the leaves, part drips straight to the ground, and a small but crucial fraction creeps along the underside of twigs, gathers on branches, and finally pours down the trunk like a miniature gutter. The smoother and less flaky the bark, the fewer obstacles that water meets and the faster the flow. Field trials show that spotted gum Corymbia citriodora and lemon-scented gum C. maculata can channel more than five per cent of a moderate rainfall event which is a surprisingly large volume for a single tree, while the heavily furrowed bark of a stringybark diverts most water away from the trunk surface.
To measure exactly what a koala might be drinking, researchers in 2024 and 2025 fitted sixty healthy trees across four eucalypt species with sterile plastic collars positioned one metre above ground. Each collar directed trunk water into dark, ice-chilled bottles so sunlight and heat could not change the chemistry. Every sample was paired with a control taken from a nearby tree of the same species that showed no sign of koala activity. Rain events ranged from brief spring showers of two millimetres to late-summer storms exceeding twenty millimetres. Recorded flow rates varied widely with both bark texture and rainfall intensity, from only half a litre per hour on rough-barked ironbark to 4.3 litres per hour on the smooth gums that koalas prefer for resting.
Putting the numbers into practical terms, an adult koala weighing six to eight kilograms needs roughly fifty to one hundred millilitres of free water on a hot, dry day. A single hour of high-quality stemflow on a smooth-barked gum can therefore meet or exceed the daily requirement for the animal perched on that trunk. The authors of the study now recommend that habitat-suitability models give explicit scores to bark smoothness, branch angle and local rainfall patterns, arguing that these micro-habitat traits directly control the availability of clean, easily accessible water for koalas and many other arboreal species (Flanagan et al. 2025).
3. When leaves run dry: learning from artificial drinkers
Extreme heat and drought can sap the moisture from eucalyptus leaves. When the water content in these leaves dips below about fifty-five percent of their fresh mass, koalas start to show higher blood salt levels and produce more concentrated urine.
To see if providing extra water makes a difference, researchers set up sixteen ground bowls and another sixteen tree-mounted troughs across two grazing properties in north-west New South Wales. Over the course of a year, cameras captured 2,452 koala visits, with each visit averaging around thirty-four seconds. Interestingly, these visits got about 11% longer for every five-degree increase in the day’s maximum temperature.
Koalas preferred the elevated troughs seventy-three percent of the time, likely because being up high feels safer than being on the ground. The trial quickly led to a reliable design: a twenty-liter, food-grade poly trough secured two to four meters up the trunk, shaded by a corrugated visor, and refilled every forty-eight hours, with a weekly scrub rinse. Flow meters in four of the troughs indicated that a typical koala drank between eighty to one hundred milliliters during each visit. Other wildlife also reaped the benefits, including brushtail possums, sulphur-crested cockatoos, and short-beaked echidnas.
Leaf moisture was a reliable indicator of demand: when the foliage dropped below fifty-five percent water content, the number of nightly trough visits more than doubled (Mella et al. 2019).
4. Heatwaves and heterothermy (a keeping cool trick)
During the record-breaking summer of 2023 to 2024 researchers fitted miniature data loggers to twelve wild koalas in northern New South Wales to see exactly how their bodies cope with extreme heat. The devices recorded internal temperature every ten minutes while thermal cameras tracked behaviour in the canopy (Mella et al. 2024).
A built-in “pre-cooling” trick
- Before sunrise on days forecast above thirty-eight degrees Celsius, each koala quietly lowered its core temperature from the usual thirty-six degrees to about thirty-two. Scientists call this strategy “heterothermy” because the animal allows its body to become temporarily cooler than normal. Starting the day chilled creates a safety margin. As the air heats up the koala’s temperature rises slowly and does not reach dangerous levels until late afternoon.
How much water does the trick save?
- By reducing the temperature difference between body and air, the koala needs less evaporative cooling. Calculations show a saving of up to one fifth of the water that would otherwise be lost through panting or licking its forearms, a common marsupial cooling method.
Why extra drinking is still vital
- Even with this clever physiology two of the instrumented koalas left their midday roosts when the mercury passed forty degrees. They climbed down to elevated troughs installed as part of the same study and drank about eighty millilitres each, measured by flow meters in the troughs. On those days fresh leaf samples contained less than fifty-one per cent water. The combination of hot air and drier leaves meant that foliage alone could not satisfy the animals’ needs.
Disease makes heat stress worse
- Koalas in the project that were already fighting urogenital chlamydiosis started the day with slightly higher blood salt levels and visited drinkers more often than healthy neighbours. The finding suggests that illness amplifies the dehydration risk posed by climate extremes.
Together these results show that heterothermy buys valuable time but cannot replace a reliable drink.
As heatwaves become longer and more frequent, natural stemflow and well-designed supplementary water stations will be essential for koala survival (Mella et al. 2024).
Some final thoughts on koala and drinking
The latest evidence is clear: koalas do drink free water, and they have some pretty clever ways of doing it.
Long-term camera studies have debunked the old myth that they only get their hydration from leaves. These studies show koalas sipping rainwater from tree trunks during chilly winter nights and humid summer evenings. Research on stemflow indicates that smooth-barked eucalypts can provide litres of clean, low-salt water every hour—more than enough to satisfy a koala’s daily thirst after just one rain shower. Tests with elevated troughs reveal that when the moisture in leaves drops during droughts or heatwaves, koalas actively seek out additional water. This behaviour helps them maintain their body weight and reduces the stress that can lead to illness. Bio-logging studies add the final touch, showing that even the koalas' smart methods for cooling down can’t fully combat extreme heat without access to water.
All these findings change the way we understand Australia’s beloved tree-dwellers. While koalas are perfectly adapted to munching on eucalypt leaves, they’re not entirely self-sufficient. They keep an eye on the water available in their surroundings and make a conscious effort to stay hydrated. Recognizing this behavior has important implications. Conservation strategies should focus on protecting smooth-barked gums that have high stemflow potential. Landowners can set up safe, tree-mounted drinkers to help koalas adapt to climate changes. Wildlife carers can confidently provide water to koalas suffering from heat stress, knowing that it’s not just a helpful gesture but a vital resource that nature offers.
In essence, koalas do drink—whether it’s from rain-soaked trunks or human-made troughs. Acknowledging this straightforward fact is crucial for ensuring their survival in an increasingly hot and dry Australia.
Finally, should you find a koala that appears heat-stressed, dehydrated or injured, do not attempt to handle or treat the animal yourself. Excessive drinking by a koala is not normal and needs further investigation. Contact your local wildlife rescue organisation or veterinarian immediately.
Learn more about excessive drinking by koalas and what this means - read about renal disease in koalas
Disclaimer:
This article summarises peer-reviewed studies published between 2019 and early 2025.
New findings may refine or revise the conclusions.
Most research cited was conducted in eastern Australia (New South Wales and Queensland). Koala behaviour and water availability can differ in other regions, especially where rainfall patterns, tree species or disease pressures vary.
In regards to water stations, installing or maintaining wildlife water stations should follow state regulations and best-practice guidelines developed by licensed wildlife carers, veterinarians and land-management authorities. Improperly designed drinkers can spread disease or attract predators. Artificial drinkers require regular cleaning (ideally ultraviolet or chlorine treatment) and secure mounting to minimise bacterial build-up and reduce the risk of transmitting pathogens such as Chlamydia pecorum.
Reference list
Flanagan, C., Krockenberger, M. B., Van Stan, J. T., Duffy, J. and Mella, V. S. A. (2025) ‘Koalas (Phascolarctos cinereus) and stemflow: drinking more than just water’, Austral Ecology, 50 (5), e70076.
Gardiner, R., Terraube, J., Frere, C. and Cristescu, R. (2023) ‘Roads and water availability influence the occurrence of koalas (Phascolarctos cinereus) in secondary habitat: a multiscale approach’, Biodiversity and Conservation, 32, 163–180.
Mella, V. S. A., McArthur, C., Krockenberger, M. B., Frend, R. and Crowther, M. S. (2019) ‘Needing a drink: rainfall and temperature drive the use of free water by a threatened arboreal folivore’, PLOS ONE, 14 (5), e0216964.
Mella, V. S. A., Orr, C., Hall, L., Velasco, S. and Madani, G. (2020) ‘An insight into natural koala drinking behaviour’, Ethology, 126 (8), 858–863.
Mella, V. S. A., Cooper, C. E., Karr, M., Krockenberger, A., Madani, G., Webb, E. B., Krockenberger, M. B. and Narayan, E. (2024) ‘Hot climate, hot koalas: the role of weather, behaviour and disease on thermoregulation’, Conservation Physiology, 12 (1), coae032.
Webb, E. B., McArthur, C., Woolfenden, L., Higgins, D. P., Krockenberger, M. B. and Mella, V. S. A. (2022) ‘Risk of predation and disease transmission at artificial water stations for a threatened arboreal folivore’, Wildlife Research, 49 (4), 324–334.