Why do koalas drink little or no water?

Koalas are unusual animals in many ways, including their drinking habits. Koalas get most of their moisture from the eucalyptus leaves they eat, so they don’t need to drink much additional water. Here’s an in-depth look at why koalas drink little or no water.

Koalas’ Diet of Eucalyptus Leaves

The key to understanding why koalas drink little water lies in their diet. Koalas feed almost exclusively on eucalyptus leaves. Eucalyptus is a genus of over 700 species of flowering trees and shrubs. Koalas mainly eat leaves from about 30 of the 600 edible eucalyptus species.

Eucalyptus leaves have several properties that enable koalas to get all the moisture they need without drinking much water:

  • High water content – Eucalyptus leaves have a water content of around 75%. This provides koalas with a lot of pre-packaged water.
  • Low nutritional content – The leaves are very low in nutrients and calories. This means koalas don’t need to drink water to help digest or metabolize the leaves.
  • High fiber content – Eucalyptus leaves have a high fiber content. This fiber absorbs water in the koala’s gut, further reducing their need for additional water.

The eucalyptus diet is key for koalas to thrive in Australia’s hot, dry climate. The high water content in the leaves provides all the moisture they require.

Low Energy Levels

Another reason koalas need little water is their low energy levels. Koalas have an extremely slow metabolism and spend most of their time dozing or resting in trees. The less active an animal is, the less water it needs to cool itself and transport nutrients around its body.

On average, koalas sleep around 20 hours per day. They rarely move very far or quickly. This lethargic lifestyle means koalas use very little energy and have minimal water losses through sweating or breathing.

Kidney Structure and Function

Koalas also have specific adaptations in their kidneys to utilize water very efficiently and avoid dehydration. A koala’s kidney structure and function differs from many other mammals in several ways:

  • Large kidney to body size ratio – A koala’s kidneys are disproportionately large compared to their overall body size. The relatively huge kidneys have more capacity to reabsorb and conserve water.
  • Low glomerular filtration rate – Koalas have a low glomerular filtration rate, meaning their kidneys filter blood more slowly. This conserves water by retaining more fluid and producing less urine.
  • High urine concentration – Koalas produce urine that is particularly concentrated. Their urine has very little water and high levels of salts and toxins extracted from eucalyptus leaves.

Thanks to these specialized adaptations, koalas use water extremely efficiently. Their oversized kidneys can concentrate urine and pull as much water back into the body as possible.

Nasal Heat Exchange

Another unusual anatomical feature helps koalas save water by cooling themselves without sweating or panting. Koalas have a nasal cavity separated by a cartilaginous septum. This allows them to recycle moisture while breathing.

When koalas inhale, the air is cooled as it passes over moist nasal turbinates. On exhalation, the air picks up more moisture and cools the nasal passage again. This effect minimizes water loss during breathing.

Low Water Availability in Eucalyptus Forests

The animals’ habitat itself also explains their limited water intake. Koalas live in eucalyptus forests, woodlands, and tree savannas. These areas typically have relatively low and unpredictable rainfall. During dry periods, there may be little free standing water available for koalas to drink.

Koalas are adapted to get by on the moisture in eucalyptus leaves, since drinking water sources can disappear for weeks or months at a time. Relying solely on leaves allows koalas to survive droughts that could otherwise be deadly.

Seasonal Variation in Water Intake

Koalas do vary their water intake somewhat at different times of year. During the hot Australian summer, koalas may drink a bit more water if it is available. Lactating female koalas also increase their water intake to produce milk for their joeys.

However, koalas still get the vast majority of their moisture from leaves year-round. Drinking is just a supplement to their main hydration source.

Water Intoxication Risk

Interestingly, koalas can actually get ill if they drink too much water. Since they are adapted to get moisture from leaves, excessive water can overwhelm their kidneys. Water intoxication causes electrolyte imbalances that can make koalas quite sick.

Koala joeys are especially vulnerable to water intoxication. Care must be taken with captive koalas to ensure they don’t overhydrate.

In the wild, koalas’ instincts keep them from drinking dangerous amounts. Their adaptations make supplemental water entirely unnecessary for their survival as long as they have access to eucalyptus leaves.

Comparison to Other Marsupials

The koala’s minimal need for water makes it unique even compared to other tree-dwelling marsupials. For example:

  • Ringtail possums often drink water in addition to getting moisture from leaves.
  • Sugar gliders get some of their water needs from nectar and insects rather than just leaves.
  • Greater gliders eat a more varied diet including Acacia gum, so they require additional water sources.

The koala is the only marsupial that can get by almost completely without drinking. Its adaptations make it capable of surviving solely on the water in eucalyptus leaves.

Reasons Koalas Venture Down from Trees

Since they don’t need to find water sources, koalas rarely come down from the safety of the trees. However, there are a few circumstances that cause koalas to venture to the ground.

  • Seeking shelter – During severe storms or heatwaves, koalas may climb down to find cooler, shadier shelter.
  • Dispersing between trees – Young koalas leaving their mother’s home range must walk across open ground between trees.
  • Escaping bushfires – Koalas will try to flee and escape burning eucalyptus forests during wildfires.

In most cases, koalas try to limit ground time to a bare minimum. They are quite vulnerable to predators and accidents when not in trees.

Consequences of Dehydration

If koalas don’t get enough moisture from leaves, they can become dehydrated. This is most common during extreme heat waves and droughts when eucalyptus trees dry out.

Dehydration impacts koalas in several ways:

  • Increased lethargy – Koalas conserve energy and move as little as possible to limit additional water losses.
  • Urinary tract infections – Concentrated urine and lack of water can lead to UTIs that can be fatal.
  • Electrolyte imbalances – Lack of water can disrupt ion levels in cells and blood plasma.
  • Constipation – Dehydration causes extremely dry stools that are painful and difficult to pass.
  • Kidney damage – Excessive dehydration can cause temporary or permanent kidney impairment.

In severe cases, dehydration can lead to organ failure and death. However, koalas do have behavioral adaptations to cope with hot, dry periods to some extent. During droughts they will:

  • Spend more time resting to conserve energy.
  • Seek out shadier, cooler areas of the canopy.
  • Choose smaller, younger leaves that retain more moisture.
  • Drink small amounts of water if available.

Why Captive Koalas Need More Water

While wild koalas can thrive drinking only minimal water, captive koalas require supplementation. There are several reasons captive koalas need additional water sources:

  • Higher energy expenditure – Captive koalas move around and expend more energy than wild koalas.
  • Water content of leaves – Leaves fed to captive koalas are not always as naturally hydrated as fresh eucalyptus.
  • Dry air – Enclosure conditions are often hot and dry with minimal shade.
  • Limited leaf choices – Not all eucalyptus species fed in captivity have ideal water content.

Zoos and wildlife parks must provide drinking water and misters for koalas to maintain healthy hydration levels.

Koala Conservation and Water

Habitat loss is a major threat to koala survival today. Clearing of eucalyptus forests for agriculture and development destroys the koala’s main food and water source. Even small disruptions to water cycles in eucalyptus groves can impact koalas.

Climate change and droughts also strain koalas by drying out leaves. Bushfires fueled by hot, dry conditions can decimate koala populations. Protecting remaining eucalyptus habitats from deforestation and climate extremes is crucial for koala conservation.

In areas where eucalypts are depleted, wildlife groups sometimes provide drinking water stations for koalas. However, restoring healthy, thriving eucalyptus forests is the only sustainable way to maintain stable koala populations that don’t rely on supplemental water.

Unique Adaptations for Water Conservation

Koalas have a range of highly specialized anatomical and behavioral adaptations that allow them to get all the moisture they need from eucalyptus leaves. Key examples include:

  • Oversized kidneys with excellent water retention abilities
  • A lethargic lifestyle with minimal energy and water needs
  • Nasal passages that cool air without losing moisture
  • Ability to concentrate urine to a very high degree
  • Instincts to seek shade and rest during hot weather

Koalas are the only mammals able to survive healthily on minimal free water. Their adaptations make them perfectly suited to Australia’s eucalyptus forests.


Koalas have a range of anatomical and behavioral adaptations that allow them to get all their moisture needs from eucalyptus leaves. Key factors that enable koalas to stay hydrated with little or no drinking water include:

  • The high water content of eucalyptus leaves
  • Koalas’ low-energy lifestyle and water requirements
  • Their highly efficient kidney function
  • Nasal cavity structure that recycles moisture while breathing
  • The typically low water availability in eucalyptus forests

Koalas’ unusual drinking habits are a result of their specialized adaptations for getting water from their food. Their ability to thrive without drinking is unique among mammals.

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