Scientists solve mystery of how koalas get water without leaving trees

Scientists have solved a lingering mystery about koala behaviour — how these tree-dwelling marsupials native to Australia consume enough water to live.

A new study describes koala drinking behaviour in the wild for the first time, finding that they lick water running down the smooth surface of tree trunks during rainfall — a phenomenon called “stemflow” — and do not rely merely on the water content of the leaves that make up their diet.

The findings, which the researchers said may be useful in koala conservation efforts, were based on 46 observations of koalas in the wild from 2006 to 2019, mostly at You Yangs Regional Park in Victoria state.

“I think the main message is that behavioural observations in the wild are very important to establish what is normal and what is unusual, and to truly understand what animals need. If we watch them carefully, they will tell us,” said University of Sydney ethologist Valentina Mella, lead author of the research published this week in the journal Ethology.

Koalas, which are not bears despite a common misconception, spend most of their lives high up in eucalyptus trees. They rely on a diet of eucalyptus leaves, normally consuming around 500 to 800 grams daily.

Name thought to mean ‘no drink’

The word koala is thought to have meant “no drink” in one of the Australian Aboriginal languages. The question of their water consumption had long been puzzling.

“Koalas have been alleged to never drink free water in the wild, or to drink only occasionally. Drinking behaviour has often been considered unusual and attributed to disease or to severe heat stress,” Mella said.

“Koalas were thought to gain the majority of the water that they require from the moisture content in the leaves that they feed on and to drink water unintentionally in the wild by eating wet leaves after rain, or when dew is present on the leaf surface,” Mella added.

A koala drinks water offered from a bottle by a firefighter during bushfires in Cudlee Creek, south Australia in December. Koalas were previously thought not to drink except when sick or under severe heat stress. (Oakbank Balhannah CFS/Reuters )

They sleep about 20 hours a day to conserve energy, because their diet requires a lot of energy to digest.

“Koalas are nocturnal animals, so they are only active at night, and they are arboreal, which means that they live in trees and rely on them for food, shelter and, as we just discovered, also for drinking,” Mella said.

“Koalas actually spend 98 per cent of their lives in trees and the only time they are on the ground is when they are trying to find another tree with a more generous food supply, or a mate.”

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