News release

From: The University of Sydney

Heat stress increases koala hospitalisation and mortality

First empirical evidence linking hot days and death among endangered populations

New research from the University of Sydney has provided the first associative link between heat stress and koala mortality.

Published in Biology Letters today, the results highlight how even moderate temperature rises can increase hospital admission and mortality risk in koalas. They underscore the need to identify and mitigate heat-related threats to wildlife as the climate continues to warm.

A team of researchers led by Dr Valentina Mella from the Sydney School of Veterinary Sciences, drew on hospital admission records of almost 12,000 koalas in New South Wales between 2020 and 2022. They mapped these (and deaths after admission) against the seven-day and 14-day average temperature leading up to, and following, each admission date, and the location of each koala rescue.

Data was collected from three koala hospitals and one rescue service in New South Wales.

“From there, we were able to assess the probability of each admission and death being associated with temperature,” said Dr Mella.

The findings show the risk of admission and mortality among adult koalas increased once the seven-day maximum daily temperature exceeded 27 degrees Celsius.

Koalas

exposed to temperatures above 30 degrees Celsius in that timeframe were 1.5 to 3.5 times more likely to be admitted to hospital or die compared to those at 25 degrees Celsius temperatures (25 degrees was the mean seven-day temperature during the study period).

Populations in inland north-west NSW were shown to be particularly vulnerable to heat stress.

“Our findings show a continued rise in air temperature and increased frequency of droughts are a serious threat to koalas in inland north-west New South Wales. Adding weight to this is confirmation the inland koala population in Gunnedah in the Liverpool Plains, once considered the koala capital of the world, is now functionally extinct,” said Dr Mella.

“This research provides the most comprehensive link between rising temperatures and koala mortality,” said Dr Mella. Previous evidence relied on observational data, which is less robust and more anecdotal and individualised, she said.

Heat stress occurs when elevated core temperature from heat generation within the body outweighs a mammal’s ability to lose body heat.  Tree dwelling species like koalas are often most vulnerable to heat stress due to their limited heat tolerance and limits to where they can forage.

Some factors favour the species. Koalas have a low metabolic rate, reducing how they generate heat internally, and their dense fur insulates them against temperature extremes. Their highly efficient kidneys conserve water, crucial for regulating high body temperatures. Koala behaviours are also adapted to deal with heat, including actively seeking shade and tree-hugging.

“Koalas can handle hot weather in the short-term but their coping mechanisms aren’t sustainable in the long term,” said Dr Mella.

Another major finding of the paper was koalas with chlamydiosis, the most significant disease among the species, and those in unsuitable environments were more likely to be admitted and die due to high temperature. This indicates temperature can compound the negative effects of disease and habitat destruction.

Understanding how temperatures affect koalas is vital for mitigating future declines during heatwaves and crucial for the conservation of declining populations. Although focused on koalas, the results are important for other tree-dwelling mammals vulnerable to thermal stress.

As a result of this research, Dr Mella said wildlife rescuers can be more prepared to monitor koala populations at risk by using forecasting tools, recognising symptoms of heat stress, and learning appropriate first-aid responses.

“Without proactive intervention, the continued rise in extreme heat events could push already vulnerable koala populations closer to extinction,” she said.