Australia Weather News

Extreme heat is a major factor in the number of deaths and hospitalisations of Australians every year.

A new study in Biology Letters suggests koalas (Phascolarctos cinereus) have faced the exact same problem when there is a seemingly moderate temperature increase.

The researchers found the risk of admissions and death climbed for the furry marsupials when the seven-day average temperature maximum hit 27 degrees Celsius or higher.

This was calculated by comparing the case notes for 11,862 koala hospitalisations in NSW with weather records from 2000 to 2022.

Koala hospitalisations or death were 1.5 to 3.5 times more likely in seven-day periods where average temperatures were above 30C in their home ranges (compared to a 25C baseline).

Valentina Mella, a behavioural ecologist from the University of Sydney who led the study, said the connection between hospitalisations and temperature thresholds was "scary".

"We found that all the koalas in inland NSW were the ones exposed the most to these high temperatures because it's just hotter there," she said.

Dr Mella said longer periods of hot weather and drought from climate change, coupled with other factors like habitat fragmentation and disease, could drive local extinctions in the north-west NSW.

"These are the first populations that are going to go," she said.

"And in fact, we have already seen this because the population in Gunnedah is now functionally extinct.

"The remaining animals are infertile. Once they die, there's going to be no new animals to take their place."

Natalie Briscoe, a University of Melbourne ecologist who was not involved in the study, said it was difficult to capture climate or weather impacts on animal survival with the exception of mass die-offs like what occurs with flying foxes.

"Using koala admissions into care in this way is a neat approach to capture this association," Dr Briscoe said.

"The findings align with what we know. We have good reasons for expecting hotter temperatures to have a negative impact on koalas based on their physiology, diet and behaviour and past studies documenting responses of individual populations to extreme heat.

"Unfortunately, things are likely only going to get worse for koalas under climate change, and particularly those in northern and inland areas."

How do koalas cope with the heat?

Koalas have a range of strategies to deal with hot weather.

"They can preserve water by producing really concentrated urine," Dr Mella said.

"They can reabsorb the water from the colon, producing really, really dry pellets or scats.

"And they can have behavioural strategies like tree hugging to try to dissipate the heat against the cooler tree trunks."

The main way koalas keep cool is panting like a dog.

Panting results in a loss of water, which can be a problem in a drought.

Dr Mella said in north-west NSW when temperatures hit 30C, koalas became severely dehydrated and were seen sitting at the base of trees.

Koalas typically draw their water needs from the eucalyptus leaves they eat.

But Dr Mella said trees reabsorbed moisture from their leaves when it didn't rain.

"And then for these animals it's like chewing cardboard, really," she said.

Kara Youngentob, a landscape and nutritional ecologist at the Australian National University who was not involved in the study, said koalas also lost their appetite when they got hot, which could be a problem even with low temperature rises.

"We [humans] can go two weeks or more without eating, as long as we have water," Dr Youngentob said.

"Koalas can only last a few days; they basically have no fat reserves and they have to eat every night."

Dr Youngentob said koalas lived in a pretty fine nutrient balance because they had to process toxic eucalyptus leaves for little reward.

"They need to meet a balance of energy and protein in a diet that has very little energy and protein," she said.

What can be done to help koalas?

Dr Mella said water stations could be left out as well as shade structures built for koalas in hot regions like north-west NSW.

"We do that for cattle and we do that for animals that we use for our needs, so we really should be doing it for wildlife as well," she said.

Other researchers such as Dr Youngentob are trying to breed eucalypts with high protein and less toxins for koalas.

"The concept we have is nutritional refugia, having climate refugia with habitat where koalas can survive and they don't have to eat as much to survive."

But koalas need more than trees with their favourite food to survive.

Bill Ellis, a University of Queensland ecologist who was not involved in the study, said non-food trees were just as vital to the survival of the species as food trees.

"They can't survive without the shady trees that they sit in during the day," he said.

"When it gets hot, they will find cooler parts of their home range and cooler trees and they hop in those trees."

Dr Ellis said it was important for private properties to retain patches of bush, especially along creek lines.

"What we really need to do is incentivise that kind of behaviour and also inform [land owners] about what's going to work," he said.