Koalas Can Predict the Hottest Summer Days and Lower Their Body Temperatures Accordingly, Study Finds

A new study by researchers in Australia has found that iconic and endangered koalas have the ability to regulate their body temperatures more than previously thought.

For the first time, scientists have observed free-ranging, wild koalas drastically lowering their core body temperatures during cool mornings in preparation for the most sweltering summer days.

“This strongly suggests koalas predict the hottest days from morning conditions and adjust their core temperatures accordingly. We have never seen this type of behaviour before in koalas,” said Dr. Valentina Mella, a zoologist with The University of Sydney’s Sydney School of Veterinary Science, in a press release from The University of Sydney.

For two weeks during the hottest part of 2019, a team of researchers from Australia observed a koala colony in northwest New South Wales.

The hottest day during the study was 105.4 degrees Fahrenheit. On that day, the research team recorded the highest ever body temperature in koalas, which was the same temperature.

That morning, the team had recorded the lowest ever body temperature for a koala, which was 90.3 degrees Fahrenheit, suggesting the marsupial regulates body temperature more than previously believed.

“This self-regulation requires individual koalas to predict days of extreme temperature from overnight and early morning conditions, adjusting their body heat regulation accordingly,” Mella said in the press release. “Our results indicate that air temperature and koala body temperature are closely aligned. What surprised us was the self-regulating animals ‘allowed’ their core temperatures to fluctuate with environmental conditions, a possible adaptive tactic to reduce evaporative cooling, saving an estimated 18 percent of water. It seems that this self-regulation of body temperature might play a more important role in surviving hotter days than known behavioural tactics, such as tree-hugging.”

So how do koalas lower their core body temperatures on cooler mornings?

“The specific mechanism is not known but it is hypothesised to have something to do with solar radiation and the redistribution of warm core blood to the cool body periphery,” Mella told EcoWatch in an email. “The greater the solar radiation intensity in the morning, the lower the minimum core temperature would be.”

Mella told EcoWatch that lowering their body temperatures may not be an entirely safe strategy for koalas in the face of the rapidly increasing temperatures associated with climate change.

“Starting with a lower body temperature in the morning provides scope for letting body heat rise with air temperature during the day, rather than attempting to keep temperature strictly in the normal range using body water and other techniques to cool down. However, as temperatures increase due to climate change, this survival technique could become quite risky. Temperatures above 40 degrees can be fatal for leaf-eating mammals like koalas. Hence, this seems to be a coping mechanism rather than a real evolutionary strategy. Koalas have no choice but to attempt to survive the heat by letting their body temperature fluctuate with environmental conditions if other strategies are ineffective or too costly,” Mella told EcoWatch.

In addition to taking advantage of cooler mornings to preemptively lower their body temperatures, koalas have techniques to cool themselves down on Australia’s stifling summer days.

“Koalas have evolved specific physiological and behavioural strategies to keep cool in hot weather. They have highly insulative fur, produce concentrated urine to preserve body water, they have low metabolic rates to minimise heat production, and they pant and lick their fur to facilitate evaporative cooling. They also seek shade and adopt tree-hugging postures that promote heat exchange and they drink free water,” Mella said.

Mella’s research team had previously documented koalas drinking water made available to them on extremely hot days, a formerly unknown behavior. 

Mella said the cooling strategy of lowering core body temperature had not been previously documented in koalas, but had been seen in another iconic Australian species.

“While this is the first time that this type of mechanism has been observed in koalas, western grey kangaroos have also been reported to predict hot days and lower their morning body temperature. This was associated with nutritional deficits, indicating that koalas may also be nutritionally challenged and may have no choice but [to allow] their body temperature to fluctuate,” Mella told EcoWatch.

^{Zoologist Dr. Valentina Mella holds a tagged koala. The University of Sydney}

Mella pointed out that tree-hugging was not a very effective strategy for koalas in combating extreme heat.

“While we did observe tree-hugging on hot days, this did not seem to lower core body temperature markedly. While this could partly be due to the type of trees, this might not be a central strategy in body temperature modulation for this koala population,” Mella said in the press release.

Koalas try to keep a core body temperature of 97.3 degrees Fahrenheit, give or take 2.4 degrees Celsius.

Mella added that the koala population in the study — in New South Wales, near Gunnedah — was older and suffered from chlamydial disease, like many koala colonies.

Six months after the study, all the koalas who the researchers observed were still alive, suggesting that modulating body temperature is a koala survival strategy.

Mella told EcoWatch that global heating presents a significant danger to koalas and their habitat and emphasized the importance of bolstering koalas’ “resilience to heat stress” by providing them with water and preserving the larger, older trees that give them shelter.

“The increase in ambient temperature combined with more frequent and severe heatwaves throughout the koala habitat expected with current climate predictions, seriously threatens koala survival and that of the trees they depend on,” Mella told EcoWatch.

The study, “Hot climate, hot koalas: the role of weather, behaviour and disease on thermoregulation,” was published in the journal Conservation Physiology.

“Global climate models forecast that dry, hot weather will escalate and drought events will increase in frequency, duration and severity. This is likely to push koalas and other tree-dwelling leaf-eating mammals towards their thermal limit,” Mella said in the press release. “Our results reinforce the importance of climate mitigations for ensuring future survival of koalas.”