The diminishing distribution of  koalas in Australia is predicted to result directly from climate change, driven by the frequency and intensity of heatwaves and drought. Little is known about the physiological responses of koalas under extreme environmental conditions. These creatures are particularly susceptible to extreme weather events because they are one of the few marsupials that do not use makeshift shelters, leaving them vulnerable to the surrounding natural environment. High mortalities of koalas have been recorded during periods of extended heat and drought, which suggests that koalas may be especially vulnerable to climate change.

 

This study set out to monitor the relationship between body temperature (Tb) of free-ranging koalas across a range of environmental conditions and ambient temperatures (Ta) across a range of environmental conditions in order to explore the physiological limits of the koala, particularly during extreme hot periods. The study was conducted at Mt Byron in Queensland, with only 14 days where Ta was > 35.0 °C at the study site. The two recorded summers were considerably mild which admittedly did not offer the best conditions to convey the impact of high temperatures on koalas, but a start nonetheless. 

 

Five free-ranging koalas were implanted with thermal transmitters and data loggers waxed together as a single package and Tb were recorded for variable periods ranging from three to twelve months. Trackers were placed on each of the koalas because they are known to travel considerable distances (> 1 km) overnight. The annual average maximum (max) Ta at Mt Byron is 20.2 °C in winter and 30.5 °C in summer, whereas the annual average minimum (min) Ta ranges between 6.4 °C in winter and 19.3 °C in summer. Thermolability became apparent immediately, the Ta temperatures were often outside the neutral zone for the koalas, which was an exciting beginning for researchers.  

 

Interestingly, the koala is considered largely nocturnal yet the peak body temperature in this study occurred during the day, whereas most nocturnal species exhibit peak body temperature during the night when animals are active. They are different to most animals, where the peak body temperature occurs during the active phase and Tb nadir during periods of rest. It became evident that cool night time temperatures appear to play an important role in heat dissipation for the koala. When minimum ambient temperature was below 22 °C, the maximum body temperature of the koala was strongly correlated to the ambient temperature. This may indicate that it was not the minimum ambient temperature that affected the diurnal Tb rhythm, but rather it appears to have been the average nighttime temperature and high daytime temperatures. It is thought that koalas become heat stressed during a period of drought and heat, causing a shift in their diurnal temperature profile so that their Tb would peak later in the day than ‘normal’.

 

Koalas are endothermic animals with an average Tb of 36.6 °C (as low as 34.2 °C and as high as 39.0 °C); Endothermy is the mechanism of generating heat by internal production. Adaptive heterothermy has typically been observed in species that have fluctuating resources such as limited access to water or unpredictable food sources. 

The advantage of adaptive heterothermy is by releasing stored heat by non-evaporative means, at night in this case, it requires less energy usage by the heat fatigued koala.

 

It is hypothesized that koalas employ adaptive heterothermy as a water saving mechanism; by gaining heat during the day and then dissipating heat during the cool off at night, koalas are able to shed this heat load, allowing for the avoidance of active dissipation of heat. The inability to release heat at night that has been absorbed during the day can lead to the surpassing of the koalas “thermal niche”. This can easily be a reality when a series of consecutive nights expose the animals with high night time temperatures. Though more studies need to be conducted to premise any conclusive temperature, this information may ultimately offer a more precise indication of the impact of climate change, thus introducing models to help sustain koala populations.

Maximum night time Ta should not exceed 28 °C for koalas in care; under this threshold, the koala can effectively release their heat and ensure promising rehabilitation. 

 

Koalas do have sweat glands but sweating as a form of thermoregulation is unlikely to be of much significance; the main form of heat dissipation occurs by panting . Most of the water requirements come from their folivorous diet, which leaves the koala restricted in terms of water influx. Long periods of hot, dry conditions leads to high mortalities in their populations. Researchers believe that this may be due to the reduced water content of the eucalypt leaves.