Controlled hyperthermia by flying-foxes in the wild: understanding mammalian tolerance to hotter summer conditions

Extreme heat events increasingly challenge the thermoregulatory capacities of wildlife, as the frequency, intensity, and duration of these events rise under climate change. Biologging can reveal the physiological and behavioural responses of wild animals to natural variation in environmental conditions, but few studies have recorded thermoregulatory patterns during extreme heat events. Flying-foxes (Pteropus spp.) are convenient bioindicators of the impacts of extreme heat events on wildlife because they often roost in accessible colonies in trees where population-level consequences, including mass mortalities, can be readily observed. To understand how flying-foxes thermoregulate in response to extreme heat, we used implanted temperature-sensitive transmitters to record the core body temperature (T_b) of 17 free-living, adult male grey-headed flying-foxes (Pteropus poliocephalus) on 142 days across two Austral summers, including six days when air temperature (T_a) exceeded 42 °C and thousands of flying-foxes died. Flying-foxes exhibited daily heterothermy, with a decrease in T_b after dawn (minimum: 35.9 ± 0.1 °C; absolute: 31.1 °C) followed by an increase in T_b during the day (maximum: 38.7 ± 0.2 °C; absolute: 44.3 °C). Above T_a of 29.5 °C, bats allowed T_b to rise above normal levels (i.e., controlled hyperthermia). On extreme heat days (T_a > 42 °C), T_b increased by up to 6 °C to a daily maxima between 40.5 to 42.4 °C, and, consequently, the T_a at which T_a exceeded T_b occurred at 40 °C, 2.5 °C higher than without controlled hyperthermia. Large variation in T_b, as exhibited by grey-headed flying-foxes, reduces the physiological costs of exposure to thermal conditions in summer. Controlled hyperthermia during extreme heat events increases the range of T_a allowing non-evaporative heat loss and hence decreases water loss. The ability to manage elevated T_b and high rates of evaporative water loss will shape the resilience of many mammals to future extreme heat events.