How do flying-foxes respond to high temperatures
The world’s largest bats are known as flying-foxes, and despite evolving in warm tropical and subtropical regions, they suffer on hot days.
Dr Les Hall
Article by Natural Areas Conservation Officer, Sunshine Coast Council
The world’s largest bats are known as flying-foxes, and despite evolving in warm tropical and subtropical regions, they suffer on hot days. In addition to their nocturnal nature, flying-foxes have evolved a number of strategies to regulate their body temperature as the ambient temperature increases. Flying-foxes generally roost in shady trees during the daylight to rest and remain cool, however they become exposed to direct sun. At temperatures above 23 °C, flying-foxes begin fanning their wings to cool. This provides radiant cooling, where warm blood flows through the tiny surface vessels in the wing and heat is lost to the atmosphere, thus cooling the animal. They actively seek shade among mid-story vegetation of their roosts at temperatures around 35 °C. At higher temperatures flying-foxes will pant and lick their wings to facilitate evaporative cooling, and they climb further down trees to the ground to attempt to escape the heat.
Flying-foxes do not sweat and their evaporative cooling is limited to licking, which causes dehydration and hypoglycaemia. Ambient temperatures above 38 °C are greater than a flying-fox’s body temperature, meaning they can no longer reduce their core temperatures. Heat stroke threatens and at these temperatures flying-foxes may begin to fall from trees. Generally, animals with higher metabolic rates such as lactating and pregnant females will have lower thermotolerance, and these individuals are the most susceptible to heat stroke. At 42 °C biological processes break down, causing heat stroke and mass mortality of flying-foxes.
A natural threat
Excessive heat is the key natural threatening processes to flying fox populations. It is not uncommon for thousands to tens of thousands of flying-foxes to die when excessive heat impacts regions of eastern Australia. The effect of high temperatures on Australian wildlife, including flying-foxes, was first recorded by several officers in charge of the infant colony of Sydney in February 1791. Temperatures at Parramatta (known at the time as Rose Hill) exceeded 40 °C with a hot-dry north-westerly wind. Captain Watkin Tench of the Royal Marines, observed, “An immense flight of bats driven before the wind…dropped dead or in a dying state…”
Captain David Collins, Judge Advocate of the colony wrote, “…immense numbers of the large fox bat were seen hanging at the boughs of trees and dropping into the water…during the excessive heat many dropped dead while on the wing….”
The Governor of New South Wales, Captain Arthur Phillip, noted, “From the numbers that fell into the brook at Rose Hill, the water was tainted for several days, and it was supposed that more than twenty thousand of them (bats) were seen within a space of one mile.”
To this very day there is a flying-fox roost at Parramatta Park, the location of the original Government house at Rose Hill, near the head of the Parramatta River.
The impact of climate change
The most recent State of the Climate Report (2022) published by CSIRO and The Bureau of Meteorology states Australia’s climate has warmed on average by 1.47 ± 0.24 °C since records began in 1910, and the frequency of extreme heat events has increased. Predictions indicate this trend will continue, where extreme heat is considered the major natural hazard causing death to humans, wildlife, and infrastructure in Australia.
Environmental consequences
Flying-fox populations are declining, and currently the grey-headed flying-fox and spectacled flying-fox are listed as vulnerable to extinction under the Environmental Protection and Biodiversity Conservation Act, 1999 (EPBC Act). These declines are a result of extensive vegetation clearing and active persecution of the animals. Heat related mass mortality across entire regions exacerbates declines. This situation is alarming because flying-foxes are critical long-range pollinators and seed dispersers of hundreds of plant species along Australia’s east and north coasts. Moreover, because roosting flying-foxes in urban areas are conspicuous, the heat impact on flying-fox populations is highly visible if they die in large numbers, which provides an indication of what may be occurring with less visible wildlife species.
What does Council do?
Local government is guided in flying-fox management by State and Commonwealth legislation, however there are no statutory requirements under Queensland State and Australian Government legislation relating to wildlife heat stress events. Sunshine Coast Council’s Regional Flying Fox Management Plan (RFFMP) does recognise the requirement for management actions during heat stress events. For this reason, Council are preparing a Flying-Fox Heat Stress Response document to guide prevention, preparation and the response procedures to heat stress impacted flying-foxes.
Ideally, we can prepare for heat stress by planting dense understory and mid-story vegetation within a roost to provide additional shade. If high temperatures are predicted, council can work with local bat rescue organisations and wildlife carers to prepare and conduct a rescue. If flying-foxes die from the heat, council also has capacity to remove dead animals from Council reserves. It is important to note that council only manages flying-foxes on council owned and managed land, and therefore our response is restricted to our reserves.
What can you do?
It’s only natural to want to help stressed animals. However, the public must not interfere with the flying-fox heat stress response. Like all wild animals, flying-foxes may carry diseases, but the risk of spreading those diseases to humans is extremely low. Queensland Health advises that while flying foxes can carry Hendra Virus and Australasian Bat Lyssavirus, disease can only be transmitted via a scratch or bite. So please, never touch a sick or injured flying-fox. Help injured bats by avoiding contact and calling trained and vaccinated wildlife rescue professionals on 1300 ANIMAL (264 625).
Finally, on hot days we can give all our wildlife friends a helping hand by providing water in shallow containers placed in shady locations and planting shade trees in home gardens. Ensure there are sticks or stones in water containers for small animals to climb out if they fall in.
