Drawn to Science – Heatwaves and Megabats

Tuesday, 31 March, 2015

Kate Cranney mixes one part science (a profile of a research scientist) with one part art (a detailed drawing of their study subject). In this edition, meet Himali Ratnayake. Himali is a PhD candidate researching how heat waves affect Australian flying foxes.


Are you sipping an espresso at this moment? Do you have Farrago in one hand and a margarita in the other? Maybe you’re nibbling on a mango, a banana or – heaven forbid – a durian. If so, it’s time to thank a bat.

Bats are famous for all the wrong reasons: blood-sucking fiction, Hendra virus and Gotham City’s caped crusader. But bats are also ecologically invaluable. They pollinate the plants that produce some of our most prized vices (tequila!), they disperse seeds (reforestation!) and some bats eat almost half of their body weight in insects each night – protecting coffee plantations and your morning mocha.



Himali Ratnayake is a postgraduate student with a keen appreciation of bats. Himali speaks fondly of our fellow mammals, though this wasn’t always the case. Like many, she had never given much thought to bats and certainly had not considered studying them in depth.

She describes how that all changed one fateful day. “My Honours supervisors took me to a temple [in her home city of Colombo, Sri Lanka]. They caught a bat and handed it to me. I thought that you couldn’t handle bats but they’re actually very docile. I remember thinking, ‘Hey, this isn’t too bad!’ It was very soft and furry, its patagia [wings] felt so fragile and delicate and its face was honestly quite cute.”

Bats make up a quarter of the world’s mammal species. In Australia, we have 58 species of microbats (small, insectivorous echolocating bats) and 12 species of megabats (nectar and fruit-eating flying foxes, with wingspans up to two metres). Himali is currently studying four megabats: the black flying fox, the spectacled flying fox, the grey-headed flying fox and the little red flying fox.



Remember the hottest days of a Melbourne summer. Remember your cognitive processing speed slowing to a saunter. Remember the bitumen sticking to the bottom of your warped soles. Well, climate change is likely to bring longer, hotter and more frequent heat waves in Australia.

On 4 January 2014, temperatures in southeast Queensland reached record highs – up to 44.6°C in Beaudesert. More than 45,500 flying foxes died in one day. ‘Die-offs’ like this followed shortly after in Victoria and South Australia. Bats were said to be ‘falling from the sky’. Corpses piled up on the ground and hung in trees. Wildlife carers were swamped with thousands of orphaned pups (juvenile bats).

But why are bats so vulnerable to heat waves? Surely they’d just fly to somewhere cooler?

Well, bats spend most of the day hanging on tree branches, often in full sun. They are social creatures and are reluctant to leave the hierarchical structure established in their colony. Since bats cannot sweat, they perform a sequence of predictable ‘thermoregulatory behaviours’ to cool down. They fan their wings, seek shade, pant and spread saliva on their fur – a last ditch attempt to promote evaporative cooling. But when temperatures soar to hellishly hot, they simply cannot beat the heat.



So, can we devise strategies to prevent future die-offs? This is where Himali’s research comes in. Over the next three years, she will measure how bats cope with extreme heat. Himali’s fieldwork will take her as far north as Ingham, in Queensland, which boasts the only colony with all four of the flying fox species she is researching.

She will gather data on environmental conditions (such as temperature) using a portable weather station, as well as information about the bats (like fur-depth and posture). All of the data will create a ‘mechanistic model’ that will predict exactly when bats become heat stressed.

Himali hopes that her research will help us understand what combination of conditions cause the die-offs so governments can develop strategies for future heat waves. And the conspicuous behaviour of flying foxes means that they can be used as a ‘bioindicator’ of heat wave impact on other species. Now that’s something to toast your tequila to!



Have you ever heard the squabble and flutter of bats at Yarra Bend? As Melburnians, this is our local bat metropolis, with 10,000 to 30,000 individuals in the colony. You can help to estimate this number at the twice-monthly Megabat Count: www.megabatcount.wordpress.com. Maybe this can be your way to thank the bats.

You can also visit Himali’s website at www.hratnayake.wordpress.com and see other ‘Drawn to Science’ profiles at www.katecranney.com.