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On Jan. 12, 2002, in the Australian state of New South Wales, biologist Justin Welbergen was observing a colony of flying foxes for his Ph.D. research. The temperatures that day on Australia’s subtropical, eastern coast reached record highs, soaring to 42.9 degrees C (109 degrees F) at the weather station closest to Welbergen’s study site – nearly 8 degrees C higher than the average summer maximum temperature.

The flying foxes, or giant fruit bats, normally just doze in the treetops through the day, but on this afternoon they were fanning themselves, panting frantically, jostling for shady spots, and licking their wrists in a desperate effort to cool down. Suddenly, when the thermometer hit 42 degrees C, the bats began falling from the trees. Most quickly died. Welbergen and his colleagues counted 1,453 flying foxes that died from the heat in one colony alone. The scorching heat that day killed at least 2,200 additional flying foxes in eight other colonies along a 250-kilometer stretch of coastline. All the deaths occurred in colonies where temperatures soared above 41.7 degrees C. 

Welbergen later documented the deaths of another 19,000 flying foxes during extreme heat waves in tropical Australia from 2003 to 2006, and thousands more have died since then. Only a few such mass die-offs were ever recorded before 1994.