Koalas use a novel vocal organ to produce unusually low-pitched mating calls.

During the breeding season, male koalas produce 'bellow' vocalisations that are characterised by a continuous series of inhalation and exhalation sections, and an extremely low fundamental frequency (the main acoustic correlate of perceived pitch) [1]. Remarkably, the fundamental frequency (F0) of bellow inhalation sections averages 27.1 Hz (range: 9.8-61.5 Hz [1]), which is 20 times lower than would be expected for an animal weighing 8 kg [2] and more typical of an animal the size of an elephant (Supplemental figure S1A). Here, we demonstrate that koalas use a novel vocal organ to produce their unusually low-pitched mating calls.