A Nasty, Brutish, and Short History of Amphibian Bioacoustics

From prehistoric times, humans have certainly been aware of the existence of sounds of frogs and toads. The significance of sounds for mating was noted by Aristotle (Historia Animalium, translated as History of Animals by D’Arcy Wenthworth Thompson Clarendon Press, Oxford, 400 BCE). However, the understanding of the nature of sound and of its transduction through any amphibian auditory system was unknown until the nineteenth century, largely because the small and inconspicuous ears of amphibians were not easily studied before the advent of suitable microscopes and histological procedures. The middle ear of frogs was first described in 1676, but the first detailed anatomical studies of their inner ear appeared only in the second half of the nineteenth century. The development of modern amphibian bioacoustics was facilitated by the development of electronics, recording devices, and computers. For example, the first sound spectrogram of a frog’s call was published in 1946. W. Frank Blair used this equipment and instrumentation to show how differences in the physical properties of calls could promote speciation. Murray Littlejohn conducted experiments proving that call differences alone allowed female frogs to choose conspecific mates. Robert Capranica was the first to electronically synthesize frog calls and to electronically modify (filter) them for use as stimuli in acoustic playback experiments. Capranica also employed behavioral studies to inform the analysis of neural processing and was one of the founders of the field of neuroethology as a powerful research strategy. We review how the pioneering work of these three individuals inspired a legion of modern bioacoustics researchers who have made major contributions to the field.