Sound production by the short-beaked echidna (Tachyglossus aculeatus)

Acoustic communication occurs in all major groups of terrestrial vertebrates, having evolved independently in early clades of mammals, birds, geckos, crocodilians and frogs, about 100-200 MYA. There is little doubt that acoustic communication was used by ancestral therian mammals, but it is ambiguous whether the reconstruction of the root of the mammalian acoustic evolutionary tree includes basal prototherian monotremes. We present here five first-hand accounts of dove-like cooing sounds and analyse the acoustics of three vocalization recordings for two observations of wild short-beaked echidnas (Tachyglossus aculeatus) to quantitatively confirm acoustic communication by this species. The 'cooing' tones were quite distinctive from exhalations, wheezes and grunts, being similar to 0.044 s down-sweeps from similar to 320 to 190 Hz, peaking at similar to 260 Hz, with at least one harmonic overtone. Tones occurred singly or as doublets, triplets (most common), quadruplets and quintuplets, with the leading tone typically the strongest and most broadband. We conclude that echidnas do produce vocalizations, but they are infrequent and acoustic communication is not their primary mode of communication. This unequivocal evidence for vocalization by short-beaked echidnas resolves a long-standing debate concerning the occurrence of acoustic communication by echidnas, which together with well-documented vocalizations by platypus, support a very early evolution of acoustic communication amongst mammals, pre-dating at least the common ancestor of monotremes and therian mammals.