Sound production in Pseudhymenochirus merlini. (a) Time series of emission of one note in a male, showing movement of throat and flanks indicative of movement of an air column (also see Aditional file 2: Movie). (b) Spectrogram and oscillogram of a male advertisement call with five notes. (c) Reconstruction under ML of ancestral character states of sound production mechanism (red without, and blue, with movement of air column) using BayesMultistate. (d) Preferred ancestral character state reconstruction of origin (red bar) and reversal (blue bar) of sound production mechanism; white bars represent the less parsimonious hypothesis of three independent origins of the implosion mechanism. (e) Same reconstruction under the alternative pipid phylogeny suggested by morphology.