The sibling species of fiddler crabs, Uca mordax and U. burgersi, which are very similar in respect to morphology and many traits of their waving display (see part I of this study), were studied bioacoustically in Trinidad (W.I.). When recording their vibration signals (mainly in the laboratory) it was, at the same time, possible to directly observe the animals' underground activities. The dominant vibration signals of the two species were found to be extremely different: males of U. mordax emit rapping sounds ("drumwhirls") by ambulatory percussion, whereas males of U. burgersi produce "howling" sounds (with a varying number of harmonics) by cheliped convulsion, i.e. inconspicuous quivering movements of this appendage. Similar interspecific differences exist in the fainter vibration signals of females. Each of the two species is capable of producing other signals in addition to the prevalent ones mentioned: U. mordax can emit cheliped vibrations as well (though percussive ones) and U. burgersi can also produce ambulatory "drumwhirls". From these and other basic similarities and from comparisons with recordings of burgersi sounds from Colombia and allopatric populations in Florida, the common starting-point and the different evolutionary pathways leading to the two species-specific termini of acoustic display are reconstructed. The extreme differences that were found for the sympatric situation in Trinidad are regarded as an example of character displacement. Apparently, the acoustic communication system of these crabs is much more affected by character displacement than the visual one.