The totally aquatic African pipid frog Xenopus borealis produces a range of acoustic signals underwater at night.
The repertoire of males in heightened reproductive condition consists of three call types. All of the calls are composed of the same impulsive, click-like components. The clicks have a rise-time of 0.5 msec, a duration of 2-5 msec, and most of their energy concentrated at 2600 Hz with a secondary peak at 1100 Hz.
The sound pressure levels average 109 dB SPL at 1 meter. The advertisement call is characterized by interclick intervals of 300-600 msec (depending on temperature) and very low coefficients of variation - 3%-10%). Phonotaxis experiments confirm that it is effective in attracting females. The approach call, produced when swimming toward or clasping another frog, has interclick intervals averaging 105 msec. Males show pronounced agonistic behavior accompanied by series of clicks with interclick intervals averaging 43 msec.
Unreceptive females sometimes produce a very weak release call when clasped, but are otherwise silent.
Comparison of this underwater acoustic communication system with terrestrial anuran systems shows surprisingly few differences, given the strong contrast in acoustic environments. The structure of the male's clicks, however, suggests that a major adaptation to underwater signalling may involve the sound production mechanism itself.