Extreme Rhythm Keeping in Long-Range Slow Click Communication of Sperm Whales

Male sperm whales produce loud, low-frequency clicks at low repetition rates. By combining measurements of source properties with sound propagation modeling, we show that sperm whale slow clicks with source levels > 200 dB re 1 µPa (peak-peak) are the loudest communication signal of any mammal, and that such loud, low-frequency clicks in deep ocean waters have an estimated active space of up to 70 km. We show that these slow clicks are highly rhythmic at rates of 0.1-0.3 Hz, which is an order of magnitude slower than other rhythmic communication signals among mammals and birds. Thus, slow clicks may allow for timing information to be maintained over extreme distances, and we, therefore, propose that sperm whale slow click production is a low entropy, long-range communication signal that may offer honest advertisement through rhythm-keeping to distant receivers. We speculate that the production of such loud, rhythmic clicks at extremely slow rates may be achieved by the sperm whales using extrinsic echo information from the environment to maintain extreme isochrony.