Raising your voice: evolution of narrow-band high-frequency signals in toothed whales (Odontoceti)

Cetaceans use sound for communication, navigation and finding prey. Most extant odontocetes produce broadband (BB) biosonar clicks covering frequency ranges from tens of kilohertz to 150-170 kHz. In contrast, the biosonar clicks of some odontocetes are unique, being narrow in bandwidth with high centroid frequency (NBHF), peak frequencies being at 125-140 kHz and bandwidths of 11-20 kHz. Thirteen species within four families (Phocoenidae, Pontoporiidae, Kogiidae, Delphinidae) are known to produce these signals, implying convergent evolution under strong selective drivers. Several hypotheses have been proposed, including acoustic crypsis to escape predation by killer whales, but none has provided comprehensive explanation of the timing of NBHF evolution and the pressures driving sound production to such extremes. Using molecular phylogenetics and the cochlea anatomy of extinct and extant taxa, we demonstrate that early NBHF adaptations occurred at least 10 Mya, and possibly up to 18 Mya, indicating that killer whales cannot have been the sole driving force of NBHF signals, but that now extinct odontocetes may have provided similar pressures. Using palaeoclimate modelling, we further demonstrate that the upper advantageous spectral window for NBHF signals at around 130 kHz has persisted throughout most of the global sea area since the mid-Miocene, covering all known instances of NBHF evolution.