Relationships of cochlear coiling shape and hearing frequencies in cetaceans, and the occurrence of infrasonic hearing in Miocene Mysticeti

Baleen whales Mysticeti are known to use low frequencies LF; 200 Hz and below and infrasound lt; 20 Hz for communication. The lowest hearing limits of toothed whales Odontoceti , which are able to produce ultrasound gt; 20 kHz , reach low frequencies. Researchers have tried to understand the evolution of LF and infrasonic hearing in mysticetes by linking the shape of the inner ear cochlea or individual cochlear measurements to known hearing frequencies and making inferences to extinct species. Using landmark based shape analysis of complete cochlear coiling, we show that cochlear coiling shape correlates with LF and high frequency HF; gt; 10 kHz hearing limits in cetaceans. Very LF 50 Hz and infrasonic hearing are associated with, for example, a protruding second turn, a descending apex, and a high number of turns. Correlations between cochlear and cranial variables and cochlear and cranial shape indicate that low LF hearing limits are furthermore connected to longer cochleae and relatively larger cranial widths. Very LF hearing in Mysticeti appeared in the middle Miocene, and mysticete infrasonic hearing had evolved by the late Miocene. Complete cochlear coiling is suitable for estimating hearing limits in cetaceans, closely approximated by cochlear length times number of cochlear turn