The extremes of mammalian hearing: The evolution of whale hearing

Abstract

The hearing of mammals spans extremes unseen in any other taxa and this diversity is best exemplified by the cetaceans. Conflicting hypotheses explain the evolution of the cetaceans’ extreme acoustic biology. Both lower- and higher-frequency hearing limits have been suggested to be ancestral for cetaceans. We investigate this intriguing problem further, through a comparative analysis across 161 extinct and extant mammal species. We show that ancestral whales and mysticetes do not have ultra-low lower frequency hearing limits, and their lower hearing limit is more typical, even slightly higher than that of land mammals of their size (driven by isometric scaling). We show that odontocetes, unlike other mammalian taxa, have reversed the mammalian body-size-hearing relationship. In odontocetes, larger body size is correlated with increased low-frequency hearing limit. We show that aquatically adapted mammals typically have higher lower-frequency hearing limits. Our results suggest that the shift to high frequencies occurred progressively over time. We ask why mysticetes did not evolve higher low-frequency hearing, in line with all other aquatically adapted ears.