The evolution of hearing and brain size in Eocene whales

Toothed whales (odontocetes) make use of high-frequency sounds to
echolocate, differing significantly from their sister group baleen whales
(mysticetes), which make use of low-frequency sound for long-distance
communication. This divergence in auditory ability has led to considerable
speculation as to how hearing functioned in the ancestral archaeocetes,
and when the specialisations of modern species arose. Numerous studies
have attempted to infer auditory capabilities from morphological
correlates valid in modern species. Here, we build upon these previous
methods with a focus on cochlear structures that have well-understood
links to function. We combine this with information on the sound
conduction apparatus to chart the evolutionary trajectory of cetacean
hearing. Our results suggest an initial move towards low-frequency
specialisation in early Eocene cetaceans, which coincides with the
appearance of new sound conduction pathways. This paved the way for the
later movement towards higher frequency hearing in protocetids, however
the ultra-high and low-frequency hearing specialisations of both modern
cetacean clades evolved after their divergence. We use this data to test
the hypotheses that evolutionary brain size increases in cetaceans were
related to the origin of high-frequency echolocation. We show that no
shift in relative brain size coincides with any changes towards
high-frequency perception. However, this does not rule out a role for
other changes in hearing ability such as some simple forms of
echolocation, similar to that suggested for hippopotamuses or bowhead
whales, which may have been present in even the earliest cetaceans.