High frequency echolocation, ear morphology, and the marine–freshwater transition: A comparative study of extant and extinct toothed whales

This study compares the bony ear morphology of freshwater and marine odontocetes (toothed whales). Odontocetes are unique among marine mammals in two important respects: 1) they use echolocation; 2) at least three lineages have independently evolved obligate freshwater habits from marine ancestries. Freshwater odontocetes include the so-called “river dolphins,” a paraphyletic group that each evolved convergent external morphological characters that distinguish them from oceanic dolphins (Delphinoidea). In addition to their convergent external morphology, “river dolphins” all have echolocation that use one peak (narrow-band) frequency around 100 kHz, compared to oceanic delphinoids which use a two peak (bimodal) frequency ranging from 40 to 140 kHz. The differences in echolocation suggest that the sensory systems responsible for detecting these different sound frequencies should also differ, although quantitative assessments of the cetacean hearing system remain understudied and taxonomically undersampled. To test if ear bone morphology reflects underlying environmentally driven differences in echolocation ability, we assembled a dataset of odontocete periotics (n = 114) from extant and fossil species. We examined 18 external and three internal linear periotic measurements, the latter of which were examined using cone-beam scanning tomography. Results from multivariate canonical ordination analyses show that periotic height, periotic thickness and pars cochlearis width collectively explain the largest amount of interspecific variation in our dataset. Because these particular ear bone measurements correspond to acoustic hearing ranges, we propose that they are also proxies for environmental preference (i.e., marine, freshwater and intermediate habitats) and may be useful for deciphering environmental preferences of extinct odontocetes.