Morphological variation among the inner ears of extinct and extant baleen whales (Cetacea: Mysticeti)

Abstract

Living mysticetes (baleen whales) and odontocetes (toothed whales) differ significantly in auditory function in that toothed whales are sensitive to high-frequency and ultrasonic sound vibrations and mysticetes to low-frequency and infrasonic noises. Our knowledge of the evolution and phylogeny of cetaceans, and mysticetes in particular, is at a point at which we can explore morphological and physiological changes within the baleen whale inner ear. Traditional comparative anatomy and landmark-based 3D-geometric morphometric analyses were performed to investigate the anatomical diversity of the inner ears of extinct and extant mysticetes in comparison with other cetaceans. Principal component analyses (PCAs) show that the cochlear morphospace of odontocetes is tangential to that of mysticetes, but odontocetes are completely separated from mysticetes when semicircular canal landmarks are combined with the cochlear data. The cochlea of the archaeocete Zygorhiza kochii and early diverging extinct mysticetes plot within the morphospace of crown mysticetes, suggesting that mysticetes possess ancestral cochlear morphology and physiology. The PCA results indicate variation among mysticete species, although no major patterns are recovered to suggest separate hearing or locomotor regimes. Phylogenetic signal was detected for several clades, including crown Cetacea and crown Mysticeti, with the most clades expressing phylogenetic signal in the semicircular canal dataset. Brownian motion could not be excluded as an explanation for the signal, except for analyses combining cochlea and semicircular canal datasets for Balaenopteridae. J. Morphol. 277:1599-1615, 2016. © 2016 Wiley Periodicals, Inc.

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

“…Therefore, within cetaceans, infrasonic hearing can be regarded as a purely mysticete trait, and ultrasonic hearing as an odontocete trait. Also, Mourlam and Orliac (2017) concluded that infrasonic and ultrasonic hearing evolved after the emergence of fully aquatic whales and within Neoceti, which contrasts with previous results of infrasonic sensitivity in basilosaurids (Ekdale, 2016;Park et al, 2017). However, no archaeocetes were included in our study, and a more detailed interpretation would be merely speculative based on our data.…”

Low-frequency hearing preceded the evolution of giant body size and filter feeding in baleen whales

“…Further indicative of low-frequency hearing are the higher number of turns in archaeocete and toothed mysticete cochleae (more than 2), as well as the large amount of overlap of the basal turn by the apical turn(s). Additionally, preliminary results of a quantitative shape analysis of cetacean cochleae by Ekdale [4] indicate that Zygorhiza and an indeterminate species of toothed mysticete (ChM PV5720) plot within the cochlear morphospace of mysticetes. A final point to note is that virtually all studies to date [3][4][5][6]9,[18][19][20][21][22] on the diversity/evolution of cetacean inner ear anatomy have sampled different taxa/specimens for scanning/analyses under differing protocols.…”

“…Although it is debated whether the earliest cetaceans (archaeocetes) were low-or high-frequency specialists [3][4][5], archaic odontocetes have been shown to use high-frequency echolocation [5 -7]. However, no descriptions of the cochlear anatomy of early mysticetes have been published, which included the critical transition from predation using teeth to filtering with baleen [8].…”

The vertebrate middle and inner ear: A short overview

“…In monotremes, the cochlear apex containing the lagena is coiled and enlarged (Schultz et al, ), while gondwanatherian mammals show a short tapering cochlear canal with a slightly curved apex (Hoffmann, O'Connor, Kirk, Wible, & Krause, ). Therian mammals (marsupials and placentals) show at least one full turn but can have more than three turns (e.g., 3.25 in domestic cats [Schellhorn, ], or 3.3 in whales [Ekdale, ]). Furthermore, the shape of the cochlear is phylogenetically informative in whales (toothless: Ekdale, ; toothed: Costeur et al, ).…”

“…Therian mammals (marsupials and placentals) show at least one full turn but can have more than three turns (e.g., 3.25 in domestic cats [Schellhorn, ], or 3.3 in whales [Ekdale, ]). Furthermore, the shape of the cochlear is phylogenetically informative in whales (toothless: Ekdale, ; toothed: Costeur et al, ).…”