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

Abstract

Abstract. Baleen whales (Mysticeti) are known to use low frequencies (LF; 200 Hz and below) and infrasound (< 20 Hz) for communication. The lowest hearing limits of toothed whales (Odontoceti), which are able to produce ultrasound (> 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; > 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 turns.

A Miocene pygmy right whale fossil from Australia

“…A tympanal recess of the cochlea is typical of Caperea and most other plicogulans, but not balaenids and more archaic mysticetes, such as eomysticetids ( Park et al, 2017b ). Two and a half turns give the cochlea a more archaic aspect than that of C. marginata , and are broadly in line with the number of turns in most other mysticetes ( Ekdale, 2016 ; Ritsche et al, 2018 ). The height and volume of the cochlea are larger than in most mysticetes, but similar to that of C. marginata .…”

An unfamiliar physeteroid periotic (Cetacea: Odontoceti) from the German middle–late Miocene North Sea basin at Groß Pampau

“…Functional conclusions, for example, about hearing capabilities, cannot be drawn at this stage. Therefore, comprehensive CT-scan data of the cochlea are one of the essential basics (see Ekdale and Racicot, 2015;Ritsche et al, 2018) and are available so far in detail only for the living sperm whale (Davids et al, 2011) and, very recently, for Acrophyseter deinodon (Galatius et al, 2019).…”

“…Both are located in this order laterally to the transverse crest. A septum, the Bill's bar (see definition in Reiss, 2003;Ritsche, 2018), is mediolaterally tilted and separates the area vestibularis superior and the foramen of the facial canal. Both the transverse crest and the Bill's bar are distinctly below the outside rim of the meatus.…”

What are the limits on whale ear bone size? Non-isometric scaling of the cetacean bulla

“…The cetacean auditory system has undergone dramatic modifications associated with at least three major shifts throughout cetacean evolutionary history: (1) the land-to-sea transition; (2) ultrasonic hearing for echolocation; and (3) infrasonic hearing in mysticetes ( Ritsche et al 2018 ; Thean, Kardjilov & Asher, 2017 ; Thewissen & Williams, 2002 ; Spoor & Thewissen, 2008 ; Thewissen et al, 2001 ; Fleischer, 1976 ; Schevill & McBride, 1953 ). Throughout these changes, cetaceans have maintained a unique auditory structure: the pachyosteoslerotic tympanic bulla.…”