The bony labyrinth of toothed whales reflects both phylogeny and habitat preferences

Abstract

The inner ear of toothed whales (odontocetes) is known to have evolved particular shapes related to their abilities to echolocate and move under water. While the origin of these capacities is now more and more examined, thanks to new imaging techniques, little is still known about how informative inner ear shape could be to tackle phylogenetic issues or questions pertaining to the habitat preferences of extinct species. Here we show that the shape of the bony labyrinth of toothed whales provides key information both about phylogeny and habitat preferences (freshwater versus coastal and fully marine habitats). Our investigation of more than 20 species of extinct and modern odontocetes shows that the semi-circular canals are not very informative, in contrast to baleen whales, while the cochlea alone bears a strong signal. Inner ear shape thus provides a novel source of information to distinguish between morphologically convergent lineages (e.g. river dolphins).

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

“…Similarly, a study using 3D geometric morphometrics of odontocete inner ear labyrinths found that the semicircular canals are not statistically significantly informative for ecological preference, whereas the cochlea alone is a decent proxy for habitat (Costeur et al. ). Riverine, coastal, and arctic environments all have different requirements for sound production and reception because of differences in spatial dimensions including pack ice; as Costeur et al.…”

“…Riverine, coastal, and arctic environments all have different requirements for sound production and reception because of differences in spatial dimensions including pack ice; as Costeur et al. () found, the cochlea may be informative for certain aspects of sensory requirements in certain habitats.…”

“…; Costeur et al. ) used avizo , avizo lite , or related software to obtain angular measurements. Consequently, the methods used to take this measurement differ between research groups.…”

“…Recent publications on cetaceans that report 90var measurements (e.g. Ekdale & Racicot, 2015;Racicot et al 2016;Aguirre-Fern andez et al 2017;Costeur et al 2018) used AV-IZO, AVIZO LITE, or related software to obtain angular measurements. Consequently, the methods used to take this measurement differ between research groups.…”

The vertebrate middle and inner ear: A short overview

“…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, ).…”