Intraspecific variation in the cochleae of harbour porpoises (Phocoena phocoena) and its implications for comparative studies across odontocetes

Abstract

In morphological traits, variation within species is generally considered to be lower than variation among species, although this assumption is rarely tested. This is particularly important in fields like palaeontology, where it is common to use a single individual as representative of a species due to the rarity of fossils. Here, we investigated intraspecific variation in the cochleae of harbour porpoises (Phocoena phocoena). Interspecific variation of cochlear morphology is well characterised among odontocetes (toothed whales) because of the importance of the structure in echolocation, but generally these studies use only a single cochlea to represent each species. In this study we compare variation within the cochleae of 18 specimens of P. phocoena with variations in cochlear morphology across 51 other odontocete species. Using both 3D landmark and linear measurement data, we performed Generalised Procrustes and principal component analyses to quantify shape variation. We then quantified intraspecific variation in our sample of P. phocoena by estimating disparity and the coefficient of variation for our 3D and linear data respectively. Finally, to determine whether intraspecific variation may confound the results of studies of interspecific variation, we used multivariate and univariate analyses of variance to test whether variation within the specimens of P. phocoena was significantly lower than that across odontocetes. We found low levels of intraspecific variation in the cochleae of P. phocoena, and that cochlear shape within P. phocoena was significantly less variable than across odontocetes. Although future studies should attempt to use multiple cochleae for every species, our results suggest that using just one cochlea for each species should not strongly influence the conclusions of comparative studies if our results are consistent across Cetacea.

Hearing from the ocean and into the river: the evolution of the inner ear of Platanistoidea (Cetacea: Odontoceti)

“…3, 4). Moreover, like Costeur et al (2018), we found no morphological similarity between Lipotes and Inia, although our sample is limited in extant representatives and might therefore be masking such characteristics, though a single specimen should be indicative of morphology (Martins et al 2020). Therefore, although numerous common adaptations are reported in the skull and postcranial skeleton of riverine dolphins (Page and Cooper 2017;Fordyce 2018;Rommel and Reynolds 2018), in agreement with Park et al (2019), we found no "river dolphin" convergent morphology for the inner ear.…”

Evolution of orbit size in toothed whales (Artiodactyla: Odontoceti)

“…Hocking et al (2017) have suggested an alternative framework for looking at prey capture and handling, in which they treat different methods of filter‐feeding as well as grip‐and‐tear feeding as alternative methods of prey processing, versus capture, and that prey capture itself for these taxa would involve elements of raptorial or suction feeding. While we do recognize that there is overlap between the categorizations of prey capture as defined by Johnston and Berta (2011), Galatius et al (2020), and Martins et al (2020), we nonetheless find that the classification used by these latter authors to be helpful when sorting whales into different ecological categories relevant to this study.…”

“…Orbit size is a ratio of orbit length to bizygomatic width of the skull. Habitat classifications based on Jefferson et al (2008) and prey capture method based upon Johnston and Berta (2011), Galatius et al (2020), and Martins et al (2020). Numbers above box plots represent sample size of species.…”

“…For prey capture method, we recognized six major categories of feeding: skim, engulfment, suction, ram, snap, and grip‐and‐tear. Categorization of prey capture method follows Johnston and Berta (2011), Galatius et al (2020), and Martins et al (2020). When feeding behavior was unknown for a given species, we used the behavior of closely related species for the assignment of prey capture strategy.…”

The so‐called foramen singulare in cetacean periotics is actually the superior vestibular area

“…The periotic or the petrosal bone contains the inner ear, which is mainly divided into three parts, a cochlea, a vestibule, and semicircular canals. Active studies have been conducted thus far on the cetacean inner ear structures (e.g., Fleischer, 1976; Geisler & Luo, 1996; Luo & Marsh, 1996), and the progress of this field of research has been dramatically accelerated and become remarkable by the introduction of noninvasive computer tomography scanning (Aguirre‐Fernández, Mennecart, Sánchez‐Villagra, Sánchez, & Costeur, 2017; Churchill, Martinez‐Caceres, de Muizon, Mnieckowski, & Geisler, 2016; Costeur, 2014; Costeur et al, 2018, b; Ekdale, 2013; Gustein et al, 2014; Martins, Park, Racicot, & Cooper, 2020; Park et al, 2017, Park et al, 2017, Park et al, 2019; Racicot, Boessenecker, Darroch, & Geisler, 2019; Racicot, Darroch, & Kohno, 2018; Racicot, Gearty, Kohno, & Flynn, 2016; Thean, Kardjilov, & Asher, 2017). However, most are focused mainly on cochlear structures associated with hearing ability with relatively few studies on the vestibule or semicircular canals associated with equilibrium (Spoor, Bajpai, Hussain, Kumar, & Thewissen, 2002).…”