Can You Hear Me Now? A Comparative Survey of Pinniped Auditory Apparatus Morphology

Abstract

Over the past century research on the morphology of the auditory apparatuses of pinnipeds, which include Phocidae (true seals), Otariidae (sea lions and fur seals), and Odobenidae (walruses) is extremely limited, in comparison to other Carnivora. Although, the auditory region and surrounding basicrania are areas that are evolutionarily conservative, most literature is unclear due to mixed terminology, inaccurate information, and indistinct, outdated illustrations. Th e lack of adequate and current studies demonstrates the need for compiling morphological information of the auditory region of modern carnivores in relation to hearing. Auditory terminology will be explored to show morphological comparisons and naming practices among the carnivoran families. By streamlining accurate terminology with straightforward illustrations, the behavioral information gleaned from auditory morphological structures will be clearer even among the diverse members of Carnivora. Th is initial assessment will detail the limited information in prior research on the hearing adaptations for transitional semiaquatic carnivores. Th is preliminary review will help to establish the adaptive patterns (from land to sea) in the basicranial morphology of early pinnipeds, especially in phocids.

The tympanic region of the skull in extant pinnipeds: A pilot study of auditory morphological disparity using linear morphometric principal component analysis

“…One of the major unresolved issues with current auditory research is how Phocidae (true seals), Otariidae (fur seals and sea lions), and Odobenidae (walruses) hear efficiently in both aquatic and terrestrial environments (Kastak & Schusterman, 1998; Reichmuth et al, 2013; Repenning, 1972; Smodlaka et al, 2018). Current auditory research focuses more on behavior, determining that pinnipeds can hear moderately well on land (about as well as humans), with otariids having the best aerial acuity, and that all pinnipeds hear at significantly higher frequencies (phocids having the best results) in the water (Ghoul & Reichmuth, 2014; Koper et al, 2021; Reichmuth et al, 2013). Audiogram results demonstrate that two completely different hearing mechanisms should be present, one for each environment, but the specific morphology of pinniped auditory apparatuses has still not been thoroughly explored (Koper et al, 2021).…”

“…Recently, Smodlaka et al (2018) hypothesized that specialized diving adaptations seen in Mirounga angustirostris (northern elephant seal) do not allow for efficient aerial hearing through the external ear, and so this pinniped may depend instead on seismic vibrations through bone conduction. Beyond these hypotheses, there have been no other current studies into the mechanisms pinnipeds employ to hear efficiently inside and outside of the water (Koper et al, 2021; Smodlaka et al, 2018).…”

“…The auditory apparatuses of the three living groups of pinnipeds reflect their origins as descendants of terrestrial carnivorans, though with secondary adaptations to the aquatic environment. Morphological differences exist among pinnipeds, with otariids having more terrestrial‐like auditory characters (possessing remnants of external pinnae and only slightly inflated tympanic bullae), phocids exhibiting slightly more derived aquatic characters (no external pinnae, exaggerated inflated tympanic bullae, and enlarged ossicles compared to body size), and odobenids possessing a mixture (no external pinnae, slightly enlarged tympanic bullae, and enlarged ossicles) of both types (Koper et al, 2021; Loza et al, 2017; 2018a; Repenning, 1972; Smodlaka et al, 2018).…”

“…In previous phylogenetic morphological studies, portions of the basicrania have been used as diagnostic characters (Berta et al, 2006; Berta & Churchill, 2012; Boessenecker & Churchill, 2015; Cozzuol, 2001; Koretsky et al, 2016; Koretsky & Holec, 2002; Koretsky & Rahmat, 2015; Wyss, 1987). This is common since the mammalian basicrania can be thought of as more evolutionarily conservative and therefore less variable in comparison to other parts of the skull (Arnaudo et al, 2014; Hunt Jr., 2001; Koper et al, 2021; Lieberman et al, 2000; Mitchell & Tedford, 1973; Neff, 1987; Turner, 1848; Wang & Tedford, 1994; Wible & Hopson, 1993; Wozencraft, 1989). This suggests that through evolutionary time, the basicranial region has traits that have high phylogenetic signal or the presentation of similar morphologies in mammals that are closely related to one another.…”

Ecological correlates of the morphology of the auditory bulla in rodents: Application to the fossil record

“…In fact, the relationship between the morphology of components of the auditory apparatus, such as the middle ear ossicles, cochlea, and bony labyrinth, and ecology has been studied before in numerous species (Bhagat et al, 2020;Kerber & Sánchez-Villagra, 2018;Mason, 2001Mason, , 2016Mason et al, 2010;Pfaff et al, 2015;Pleštilová et al, 2021). Tympanic bullar morphology itself has been investigated in specific species of mammals (Basso et al, 2017;Groves et al, 2021;Koper et al, 2021), including rodents (Alhajeri et al, 2015;Momtazi et al, 2008;Pleštilová et al, 2021;Potapova, 2019;Schleich & Vassallo, 2003;Tabatabaei Yazdi et al, 2014;Zherebtsova & Potapova, 2019) and has been shown to be associated with habitat use. In particular, inflated tympanic bullae and the associated sound amplification have been linked to open and arid environments (Alhajeri et al, 2015;Tabatabaei Yazdi et al, 2014).…”