Morphological Trends of the Pinniped Tympanic Bulla, with Emphasis on Phocidae

Abstract

Research over the past century is extremely limited on the morphology of the auditory apparatuses of pinnipeds, which include the Families Phocidae (true seals), Otariidae (sea lions and fur seals), and Odobenidae (walruses). An extensive literature review revealed in accurate terminology of this region, incorrect information, mislabeled figures, and details that only correspond to terrestrial taxa. Pinnipeds are unique semiaquatic taxa, possessing adaptations that allow them to hear efficiently both in water and on land. The severe lack of information on this region, as well as nonexistent current studies, demonstrates the need for evaluation and comparison of the auditory morphologies of modern carnivorans in relation to hearing. Initial questions addressed in this study include: comparing the tympanic bulla morphologies of Phocidae to other pinnipeds; comparing distinct morphologies of the tympanic bulla found within phocid subfamilies; and demonstrating any discernable trends in regard to ontogeny or sexual dimorphism. This project explored the morphological correlations of the auditory region of the basicrania of the following representatives of the carnivoran families: Phocidae (including looking at individual subfamilies of Cystophorinae, Monachinae, and Phocinae), Otariidae, Odobenidae, Ursidae (bears), and Mustelidae (weasels). Twelve measurements of the skull and nine calculated ratios were included in multiple principal component analyses (PCA) to determine what areas of the auditory apparatus had the most significant variation in morphology. This is the first study using this type of methodology, especially in reference to the hearing adaptations of pinnipeds, and specifically in phocids. The results of the principal component analyses did exhibit distinct trends in phocid bullar morphology in relation to the other pinnipeds. Phocids tend to have larger tympanic bullar ratios in comparison to other carnivorans and otariids present tympanic bullae that are wider apart and medially parallel. Phocid subfamilial trends include: Cystophorinae on average have wider tympanic bullar ratios; Monachinae have greater distances between their tympanic bullae; and Phocinae overall presents with more generalized morphology. There are distinct trends in the auditory morphology of juveniles (their tympanic bullae make up a greater portion of their skull), but only phocid species that have extreme differences between males and females exhibit distinct morphology within the PCA. To establish more significant trends in ontogeny and sexual dimorphism, more taxa need to be included in future studies. Subsequent studies on auditory morphology will be able to incorporate behavioral hearing information so that a clear, accurate picture of pinniped hearing may emerge. Understanding the modern semiaquatic pinniped ear will help clarify the evolutionary process of aquatic adaptations found within fossil carnivoran taxa, a significant and unknown field of study.