The tympanic region of Otaria byronia (Otariidae, Carnivora) – morphology, ontogeny, age classes and dimorphism

Abstract

Here we describe and explore for the first time the ontogeny and sexual dimorphism of the auditory region of Otaria byronia. We studied the tympanic region of skulls of 237 specimens of different ages and sexes. Geometric morphometric methods were used to analyze the tympanic bulla. In addition, 3D reconstructions of the tympanic bulla were performed using computed tomography analysis scans and a serial wearing technique. We provide a description of the external and internal morphology of the tympanic bulla in both sexes and across different stages (bioclasses). The average shape of the bulla in O. byronia has a subtriangular contour, with variations between sexes and ages. Each stage (bioclasses I, II, and III) is characterized by the respective mean shape of the tympanic bulla and designated as a morphoclass (1, 2, and 3). In all cases, the ectotympanic shows greater surface area than the endotympanic, as in other otariids, in contrast to Phocidae. During ontogeny, the relative size of the ectotympanic increases, growing in all directions and covering the endotympanic. This pattern is seen to the greatest extent in adult males, in which the ectotympanic forms an extremely well-developed apophysis jugulare. No differences in internal morphology of the tympanic cavity were recorded between ages and sexes. The bulla does not increase in thickness in successive age classes; in fact, the walls are extremely thin in the adult stages, despite the extensive development of its processes. This pattern is opposite that observed in Phocidae. In morphoclass 3, adult males older than 7 years undergo hypermorphic change that results in a peramorphic condition when compared to adult females. These changes probably follow the same pattern shown by the rest of the skull and contribute to the marked sexual dimorphism of the species.

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

“…This is not meant to be an embryological study, but the portions of the tympanic bulla are mentioned for thoroughness. This measurement was modified from Koretsky (2001) and Loza et al (2015, 2018b).…”

“…Depending on the specimen, this measurement may or may not terminate on the division between the entotympanic and tympanic contributions to the bulla, even though this study did not focus on this presentation. This measurement was modified from Koretsky (2001) and Loza et al (2015, 2018b).…”

“…This mastoid width is not to be taken from the zygomatic arches if they appear to be the widest portion. This measurement was adapted from Koretsky (2001) and Loza et al (2015, 2018b).…”