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

LOZA, C. M.; SCARANO, A. C.; GALLIARI, F. C.; SOIBELZON, L. H.; NEGRETE, J.; CARLINI, A. A.

JOURNAL OF ANATOMY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2017 vol. 232 p. 54 - 71

0021-8782

Here we describe and explore for the first time the ontogeny and sexual dimorphism of the auditory region ofOtaria 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 ofthe 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 andacross 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 respectivemean shape of the tympanic bulla and designated as a morphoclass (1, 2, and 3). In all cases, the ectotympanicshows greater surface area than the endotympanic, as in other otariids, in contrast to Phocidae. Duringontogeny, the relative size of the ectotympanic increases, growing in all directions and covering theendotympanic. This pattern is seen to the greatest extent in adult males, in which the ectotympanic forms anextremely well-developed apophysis jugulare. No differences in internal morphology of the tympanic cavitywere recorded between ages and sexes. The bulla does not increase in thickness in successive age classes; infact, the walls are extremely thin in the adult stages, despite the extensive development of its processes. Thispattern is opposite that observed in Phocidae. In morphoclass 3, adult males older than 7 years undergohypermorphic change that results in a peramorphic condition when compared to adult females. These changesprobably follow the same pattern shown by the rest of the skull and contribute to the marked sexualdimorphism of the species.