Skull development of the California sea lion (Zalophus californianus californianus)

Abstract

This work describes the skull development in males and females of the California sea lion (Zalophus californianus californianus) examined in 87 specimens collected in the Gulf of California between 1982 and 1991. Forty-three measurements, nine qualitative characters and the number of upper-canine-tooth dentine layers, in addition to the natal layer, were recorded. Growth parameters were calculated with the van Bertalanffy equation and Cartesian distortions were used to schematize the skull development. To simplify the description of development, the skull was divided into the following parts: (I) braincase, (2) auditive apparatus, (3) masticatory apparatus and (4) nasal apparatus. Males with zero tooth dentine lasers have skulls slightly smaller than those of females but they grow faster. Male skulls grow larger than those of females but the older males in the sample examined seem to have bone reduction. The mast developed part of the skull at birth is the braincase, which also shows the smallest change thereafter. The frontal ridge shows the most changes during postnatal growth. The frontal ridge and sagittal crest are structures with evident asymmetry. Sexual dimorphism may be recognized in the skull proportions since birth. The female skulls grow fairly uniform in time, as well as in length and width. In the males, the skull grows more in length after birth and more in width thereafter. This heterogeneity is more marked in the masticatory apparatus. The capacity of bone remodelling in the different parts is not related to their degree of sexual dimorphism. The variation in the sample examined shows the need to study skull development in relation to geographic and environmental variations, as well as in relation to the habits of the animals throughout their life.