Sex- and age-related variations in cranial measurements and suture closure in the Australian sea lion, Neophoca cinerea (Peron, 1816)

Abstract

A total of 65 skulls of the Australian sealion, Neophoca cinerea, was examined to investigate the extent to which sexual dimorphism is reflected in cranial dimensions (n = 32) and skull growth, and to determine whether cranial sutures (n = 18) can be useful in age determination. All adult skull dimensions studied display significant sexual dimorphism. Skull growth ceases close to 4–7 years of age for females (Suture Fusion Rating, SFR 25–34) but skull growth in males continues until at least 16 years of age. In animals with a SFR ≥  25, male skulls have a minimum condylobasal length of 272 mm, whereas female skulls have a maximum condylobasal length of 259 mm. The relatively early closure of the cranial vault sutures (cessation of brain growth) is balanced by the continued growth of the bony projections that provide muscle attachment (e.g. mastoid width). The later fusion of the snout and palate sutures corresponds with the continued growth of the snout and palate to match the prolonged growth of the mandibles. The upper sixth postcanine tooth was present in 43% of the adult female skulls, but only 15% of the adult male skulls. The data suggest that it may be possible to determine age(s) from examination of the sequence of fusion of cranial sutures as well as by calculation of an overall suture fusion rating for the skull.