Age determination and growth in the male South African fur seal Arctocephalus pusillus pusittus (Pinnipedia: Otariidae) based upon skull material

Skull remains are the most commonly found material of marine mammals and the most likely to be kept in natural history collections. Morphology, relative size and growth of the skull in 83 South African fur seals, Arctocephalus pusillus pusillus, from the coast of southern Africa are described. The South African or Cape fur seal is very closely related to the Australian fur seal (Arctocephalus pusillus doriferus). Age structure of populations is important in understanding the conservation status of an animal population and the impacts of human activity upon the survival of viable wild populations of animal species. Skull measurements (n = 32 variables) were examined in relation to standard body length (SBL - defined as the length from the nose to the tail in a straight line with the animal on its back), condylobasal length (CBL) and chronological age (y) using linear regression. Animals ranged from 10 months to ≥ 12 y (12⁺ y). Twenty four animals were of known-age, while 39 were aged from counts of incremental lines observed in the dentine of tooth sections. Morphological observations were generally consistent with earlier studies. Condylobasal length was highly, positively correlated with SBL and age. Overall, skull variables grew at a slower rate than SBL, apart from height of mandible at meatus and angularis to coronoideus, which expressed isometry relative to SBL. Condylobasal length continued to increase until at least 12 y, with no obvious growth spurt between 8-10 y, when social maturity (full reproductive capacity) is attained. Mean CBL was 19.4% of SBL in yearlings; 15.5% in subadults, and 13.7% in adults. Apart from the dentition, all variables of the facial skeleton followed a somatic growth trajectory. Most variables expressed positive allometry relative to CBL, with greatest growth occurring in the vertical part of the mandible. Mastoid breadth, and gnathion to middle of occipital crest, expressed a strong secondary growth spurt at 10 y. Breadth of brain case, and basion to bend of pterygoid, followed a neural growth trajectory, scaling with negative slope relative to CBL. Sutures of the brain case (i.e., basioccipito-basisphenoid, occipito-parietal, interparietal and coronal) closed before those of the facial skeleton. Condylobasal length was found to be a 'rough indicator' of SBL and age group (adult, subadult), but not of absolute age. Suture age was not a good indicator of absolute age or age group. A comparison is finally made between skull data on the South African fur seal (A. pusillus pusillus) with available data on the Australian fur seal (A. pusillus doriferus).