Suture index as an indicator of chronological age in the male South African fur seal, Arctocephalus pusillus (Pinnipedia: Otariidae)

The South African fur seal (Arctocephalus pusillus pusillus) is very closely related to the Australian fur seal (A. pusillus doriferus). We examine skull suture index (SI^*) as an indicator of chronological age in the male South African fur seal, based on 42 animals of known age ranging from 10m to 11 y 11m. Twenty one (21) animals were aged based upon tagging as pups and 21 were aged based on dentine growth layers (1 to 11 y). Age is approximately directly proportional to suture index [Age = (0.7990 ± 0.02354) × SI^*, r = 0.8887, n = 42, valid SI ^* range 0-16, useful predictive range 0-≈14 y)]. We describe the sequence of cranial suture closure (n = 11 sutures, 69 animals) and determine whether suture index (SI^*) reliably corresponds to chronological age. Sutures do not close in a definitive order in all individuals and some sutures take longer to close than others. In animals ≤ 12 y, the general sequence of full suture closure was the Basioccipito-basisphenoid, Occipitoparietal, Interparietal, Coronal and finally the Squamosal-jugal. The Maxillary, Squamosal-parietal, Interfrontal, Basisphenoid-presphenoid, Internasal were used in the SI^* calculation even though none showed any sign of closure in the known-age individuals but did show some closure in very old animals of indeterminate age. Suture closure criteria are useful in classifying males into juveniles, subadults and adults. Multiple linear regression might also prove to be useful to predict chronological age from suture closure data but its utility was limited in the present study by both the size of the data set available and the lack of animals with a known age older than 12y. More data is needed on old animals of known age but the relationship between skull sutures and age found in the present study would be sufficient for aging most male skull material because very few males are likely to reach ages greater than about 12-14 y.