Bacular measurements for age determination and growth in the male South African Fur Seal, Arctocephalus pusillus pusillus (Pinnipedia: Otariidae)

Morphology, relative size and growth of the baculum in 103 South African fur seals, Arctocephalus pusillus pusillus, from the Eastern Cape coast of South Africa are described. Bacular measurements (n = 8 linear variables and mass) were examined in relation to standard body length (SBL), bacular length (BL) and chronological age (y) using linear regression. Animals ranged from < 1 month to ≥ 12 y Bacular shape was most similar to Callorhinus ursimts (Northern fur seal) and Zalophus californianus (California sea lion). For the range of ages represented in this study, the baculum continued to increase in size until at least 10 y; with growth slowing between 8-10 y, when social maturity (full reproductive capacity) is attained. Growth in bacular length (BL), distal height and bacular mass peaked at 8 y; middle shaft height and distal shaft height peaked at 9 y; proximal height, proximal width, distal width and proximal shaft height peaked at 10 y. In the largest animal (age ≥ 12 y), maximum bacular length was 139 mm and mass 12.5 g. Relative to SBL, bacular length (BL) increased rapidly in young animals, peaked at 9 y (6.9%), and then declined. Bacular mass and distal height expressed greatest overall growth, followed by proximal height, proximal shaft height and bacular length. At 9 y, mean bacular length and mass was 117 ± 2.7 (± SE, n = 4) mm and 7 ± 0.7 (4) g; growth rates in bacular length and mass were 311% and 7125% (relative to age zero), and 5% and 27% (between years); and bacular length (BL) was about 6.9% of SBL. For all males ≥12 months, most bacular variables grew at a faster rate than SBL and BL. Exceptions included proximal width which was isometric to SBL; distal width and distal shaft height which were isometric to bacular length; and proximal width which was negatively allometric relative to BL. Bacular length (BL) was found to be a useful predictor of SBL and seal age group (pup, yearling, subadult, adult), but only a 'rough indicator' of absolute age.