Age determination and growth in the male South African fur seal Arctocephalus pusillus pusillus (Pinnipedia: Otariidae) using external body measurements

Morphology, relative size and growth of the South African fur seal or Cape fur seal, Arctocephalus pusillus pusillus, from the coast of southern Africa are described and comparisons made to data available on the closely related Australian fur seal (Arctocephalus pusillus doriferus) and the New Zealand fur seal (Arctocephalus forsteri). Useful information can be gained from body measurements of seal carcasses provided canine teeth are extracted for aging. External body measurements (12 linear variables) were examined in relation to standard body length (SBL) and chronological age (y) using linear regression and non-linear least squares fitting as appropriate. Animals ranged from < 1 month to > 12 y. Of the 149 animals in the study, 39 were animals of known-age based on tagging; 34 were aged from highly reproducible counts of incremental lines observed in the dentine of upper canines (i.e., range 1-10 y); 10 were identified as adults 12 y (i.e., pulp cavity of the upper canine closed); and 66 were not aged. At birth, male South African fur seals are 35% (c. 69 cm) of their mean adult size. At puberty, they are 57% (c. 113 cm). The foreflippers measure 25-26% (c. 18 cm) of standard body length (SBL) in pups, and 24% (c. 48 cm) of SBL in adults. The hind flippers are considerably shorter, measuring 19% (c. 13 cm) in pups, and 14.5% (c. 29 cm) in adults. Axillary girth is usually about 57-67% of SBL. Growth of SBL was rapid during the early postnatal period with a significant growth spurt occurring at the onset of puberty (2-3 y). The rate of growth slowed significantly between 6 and 7 y. Social maturity was reached at about 9 to 10 y. Growth slowed thereafter. The mean SBL for aged males >10 y and unaged animals > 200 cm was 199 cm. Relative to SBL, facial variables and the fore/hind limbs scaled with negative slope relative to SBL or were negatively allometric; tip of snout to genital opening scaled with positive slope; and tip of snout to anterior insertion of the foreflipper was positively allometric. Relative to age, body variables scaled were negatively allometric. SBL was found to be a rough indicator of age and age group. The growth kinetics of juvenile and adult the South African fur seal and the Australian fur seal are best described by the logistic and double exponential (Gompertz) models rather than the exponential von Bertalanffy model. Australian fur seals grow at a faster rate but asymptotic maximum sizes are similar in South African and Australian fur seals.