Bacular and Testicular Growth, Allometry, and Variation in the Harp Seal (Pagophilus groenlandicus)

Abstract

We investigated quantitative relationships of bacular size to age, length of body, and testicular size in the harp seal ( Pagophilus groenlandicus ), based on collections made in the northwestern Atlantic from 1985 to 1992. Bacular growth was faster than growth in length of body except in old seals (>9 years of age), in which relative growth was isometric. A pubertal spurt in growth in bacular size occurred between 3 and 4 years of age when length increased by 48.3%, mass by 331%, and density by 185%. Concurrently, length of body showed a spurt in growth of 7.2%. Bacular length reached 90% of its asymptotic size of 17.4 cm at ca. 9 years of age, and bacular mass reached that point relative to its asymptote (49.6 g) later at ca. 20 years. In contrast, length of body reached 90% of asymptotic size (176 cm) at only 5–6 years of age. In old seals, bacular length was correlated positively with length of body and age independently. It averaged 9.9% of length of body in old seals. Bacular size was variable: CV = 8.3% for bacular length and 32.8% for bacular mass in old seals. Testicular size peaked in February and March. For young seals (≤9 years old), growth in bacular length was isometric, and growth in bacular mass was positively allo-metric relative to testicular length and mass. In contrast, testicular length and mass of breeding old seals were not correlated significantly with bacular size, length of body, or age. In breeding old seals, testes averaged 10.1 cm in length (range = 7.4–13.2 cm, CV = 10.9%) and 128 g in mass (range = 60.6–204 g, CV = 24.9%), and testicular length averaged 5.7% of length of body. Variation in bacular and testicular size suggests that males have variable reproductive strategies. Comparative analyses on phocid species with known mating systems are needed to test this interpretation.