Baculum and testes of the hooded seal (Cystophora cristata): growth and size-scaling and their relationships to sexual selection

Growth and size-scaling of the baculum and testes in the moderately porygyn ous hooded seal (Cystophora cristata) were studied using 107 specimens of k nown age (1 month to 28 years) from the northwestern Atlantic. Bacular grow th was rapid between 2 and 5 years of age: length increased 150% and "densi ty" (i.e., mass/length) increased 8-fold and mass 20-fold. Growth continued throughout life. In large, old (>14 years) males, the baculum averaged 20. 7 cm in length, 2.1 g/cm in density, and 44.4 g in mass. Bacular length inc reased relative to body length until seals were about 5 years of age, after which it averaged 8.2%. Testicular growth continued until the seals were a bout 12 years of age. Testes from breeding males >12 years old averaged 11. 2 cm in length, 4.6 cm in width, and 138 g in mass; length averaged 4.9% of body length. In males 2-5 years of age, bacular and testicular sizes were positively allometric relative to body length; in older males, bacular mass and density were positively allometric, and bacular length and testicular size isometric, relative to body length. Bacular size was mostly positively allometric relative to testicular size (bacular length exhibited some isom etry). Compared with that of the related and ecologically similar harp seal (Pagophilus groenlandicus), which is presumed to have a promiscuous mating system, the baculum of the hooded seal was structurally simpler and grew m ore quickly but reached a relatively smaller size in adults (8.2 vs. 9.9% o f body length). Relative testicular length was also smaller (4.9 vs. 5.7% o f body length) and bacular density lower (2.1 vs. 2.8 g/cm) than in the har p seal. These observations suggest that intra- or inter-sexual competition via copulation is weaker in the hooded seal.