Breeds in spring or early summer. Gestation lasts 15-16 months (also reported as 14.5 months). One calf is produced about once every 3-4 years, in late summer in the North Atlantic. Young are weaned in about 2 years. Females first reproduce at about 5 years (also reported as 6-10 years). Spermatogenesis begins in mles at 5 years, but functional maturity is not attained before about 15 years.