In Alaska, single pups are born between May and mid-July. Young pups are able to swim almost immediately after birth. They normally remain with their mothers about one month, after which they are weaned and separate from their mother. At that time over half their body weight may consist of fat, providing them a head start on self-sufficiency. Sexual maturity occurs at between 3 and 7 years. Mature females mate shortly after weaning their pups. Development of the embryo is suspended for about 11 weeks; a trait called embryonic diapause (i.e., delayed implantation). Active fetal development is about 8½ months.
Births of harbor seal pups are not restricted to a
few major rookeries (as is the case for many species of
pinnipeds) but occur at many haul-out sites. The sex ratio
of harbor seals at birth is approximately equal and
remains so until about 5 years of age. Thereafter
mortality rates for males are higher, and females become
relatively more abundant. Maximum ages estimated from
annual rings in their teeth are 26 years for a male and 35
years for a female.