Reproductive behavior occurs underwater. The female is mounted by the male from behind while her sides are held by his foreflippers. Quite often she is bitten on her neck while copulation occurs. Male-male fighting occurs, which suggests that mating systems are polygynous.

Mating System: polygynous

From mid-September to late December active spermatogenesis occurs. In late November to mid-December females are impregnated and about mid-January implantation occurs (Stirling 1971). Gestation last 9 to 10 months. Young pups are born with their permanent dentition. Birth occurs onto the sea ice, which often results in a change of external temperature in newborn pups. Pups are usually born singly, and the time of birth usually varies with latitude from early September at latitude 60 degrees south to late October at latitude 78 degrees south (Stirling 1971). Weddell seal pups weigh about 29 kg at birth. They have a gray lanugo, which after 3 to 4 weeks turns to a dark coat. Weaning takes place at 6 weeks of age and maturity at 3 years. First breeding of females may be denied for 1 or 2 years under some population conditions and males usually don't mate until 6 to 8 years of age because of social pressures (Stirling 1971).

Breeding season: In late November to mid-December females are impregnated and about mid-January implantation occurs.

Average number of offspring: 1.

Range gestation period: 9 to 10 months.

Average weaning age: 6 weeks.

Average time to independence: 6 weeks.

Range age at sexual or reproductive maturity (female): 3 to 5 years.

Range age at sexual or reproductive maturity (male): 6 to 8 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous ; delayed implantation

Average birth mass: 29000 g.

Average number of offspring: 1.

Parental Investment: precocial ; pre-fertilization (Provisioning); pre-hatching/birth (Protecting: Female); pre-weaning/fledging (Provisioning: Female)