An intruding seal may be physiologically forced to take a breath which would override territorial aggression. Breeding and pupping occur in the summer months. Breeding Weddell sea bulls set up under-ice territories of twenty meters diameter and tend to remain in the water where breeding takes place. Female Weddell seals move freely through the territories of the bulls. Subordinate males have their activity restricted by the dominant bull when moving through a territory.
Females claim less well defined territories, individually or jointly with other females.
The pup is born at 29 kilograms and gains 10-15 kilograms per week. By seven weeks of age, the pups can remain submerged for five minutes and dive down to 92 meters. The moulting of the pup's fur is complete in thirty days. Weddell seals have high juvenile survival due to a lack of predation in fast sea ice. The Weddell seal vocalizes underwater and the diver is very aware of their presence even when they cannot be seen.