Most male walruses are sexually mature at about eight to ten years. Successful reproduction, however, probably doesn't occur until 15 years when a male attains full physical size and is able to compete for females.
Most females are sexually mature at about five to six years. Successful reproduction probably begins at about ten years.
Only a portion of the female population mates each year, as some are pregnant from the year before. Non-pregnant females may go into estrus some time between December and June and most ovulate in February.
In the Pacific, female herds meet male herds as they move south into the central and south Bering Sea in January. Estrous females gather in herds separate from pregnant females and are attended by males displaying nearby in the water.
Most mating probably occurs from December through March, when most sexually mature males produce viable sperm. Mating takes place off the pack ice, underwater and remote from shore; breeding locations are thus largely inaccessible for observation.
Each herd of estrous females is attended by one or more large adult males. According to one study, the ratio of males to females averaged 1 to 23.
Males display visually and vocally from the water while the females rest. A display occurs both at and below the surface and lasts about two to three minutes. The males' displays include clanging bell-like sounds, pulses, and clicks under water, and teeth clacking and whistles at the surface.
Bulls either maintain a distance of about 7 to 10 m (23-33 ft.) or fight violently with each other. When displaying males are present, subadult males are scarce or absent. Those present remain on the fringes of the group and do not display.
Females leave the ice to join a displaying male in the water, where copulation takes place.
After the mating season, mature bulls return to all-male herds.
Total gestation is 15 to 16 months.
Gestation includes a period of delayed implantation. The fertilized egg divides into a hollow ball of cells one layer thick (blastocyst), and then it stops growing and remains free-floating in the uterus for four to five months. The blastocyst then implants on the uterine wall and continues to develop.
Delayed implantation allows the mother time to recover from her last pregnancy and devote her energy to nursing and caring for her calf. It also ensures that the calf will be born when environmental conditions are optimal for its survival.