Northern fur seals are polygynous. Males compete for territories and successful males can mate with many females.
Males arrive at the mating island prior to the females, at which time they begin to claim a mating territory. The mating islands in the Pribilofs, St. George and St. Paul,the southern California island San Miguel, and additional islands in the Bering Sea are the most common mating grounds.
Non-mating males will also travel to the island and usually end up on a territory on the outer edge. Non-mating males learn from their experiences until they reach mating age and are able to successfully compete with other males.
When females arrive it is believed that they choose a mate not based on the male's appearance, but the specific characteristics of the mating territory and possibly the presence of other females.
Males will participate in aggressive, but usually not lethal, behavior with other males. Females are more likely than males to be injured during the mating selection process. Occasionally, two males with fight over a female if she attempts to leave a mating territory, which could result in a tug of war. While males are sparing they will make attempts not to injure any pups in the area.
Mating System: polygynous
Northern fur seals are extremely polygamous. In some cases a single male may mate with up to 50 females during one breeding season. However, more typical is 15 to 20 females. The number of females a male mates with is dependent on the number of available females and the male’s ability to control a mating territory.
The female arrives at the mating island, gives birth to a pup which is a result of the prior year's mating, then mates. Mating occurs during summer. The fertilized egg undergoes delayed implantation. After the blastocyst stage occurs, development halts for approximately four months. Implantantion of the embryo occurs four months after fertilization.
Gestation lasts for approximately one year. Pups are born between June and July, and remain on land for 60-70 days. Peak pupping season is mid-June to mid-July, and peak mating occurs just after this, from late-June to late July.
Females typically become sexually mature between 3 and 6 years of age. Males reach sexually maturity between the same ages, but typically will not participate in mating until three years later.
The same group of males and females tends to return to the same mating island every year. However, male pups born on a particular island will go to a different island when they reach reproductive age. This can be considered dispersal of males.
Breeding interval: Northern fur seals breed annually.
Breeding season: Mating occur in summer, with the peak of mating falling in late June through mid-July.
Average number of offspring: 1.
Range gestation period: 11 to 12 months.
Average weaning age: 4 months.
Range age at sexual or reproductive maturity (female): 3 to 6 years.
Range age at sexual or reproductive maturity (male): 3 to 6 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; delayed implantation
Average birth mass: 5281 g.
Average number of offspring: 1.3.
Pups are born unable to swim or to move around much. Females care for their pups. They nurse the pups, providing milk to sustain them. During the nursing period, the mother makes feeding trips to the ocean. She may leave her pup for four to nine days! The mother returns to land to nurse the pup for about two days between feeding trips. Weaning occurs between three and four months of age, and is apparently begun by the pup. After the pup is weaned, there are no long-term social bonds in this species.
Parental Investment: altricial ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female)