There is a definitive hierarchy structure to the mating system of these animals because they are polygynous and they aggregate in colonies on land during the breeding season. Each dominant male controls access to mating opportunities with a group of females. Bonner (1990) calls this mating system "female defence polygyny". Less dominant males are restricted to the fringes of a colony and continually try to gain access to females, resulting in battles between males and aggressive charges by the dominant male. Sub-dominant males usually run away but occasionally a male will challenge the dominant male in an attempt to take over the harem. Females release an audible "bawling" sound when a non-dominant male tries to mate with her. This results in a defense attempt by the dominant bull, who chases the less dominant male away. Occasionally the less dominant male becomes defiant and this can result in spectacular displays of threats and sometimes violent fighting. When a male wants to mate, he throws a flipper over the side of a female, grips her neck in his teeth and begins copulation. Resistance by a female only results in the male moving his large and heavy body on top of the female so she is unable to move. Aggressive interactions among males often result in elephant seal pups being killed by trampling. (Bonner 1990)

Mating System: polygynous

Northern elephant seals haul out for birthing and breeding from December to March. The females come into heat only 19 days after giving birth. They remain receptive for about four days, during which mating occurs. Females become sexually mature at 2 years of age, but usually begin giving birth in the 4th year of life. Males are sexually mature at the age of 6 or 7, but only occasionally are allowed to mate before they reach the age of 9 or 10 because of the hierarchy system of mating exhibited by these animals. (Reeves, et al. 1992; Bonner 1990) These animals display a phenomenon in their development cycle called delayed implantation. Delayed implantation lasts for about 3 months, resulting in a total gestation time of nearly one year. This allows both birthing and mating to occur in the same time frame, during the short period of the year when these animals are aggregated in terrestrial colonies. Interestingly, the embryo is never actually implanted, by definition of most mammals. Instead, it attaches only outwardly to the uterine wall throughout its development. (Bonner 1990; Mathews 1952)

Breeding interval: Females may breed as often as once yearly.

Breeding season: Breeding and birthing occurs from December to March.

Range number of offspring: 1 to 2.

Average number of offspring: 1.

Range gestation period: 10 to 12 months.

Range weaning age: 23 to 27 days.

Range age at sexual or reproductive maturity (female): 2 to 10 years.

Average age at sexual or reproductive maturity (female): 9 years.

Range age at sexual or reproductive maturity (male): 2 to 10 years.

Average age at sexual or reproductive maturity (male): 9 years.

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

Average birth mass: 37500 g.

Average number of offspring: 1.

The pregnancy will ultimately last just under one year, as a result of delayed implantation. Parturition, which results in one offspring per year (although there have been occurrences of twins), will occur the following winter and lactation will follow for about 27 days before the pup is weaned. Pup weight gain during the period of lactation is phenomenal, the milk is extremely high in fat. During weaning the pup remains close to the mother until such a time that the mother leaves the pup behind to return to sea. Young pups left alone form groups or "pods", which remain on shore for up to 12 weeks without parental care. They learn to swim in the surf and eventually swim further out to sea for a short time to feed. An interesting phenomenon displayed by young male pups left to fend for themselves is that of "milk stealing". An attempt to nurse from lactating females still on the beach raising their young can give the successful pup a significant advantage in survival and higher ranking later on in his life by increasing his weight and overall health.

Parental Investment: no parental involvement; precocial ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)