Because walruses breed during the harsh Arctic winters, little is known about their mating systems. The walrus mating system is believed to be a female-defense polygyny. Large mature males have exclusive access to a herd of females for 1-5 days at a time. Courtship behavior is described in the next section. Mating takes place in January and February, most likely underwater. Walruses are interesting because implantation of the blastocyst is delayed for 4 or 5 months, until June or July. Birth occurs 10-11 months later, from mid-April to mid-June, meaning that the total gestation period is 15-16 months. Females give birth to a single, precocial offspring. The calf is about 113 cm long and weighs approximately 63 kg. It is grey in color and can swim at birth. The social bond between the mother and calf is very strong, and cows are extremely protective of their offspring. Lactation generally lasts for 2 years, but calves are often able to find food before they are finally weaned. Young bulls become sexually mature at 8-10 years, but are often unable to compete successfully for females until they are at least 15 years old. Females become sexually mature at 6-7 years, and are full grown at 10-12 years old. Female fecundity is greatest when cows are 9-11 years old, and at this age they can produce a calf every other year. The interval between births is longer in older females. In the wild, walruses have been known to live for over 40 years (Sjare and Stirling 1996, Nowak 1991).

Average birth mass: 60000 g.

Average gestation period: 331 days.

Average number of offspring: 1.

Average age at sexual or reproductive maturity (male)

Sex: male: 2635 days.

Average age at sexual or reproductive maturity (female)

Sex: female: 1745 days.