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.
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.