Walruses are among the most distinctive and massive marine mammals of the Arctic, easily recognized by their prominent tusks and considerable bulk. These pinnipeds inhabit the frigid, shallow waters of the Northern Hemisphere, spending significant time on sea ice or land. Their life history involves slow growth and late sexual maturity, which is common among long-lived species in demanding environments. This unique life cycle includes a drawn-out reproductive strategy designed to maximize the survival of few offspring.
The Core Answer: Walrus Litter Size
Walruses almost exclusively give birth to one calf, or pup, per successful pregnancy. The production of a single offspring reflects a strategy of high parental investment in a harsh environment. Newborn calves are substantial, weighing between 99 and 165 pounds (45 to 75 kg) at birth, and are immediately capable of swimming alongside their mothers.
Cases of walrus twins are extremely rare and are primarily reported in captive settings. Even when twins are conceived, the survival rate is exceptionally low, with only a few live twin births ever documented. The energetic demand of gestating and nursing two large calves is likely unsustainable for the mother in the wild.
The Long Reproductive Cycle
The infrequency of reproduction in walruses is dictated by a long timeline leading up to birth. Mating typically occurs in the water during the late winter and early spring, peaking around February and March. Females only ovulate once during this mating season, and only a portion of the adult female population mates in any given year.
Following fertilization, the walrus reproductive cycle involves delayed implantation. The fertilized egg, or blastocyst, remains free-floating for approximately four to five months. This pause ensures that fetal growth and birth occur at the most advantageous time of year, offering the best chance for the calf’s survival.
The total gestation period, including the delayed implantation phase, lasts 15 to 16 months, with the majority of calves born between April and June. This prolonged pregnancy results in a long calving interval, which is the time between successful births. A female walrus typically gives birth only once every three years, though this interval can range from two to five years.
Calf Dependency and Parental Care
The lengthy interval between births is a direct result of the extensive maternal investment in the calf. Walrus mothers are fiercely protective of their single offspring, often sheltering them under their chest between their foreflippers when threatened. This intense care is necessary because young calves are vulnerable to predators like polar bears and killer whales.
The mother provides nourishment through a long nursing period, which can last for up to two years. Walrus milk is rich in fat and protein, though its fat content is lower than that of some other seals. This contributes to the calf’s slower growth rate and the need for prolonged nursing. Walruses are also the only pinnipeds known to exhibit aquatic suckling, as the mother and calf spend much of their time at sea.
The calf’s dependency extends beyond weaning, with the young often remaining in close association with the mother for up to three years. In some instances, the calf may stay with its mother for as long as five years, achieving full independence when the mother is ready to give birth to another calf. This extensive, multi-year bond is one of the longest observed among pinnipeds.