Walruses are long-lived animals with a low reproductive rate. A walrus can live to be 40 years old. Females reach sexual maturity at from 4–10 years old, males at 6-10 years, although males likely cannot compete successfully for females until they are older, about 15 years of age. Mating takes place in the water, and occurs in areas of drifting pack ice or polynyas (areas of open water surrounded by stable landfast ice) during the winter, usually from January to April (Sjare and Stirling 1996). The implantation of the embryo is delayed for 3 or 4 months, after which the gestation period is 12 months (Richard 2001). The pregnancy therefore lasts 15 or 16 months, meaning that females can only give birth a maximum of once every two years, though it is more commonly 3 years between calves. The result is that the walrus pregnancy rate is much lower than that of other pinnipeds (seals).
Walrus calves are born on land or on the pack ice between late April and early June. The calf is nursed solely on milk for the first 6 months or so, before beginning to eat solid foods. Nursing mostly takes place in the water, but also sometimes on land or ice. The nursing period typically lasts for 2 years, with weaning occurring gradually over a period of time.
Walruses are primarily bottom feeders, foraging in sediments on the ocean floor for bivalve molluscs (clams) and other invertebrates (Outridge et al. 2003). The soft parts of clams, the feet and siphons, have been found to make up 95% of both numbers and weight of walrus food intake. The main species taken in Greenland are the bivalves Mya truncata, Serripes groenlandicus, Hiatella arctica and Macoma baltica (Dietz et al. 2013). A study of the fatty acid composition of walrus inner blubber from Svalbard found that its composition closely resembled the fatty acid composition of the lipids in the bivalves Mya truncata and Buccinum spp. (Lydersen and Kovacs 2013). Other invertebrates are eaten to varying extents, and fish may be rarely taken (Born et al. 1995).
Walruses need to eat a lot of clams to meet their energy needs. One study in northeast Greenland was able to quantify the amounts of food eaten be walruses using a combination of divers observing the walruses and satellite-telemetry data on movement and diving activity (Born et al. 2003). On 10 occasions, divers were able to observe walrus feeding, and collect empty bivalve shells from the sea floor. An average of 53.2 bivalves were consumed per dive, corresponding to 149.0 g shell-free dry, or 2,576 kJ per dive (Born et al. 2003).
In the same study, one walrus was fitted with a satellite tag to record its foraging and diving activity. During a 74 hour period of foraging at sea, the walrus spent 57% of its time diving to depths of between 6 and 32 m, and made a total of 412 dives, each lasting between 5 and 7 minutes. It was estimated that this walrus ingested 57 kg (95% CI: 41–72 kg) wet weight bivalve biomass per day, or close to 5% of his total body mass (Born et al. 2003).
This walrus has a satellite tag attached to it's tusk. Read more about walrus research here. Photo: K. Kovacs and C. Lydersen / Norwegian Polar Institute
Although they are primarily benthic (bottom) feeders, some walruses also eat seals and other large animals. Such predation seems to be largely carried out by older males. Ringed seal, bearded seal, as well as beluga and narwhal remains have all been found in adult walrus stomachs (Richard 2001). Walruses have also been documented hunting various species of birds. In Svalbard, walruses were observed hunting flightless pinkfooted geese (Anser brachyrhynchus) as well as bearded and ringed seals (Lydersen and Kovacs 2013).
|Watch a video of a walrus feeding here!|
Apart from humans, the main predators of walrus are killer whales (Orcinus orca) and polar bears (Ursus maritimus). Mainly calves and younger walrus are susceptible to such predation.
Left: A walrus eats a ringed seal. Photo: M. Forsberg / Norwegian Polar Institute
Right: A polar bear eats a walrus carcass on Svalbard. Photo: K. Kovacs and C. Lydersen / Norwegian Polar Institute