The “Ice Walkers” – Walruses in Perspective
Walruses are extremely large pinnipeds with highly distinctive tusks, native to the Arctic and sub-arctic oceans. There are three species, the largest of which is the pacific walrus – Odobenus rosmarus divergens (which means “tooth walking seahorse” in Latin). Weighing in at up to 2000 kg, and measuring up to 3.5 m in length the only pinnipeds bigger are the Northern and Southern species of elephant seal (Mirounga angustirostris & Mirounga leonine). The other species: Odobenus rosmarus rosmarus (the Atlantic Walrus) does not grow as large and weigh 10-20 % less than the pacific species, O. rosmarus divergens.
Walruses spend the majority of their lives at sea but haul on to land, or more commonly snow covered ice floes, to rest, bear their young and moult. In general they avoid dense pack ice, in favour of newer thinner ice, in which their movement is almost unrestricted, due to the ability to break through ice up to 20cm thick with the use of their tusks, to create breathing holes. The tusks are also used as “ice axes” to help haul walruses out onto pack ice, which has led to their colloquial name – ice walkers. It is important to note that dry land is no replacement for ice floes for the walrus, as ice floes serve places to rest between foraging dives to the seabed.
Diet & Feeding
In general walruses prefer to feed on clams, particularly Mya truncate, at depths of around 30 m but also feed on 60 genera of marine organisms including: shrimp, crabs, tube worms, soft corals, tunicates and sea cucumbers. Walruses feed by rooting along the sea bed with their muzzles, similar to pigs. Sucking clams out of their shells by sealing its mouth to the shell to form a seal and then using a “vacuum pump”, generated by a piston like motion created by the tongue moving rapidly in and out, inside the mouth. An adult male pacific walrus must consume 6- 8 % of its body mass per day to survive. This equates to 4000 – 6000 clams per day, requiring 7 -17 hours foraging time.
A minute percentage of adult, exclusively male walruses, diverge from their typical benthic diet and instead develop the habit of eating seals and young walrus pups. For some individuals, in the presence of deep water where it is not possible to feed on the seabed, this change in diet is temporary. However, arctic aborigines talk of solitary bulls with different body morphologies, (the chest, neck and chin are a distinctive amber colour due to the frequent contact and oxidation of seal blubber) which feed exclusively on vertebrates.
Besides man, walruses only have two natural predators due to their huge size and sharp tusks: polar bears and orcas. While a 1000+ kg adult walrus would provide significant sustenance to either predator (one walrus may feed an adult polar bear for several months), they do not make up a significant proportion of either animals diet. This is probably due to the risk involved with capturing and killing an adult walrus, which have been known to fatally wound polar bears and even fight off orca attacks.
Walruses are relatively long lived among pinnipeds; typically surviving to 20-30 years of age in the wild. Males reach sexual maturity at around 7 years but are unable to mate due to the competition with dominant older males, it usually takes over 15 years before males are mature enough to mate. In contrast females reach maturity at a younger age, between 6 and 7 years of age. The mating season runs between January and April but peaks in February. Males gather in the water around ice-bound groups of females and compete vocal displays, the females join them and copulate in the water. The gestation period takes 15 to 16 months; longer than other pinniped species. This means that Walruses can effectively only breed once every two years.
As mentioned earlier, Walruses forage by rooting along the seabed with their muzzles. This creates furrows and feeding pits along the sea bed. Due to the large consumption of benthic infauna per Walrus and the high volume of time spent foraging (up to 17 hours per day) this is has great repercussions on much of the shallow shelf seas in the artic. Walruses consume annually an estimated 3 million metric tons of benthic biomass (in the Bering Sea), which affects an area in the order of thousands of square kilometres per year. These furrows average 47 m long, 0.4 m wide and 0.1 m deep. The Walruses feeding behaviour has four main ecological effects: food consumption (3 million metric tons of benthic biomass eaten per year); benthic community changes (Walruses quickly deplete areas of target species such as clams and other bivalves, the furrows are then colonised by other organism, such as polychaetes and tunicates); sediment restructuring (as Walruses dig deep into the sediment looking for clams they mix, oxygenate and restructure the sediment); and nutrient mobilisation (the digging of Walruses releases trapped organic matter into the water column, the scale of which is great enough to trigger a localised phytoplankton bloom).