After a brief absence whilst I was away in Liverpool last week, Marine Critter Thursday returns in the unmistakable shape of the walrus. With its large pronounced tusks, whiskered snout and large bulk, the walrus must rank as one of the most recognisable marine creatures around.
The name walrus is thought to have several possible origins. The first is a combination of two Dutch words, either walvis (‘whale’) and ros (‘horse’) or wal (’shore’) and reus (‘giant’). However, the most likely origin of the name walrus comes from Old Norse, from the word hrossvalr (‘horse-whale’), which was passed in a juxtaposed form to Dutch and North-German dialects as walros and Walross. The species name for walrus is Odobenus rosmarus, with the genus name derived from the Greek odous (‘tooth’) and baino (‘walk’), based on observations of walruses using their tusks to haul themselves out of the water.
Walruses are distributed around the Arctic Ocean and sub-Arctic seas in the Northern Hemisphere. They can be found around shallow shelf sea regions, spending a large part of their life on sea ice, from which they dive down to the ocean floor to find bottom dwelling molluscs to eat. There are three subspecies of walrus, the Atlantic walrus, the Pacific walrus and the Laptev walrus, found (unsurprisingly) in the Atlantic, Pacific and Laptev Sea respectively. Walruses are big – an adult male can weigh up to 2,000 kg, although most are weigh in somewhere between 800 – 1810 kg. Females are typical about two thirds the size of males. Out of the other pinnipeds (finned mammals including seals, sealions etc), only the elephant seal is larger than the walrus.
The tusks of the walrus are actually elongated canines, and can grow up to a meter in length. Both sexes have tusks, although they are slightly longer and thicker in males than females. Male walruses use their tusks in dominance displays and fighting. Those males with the most impressive tusks tend to dominate social groups. Tusks were once thought to be used to dig out prey items, but studies have shown that the upper lip is actually used to dig whilst the tusks are just dragged through the sediment. Tusks are also used to enlarge holes in the pack ice, and to help walruses to pull themselves out of the water. The tusks are surrounded by an array of stiff bristly whiskers known as vibrissae. A walrus can have between 400-700 vibrissae on its snout, and they function as a highly sensitive touch organ. As the walrus digs for food on the sea floor, visibility is extremely reduced by clouds of sediment they kick up, and the vibrissae are used to feel out prey. Walruses feed on a variety of different organisms, but they have a great preference for bivalve molluscs, especially clams. Once a clam has been found, the walrus seals its lips around it and sucks the flesh out by creating a vacuum by piston movements of the tongue. The suction is so powerful that captive walruses have been known to suck holes in plywood and strip the paint off walls. A grown walrus can consume an incredible 3,000-6,000 clams in one sitting, and they consume around 4-6% of their body weight per day. Considering how much a walrus weighs, this is a tremendous amount of food. The walrus also possess an air sac under its throat, which acts as a flotation device and allows them bob vertically at the surface of the water and sleep there.
Due to its huge size and formidable nature, the walrus has only two natural predators, the polar bear and the orca (killer whale). Polar bears rarely lauch direct attacks on walruses, only preying on injured or juvenile walruses. However, walruses have a long history of human exploitation, especially throughout the 18th and 19th centuries. Pretty much none of the animal went to waste. The tusks represented a valuable ivory commodity, whilst the blubber was rendered down for oil. Even the guts and intestines were used to create waterproof clothing. Although the commercial hunting of walruses is now banned, the potential impact of global climate change on walrus populations is of great concern.
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