Monotypic Species - Keystone Species - The Walrus Is Both
The walrus is a monotypic species as it is the only living species in the genus Odobenus. There are a number of animals that are the sole representatives of their particular genera. Some of these are the moose, pronghorn, okapi and numbat.
The genus name came about as a combination of two Greek words – odous meaning 'tooth' and baino meaning 'walk'. This refers to the habit of the walrus of pulling itself out of the water with its tusks. The species name 'rosmarus' means 'sea-horse'.
The walrus is also considered a keystone species of Arctic marine ecosystems. Keystone species are those organisms which have an influence on their environment which would seem out of all proportion to their size and/or number. Other keystone species include the prairie dog, cassowary and mountain tapir. The value of the walrus is that, by its foraging, it has a huge peripheral impact on benthic communities, meaning those collections of organisms on the sea bed. By disturbing the sea floor, nutrients are released, organisms are amalgamated and the benthos refreshed.
The walrus belongs to a diverse group of animals known as pinnipeds. Pinnipeds are fin-footed, semi-aquatic mammals such as the eared or true seals, the earless seals and the walrus. Only the two elephant seal species out-weigh the walrus.
The walrus cannot be mistaken for anything else, what with its great bulk, prominent tusks and bewhiskered snout.
There are three subspecies of Odobenus. The Atlantic walrus (O.rosmarus rosmarus) is found in the Atlantic Ocean, the Pacific walrus (O.r.divergens) in the Pacific Ocean and O.rosmarus laptevi in the Laptev Sea.
The bulk of the Pacific walruses spend their summer north of the Bering Strait off the northern shores of Russia and Alaska. Smaller numbers are found on the south shores of the Chukchi Peninsula and southern Alaska. Winters are spent in the Bering Sea.
There are much smaller numbers of Atlantic walruses which inhabit coastal regions from north-eastern Canada to Greenland.
The Laptev population is calculated at between 5,000 and 10,000 animals, confined to central and west areas of the Laptev Sea, eastern parts of the Kara Sea and western parts of the East Siberian Sea.
The walrus spends most of its time in the shallow oceanic shelf habitat, foraging for its preferred food of benthic bivalve molluscs.
Most Pacific males weigh between 800 and 1,680 kg with Atlantic males weighing 10 to 20 % less. The Atlantic also has somewhat shorter tusks and a more flattened snout. Females weigh about 2/3 rds as much as males. The walrus can turn its rear flippers forward and move on all fours. Like the true seals, it tends to rely on whole body movements to move through the water rather than using the flippers to swim with. It also shares with true seals an absence of external ears.
The most prominent feature is the tusks which are really elongated canines. Both males and females have tusks which may grow to a metre in length and weigh up to 5.4 kg. Males use their tusks, which are slightly longer and thicker than the females', for dominance and display. The male with the largest tusks will usually be the dominant animal is a social group and will use his tusks to defend his harem. The walrus uses its tusks to make and maintain air holes. They will also poke the tusks through an air hole and rest on them while keeping the nostrils above the level of the ice and water. Apart from the tusks, walruses generally have few teeth.
The characteristic whiskered appearance comes from the mystacial vibrissae, a broad mat of stiff bristles. These 400 to 700 vibrissae are arranged in 13 to 15 rows. While they can grow to a length of 30 cm, they are usually much shorter due to their constant use in foraging. Muscles are attached to the vibrissae which contain blood and nerves. They form a highly sensitive organ.
Although the walrus is covered with fur, it is so sparse that he appears bald. Beneath the thick, highly wrinkled skin lies a layer of blubber which can be 15 cm thick. Nodules appear on the males, particularly around the neck and shoulders. These are called bosses.
When young, walruses are a deep brown, becoming more cinnamon coloured as they age. Old males can appear almost pink. They can appear to be nearly white when swimming as the cold water causes the blood vessels to constrict.
An air sac under the throat acts as a flotation device and allows the walrus to bob vertically. In this way, it can even grab a nap in the water. Males have a baculum or penis bone which can be 63 cm in length and is the largest of any land mammal.
It has a preference for shallow shelf regions and forages mainly on the sea floor. They can stay submerged for up to half an hour but are not particularly deep divers like some of the pinnipeds. The deepest recorded dive is about 80 metres. Whales are capable of slowing down their heartbeats which allows them to more easily withstand the polar temperatures.
The walrus is relatively long-lived and is a social animal. From late summer to autumn, tens of thousands of animals gather on rocky beaches or outcrops.
The walrus feeds on over 60 genera of marine organisms but has a preference for benthic bivalve molluscs such as clams. It identifies prey via the sensitive vibrissae and clears murky areas by powerful jets of water and by flapping its flippers. The palate is vaulted and enables a powerful suction action when the walrus seals its lips on an organism and rapidly withdraws the tongue, sucking the meat out of a shell. The importance of seals as a prey species is still under debate.
This is still much to learn about the reproductive cycle of the walrus. Females come into heat twice a year but males are only fertile around February with mating occurring from January to March. Males gather around ice-bound groups of females and compete vocally for their favours. Copulation takes place in the water. Females are sexually mature at 4 to 6 years old and males at about 7 although they do not typically mate until fully developed at about 15 years. In the wild, the life span is 20 to 30 years.
Gestation takes 15 or 16 months. Just as marsupials practise embryonic diapause, walruses have a period of delayed implantation with the suspended development of the blastula (the very earliest beginnings of the embryo) for a period of 3 to 4 months. This process is common among pinnipeds. Calves are born during the spring migration and weigh between 45 and 75 kg. They are able to swim immediately and nurse from the mother for over twelve months before being weaned. They remain with the mother for 3 to 5 years.
The walrus has the lowest reproductive rate of the pinnipeds as the females do not ovulate until the previous calf is weaned.
The walrus has been hunted for centuries by the indigenous people from the Arctic circle. Meat, fat, skin, bone and tusks were all utilised by the hunters. Traditional hunters of the walrus make full use of their kills. The meat was often preserved, fermented flippers were considered a delicacy, blubber was rendered down for oil, bone and tusks used for tools, pelts used for rope and as a covering for boats and houses and the intestines and gut linings were used to make waterproof linings for jackets. Carving of the tusks was a vital part of native culture.
Then in the 19th and early 20th centuries, there was gross commercial exploitation of the walrus for its blubber and ivory. The walrus was hunted to extinction in the Gulf of St Lawrence and around Sable Island.
Polar bears and orcas are natural predators but will usually tackle only walrus calves. Polar bears may rush a group in the expectation that the sudden stampede will result in an injured animal. Otherwise they look for younger animals or infirm adults. Even a wounded walrus is a dangerous foe for a polar bear.
Orcas regularly attack walruses.
Although numbers of Atlantic and Laptev walruses have grown, the populations remain fragmented and low in number. Two of the species are listed as 'of least concern' by the IUCN while the third is classified 'data deficient'.