Walruses

(Odobenidae)

Class Mammalia

Order Carnivora

Suborder Pinnipedia

Family Odobenidae

Thumbnail description
Large marine mammal that waddles on fore and rear flippers on land and propels itself with rear flippers in water; brown in color with short hairs over pinkish skin beneath and more visible around the neck; canines have evolved into long tusks in both males and females

Size
44–126 in (112–320 cm); 139–2662 lb (63–1210 kg)

Number of genera, species
1 genus; 1 species

Habitat
Shallow marine areas, polynyas (open water within large ice masses), drifting ice floes, island beaches (sand, cobble and rock)

Conservation status
Data Deficient for both the Atlantic and Pacific subspecies

Distribution
Circumpolar waters, including the Bering and Chukchi Seas (around Alaska and Russia), the western and eastern Atlantic Ocean (around Canada, Greenland, Scandinavia and Russia), and the Laptev Sea (around Russia)

Evolution and systematics

There has been a long standing controversy as to whether the Pinnipedia is diphyletic (arising from two lines) or monophyletic (arising from a single line). The majority of morphological and DNA evidence now supports monophyly, with an arctoid ancestor (probably ursid, but could be mustelid or procyonid). However, still uncertain is where odobenids fit within this lineage. Most morphological evidence supports a closer relationship between odobenids and phocids although at least one morphological study places the odobenids and otariids closer together. Molecular genetic studies generally support a closer relationship between the odobenids and otariids, but there are some studies that suggest the relationship with phocids. In short, additional study is required to clarify the evolutionary position of the Odobenidae within the Pinnipedia. Currently three subspecies of walrus are recognized, the Pacific, Atlantic, and Laptev walruses, although there is disagreement on whether the Laptev group is distinct enough to be considered a subspecies.

The taxonomy for this species is Odobenus rosmarus (Linnaeus, 1758), "intra zonam arcticam Europae, Asiae, Americae."

Physical characteristics

Walruses are large mammals. They are probably most noted for the extended size of their upper canines that protrude externally from the mouth as tusks. The tusks occur

in both males and females, although those of males tend to be larger in diameter and shorter than those of females. The snout of the walrus distinguishes it from other seals in that it is flat or "pug nosed" and contains a pad of mustacial vibrissae (whiskers) that are short and stout. These whiskers are used in detecting prey. There are no external pinnae, i.e. ears.

Walruses have a coat of hair covering most of their body, although some regions such as the neck and flippers are either sparse with hair or devoid of it completely. When dry, the hair is a light brown but it is darker in appearance when wet. Male walruses have large fibrous tubercles (bumps) around the neck region. The absence of these among females suggests a secondary sexual function for them.

The rotund appearance of walruses is a result of the thick layer of subcutaneous blubber. Adult females have a slightly thicker layer of blubber, being nearly twice as thick as males during the breeding period.

Like the otariids, walruses rotate the rear flippers under their body and lift themselves off the ground to walk on land. However, for swimming at sea, walruses are more like phocids in that they move their rear flippers from side to side to propel themselves. Unlike phocids, the fore flippers are used to steer and maneuver.

Distribution

Walruses are essentially circumpolar in their distribution. The Atlantic subspecies is probably widest ranging in their distribution from the Canadian Arctic to the Kara Sea of Russia. They were once found as far south as Sable Island, off the coast of Nova Scotia and at the Magdalen Islands and Prince

Edward Island in the Gulf of St, Lawrence. However, northern Hudson Bay is as far south as they now occur.

The Pacific subspecies primarily inhabits the Bering and Chukchi Seas. As this subspecies moves with the pack ice or floating ice, the majority of Pacific walruses are in the Bering Sea in winter and the Chukchi Sea in summer. Births occur on ice usually in the region between Nunivak and St. Lawrence Islands.

The Laptev walrus is found in the eastern Kara Sea, the Laptev Sea, and the Siberian Sea.

Habitat

The feeding habits of walruses restrict the habitats they use to relatively shallow water of less than 328 ft (100 m) for foraging. During the breeding season, female walruses are found on ice floes, where they give birth to their calves. Females and calves usually remain associated with floe ice, but will haul out on land when the ice is unsuitable to support their large mass or too thick (more than 8 in; 20 cm) to break through. When females are receptive, males will be found in the water off ice floes containing females and will spend long periods of time without hauling out. Outside the mating period, males have traditional haul out sites on islands with sand, cobble or boulder beaches. Round Island in Bristol Bay, Alaska is one such site where thousands of males can be seen huddled side by side, forming a churning mass of walruses. In a few places, such as Coats Island, Northwest Territories, mixed herds of males, females, and calves can be found on land in the summer.

Behavior

Walruses are gregarious, traveling in small groups at sea or on ice pans and in large groups when resting or molting on land or ice. Despite the gregariousness the primary social bond seems to be that between a mother and her calf. Based on their behavior males appear to be polygynous although mating by identified animals or genetic paternity results to confirm polygyny are not available. In some situations males appear to follow females, competing directly with one another for the chance to mate. In other situations, males cluster around ice pans containing females that are likely receptive and compete through visual and vocal displays. The displays include in-air whistles and roars and underwater vocalizations that sound like taps, knocks, pulses and bells. It is thought that these sounds are made to attract females, which choose

among the displaying males, but it is possible that some of the vocalizations also are threats aimed at other males.

Mother walruses, unlike any other pinniped species, nurse their young at sea as well as on land. In some instances the female floats on her back and the calf suckles on top of the female. In others, the female floats vertically in the water and the calf dives underwater, upside down to suckle. Young calves sometimes ride the backs of females while traveling at sea.

Both walrus males and females use their tusks for multiple purposes. They may be displayed as threats to gain a prime position in a resting or molting group or they may strike another with them in a fight. Tusks are useful defensive weapons against predators such as polar bears and killer whales. They may also be used to produce holes in the ice and to assist in hauling out of the water onto the ice. There is no evidence that the tusks are used to dig for food in the bottom substrate.

Digging or rooting for food at the bottom is done with the snout and mustacial vibrissae. Walruses use their tongue and lips to create a suction and suck the soft parts of prey from their hard shells.

Feeding ecology and diet

Walrus calves appear to consume their mother's milk for the first 12–18 months. After that they begin to eat invertebrates as well. Walruses are generally benthic feeders (i.e., feed on organisms in the bottom substrate). The most common food items are bivalve mollusks. However, Pacific walruses have been documented to eat more than 60 genera of marine organisms. Among these organisms are corals, worms, polychaetes, crustaceans (e.g., crabs and shrimp), sea cucumbers, and other seals (e.g., ringed and spotted seals).

Walrus diets vary both seasonally and by region. This is common among other pinnipeds too. Food resources vary with differing habitats and seasons and walruses adapt to these differing conditions. There also appears to be a difference in the diets of male and female walruses. The size of prey in the stomachs of males is more than five times greater than that of females. Although based on a relatively small sample, eating other seals tends to be done more by male walruses than by females.

Reproductive biology

Walruses are long-lived animals and in keeping with such a life history, they do not begin to reproduce for several years. Males begin to produce sperm around 7–8 years of age, but do not appear to be capable of fertilizing a female until 10–11 years old. They may not actually become competitive socially until as late as 15 years. Females become reproductive at an earlier age than males, as is often the case in mammals. The youngest females to ovulate for the first time are four year olds and most have their first ovulation between 5 and 7 years of age. First ovulations do not always lead to pregnancy, however, so a females first calf might occur slightly later. Walruses that have been studied display a polygynous mating system.

Walruses are unusual among pinnipeds in that they have a gestation that lasts for longer than a year, about 15 months. Like other pinnipeds they produce a single young during a reproductive event. Most females nurse their calf for between two and three years. As a result the interval between births for individual females is about three years. The fat content of walrus milk, based on a few samples, appears to be around 25–32% fat. This is high compared to the fat content of a lot of mammals, but low for seals. The low fat content is probably related to the long lactation period, which requires a slower rate of fattening to sustain a fast before an abrupt weaning period, as is the case with phocids or true seals with short lactation periods.

In most pinnipeds, mating immediately follows the period of births and lactation. The birth and mating periods are separated by several months in the walrus, however. Maximum known longevity in the wild is just over 40 years.

Conservation status

The major recent threat to walruses has been hunting by humans. All subspecies were hunted intensely in the 1700s and 1800s. As noted above, the Atlantic walrus used to occur in the Gulf of St. Lawrence and on Sable Island, but was hunted to extinction at these sites in the 1700s. Walruses are hunted for subsistence by indigenous peoples, but are otherwise protected globally. Threats to this species today are indirect, such as through pollution, possibly competition with fisheries, fisheries bycatch and habitat destruction from bottom fisheries. Pacific walrus recovered better from the intense hunting than did the Atlantic subspecies for unknown reasons. However, in recent years the Pacific walrus population appears to be declining. The reason for this decline is not yet known.

Significance to humans

Indigenous subsistence hunting was done initially to provide food and other materials (e.g. skin for umiak or traditional Eskimo boats, oil for fuel, and tusks for artwork). European hunters took large numbers of walruses for similar reasons, but far in excess of what could reasonably be used. Today walrus meat is eaten little by people but still used to feed dogs. Tusks and bone are still carved for artwork and sold on the world market, but under strict control by international convention.

Resources

Books

Kastelein, Ronald A. "Walrus." In The Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Wursig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Rice, Dale W. Marine Mammals of the World. Lawrence, KS: Allen Press, 1998.

Periodicals

Fay, F. H. "Ecology and Biology of the Pacific Walrus, Odobenus rosmarus divergens Illiger." United States Fish and Wildlife Service, North American Fauna 74 (1982): 1–279.

Miller, E. H., and Boness, D. J. "Summer Behaviour of the Atlantic Walrus, Odobenus rosmarus rosmarus on Coats Island, NWT." Zeitschrift für Saugetierkunde 48 (1983): 298–313.

Daryl J. Boness, PhD