View Full Version : Week 16 - Walrus

September 5th, 2007, 12:57 AM
Order -- Pinnipedia.
Pinnipeds are seals, sea lions, and walruses. Some scientists classify the pinnipeds as a suborder of the order Carnivora.

Family -- Odobenidae.
While the odobenids share some characteristics with the other two pinniped families, behaviorally they more closely resemble the Otariidae (the eared seals). Some researchers divide the Odobenidae into two subfamilies: the Odobeninae (living walruses) and the Dusignathinae (fossil walruses).

Genus, species -- Odobenus rosmarus.
Most scientists recognize two subspecies of walruses: Odobenus rosmarus rosmarus (Atlantic) and Odobenus rosmarus divergens (Pacific). Odobenus comes from the Greek "tooth walker," and refers to the walruses' method of pulling themselves up onto the ice with their long tusks.
These two subspecies are physically and reproductively isolated: O. r. divergens lives in the Pacific Ocean and O. r. rosmarus lives in the Atlantic Ocean.
The Pacific walrus is larger, with longer tusks and a wider skull.
A third subspecies, Odobenus rosmarus laptevi, has been suggested based on specimens from the Laptev Sea in the Pacific Ocean. O. r. laptevi has skull characteristics similar to the Pacific walrus. Its size is intermediate to the Atlantic and Pacific subspecies.
The common name, walrus, originated with the Danish word hvalros, meaning "sea horse" or "sea cow." The Russian word for walrus is morzh. Eskimos call the walruses aivik (Inuit) or aivuk (Yu'pik).
Fossil record.
The earliest of the odobenid fossils date back to the middle Miocene, about 14 million years ago.
The Dusignathinae, or fossil walruses, were abundant in the North Pacific 11 to 14 million years ago. Unlike the modern walrus with its elongated upper canines (tusks), the upper and lower canine teeth of these fossil walruses were about the same size.
Ancestors of the Odobeninae, or modern walrus, probably made their way from the northern Pacific Ocean to the Atlantic during the late Miocene, 6.5 million years ago, by way of a Central American seaway.
Within the last one million years, walruses re-entered the Pacific via the Arctic. The modern Pacific walrus originated from this Atlantic stock.

Walruses are circumpolar, but they are concentrated in several geographically separated areas, with little or no chance of interbreeding.
Pacific walruses inhabit the Bering, Chukchi, and Laptev seas.
Atlantic walruses inhabit coastal areas of north eastern Canada and Greenland.

Walruses are generally found where the water is not more than about 80 m (262 ft.) deep. They prefer a habitat with a gravelly bottom.
Walruses spend about two-thirds of their lives in the water.
Walruses haul out to rest and bear their young.
Walruses are adapted to a habitat of sea ice and prefer snow-covered moving pack ice or ice floes to land. They haul out on small rocky islands when ice is not present.
Eskimos call a traditional walrus haul-out area an ooglit.
Walruses are not generally found where heavy, snow-covered ice covers more than 80% of the sea's surface.
Most walruses live where the air temperature is about -15 to +5 degree C (5 to 41 degree F).

The walruses' migration follows the extent of the pack ice. Throughout the year, they occur primarily in or near the southern periphery of the pack ice.
Pacific walruses winter in the central and south Bering Sea and summer in the Chukchi Sea.
Migration of the Canadian population is less well-known. They seem to remain in the same general vicinity all year.
Walruses migrate primarily by swimming, but they may also ride ice floes.
Some walruses may migrate more than 3,000 km (1,863 mi.) each year.
Pacific walrus adult females and young walruses are more migratory than adult males.
Pacific walrus calves are born on the northward migration to the Chukchi Sea.
Several thousand Pacific walrus bulls remain in the south Bering Sea during the summer. When the ice melts, these bulls haul out on islands.
Some researchers have suggested that the hormones that control sperm development may also inhibit migration in adult males.

Total world walrus population is about 250,000 animals.
Pacific walrus population is more than 200,000 animals.
The Pacific walrus population has been hunted to depletion and allowed to recover several times.
After the latest population depletion, which began in the 1930s, Pacific walruses were given protection by Russia, the State of Alaska, and the U.S. federal government. This protection led to the eventual recovery of the Pacific walrus population (more than 200,000 animals). Walruses reoccupied areas where they had not been seen for several years.
By the early 1980s, walruses appeared leaner, they increased their consumption of alternate foods such as fishes, natural mortality increased, and birth rates decreased. This evidence supports the theory that the Pacific walrus population may have approached the carrying capacity of its environment.
As the Pacific walrus population has grown, annual subsistence catches by indigenous Arctic peoples has increased. Some scientists predict that, without adequate long-term management, natural and human-related mortality factors will rapidly reduce the population once more.

Male Pacific walruses weigh about 800 to 1,700 kg (1,764-3,748 lb.) and are about 2.7 to 3.6 m (9-12 ft.) long.
Female Pacific walruses weigh about 400 to 1,250 kg (882-2,756 lb.) and are about 2.3 to 3.1 m (7.5-10 ft.) long.
Atlantic walruses are slightly smaller: males weigh about 908 kg (2,000 lb.) and reach lengths of 2.9 m (9.5 ft.). Females weigh about 794 kg (1,750 lb.) and reach lengths of 2.4 m (8 ft.).
The northern and southern elephant seals are the only pinnipeds that, full-grown, can be larger than the walrus.

Body shape.
A walrus has a rounded, fusiform body.

Generally, walruses are cinnamon-brown overall.
Walruses appear quite pale in the water; after a sustained period in very cold water, they may appear almost white. They are pink in warm weather when tiny blood vessels in the skin dilate and circulation increases. This increased skin circulation sheds excess body heat.
Calves at birth are ashen gray to brown. Within a week or two, calves become tawny-brown. The coloration pales with age. In general, younger individuals are darkest.

Limbs are modified into flippers.
Flippers are hairless. The skin on the soles of a walrus' flippers is thick and rough, providing traction on land and ice.
The foreflippers, or pectoral flippers, have all the major skeletal elements of the forelimbs of land mammals, but they're foreshortened and modified.
A walrus' foreflippers are short and square. Each foreflipper has five digits of about equal length. Each digit has a small and inconspicuous claw.
While swimming, a walrus holds its foreflippers against its body or uses them for steering.
On land, a walrus positions its foreflippers at right angles to the body for walking.
Walruses have triangular-shaped hind flippers. Hind flippers have five bony digits. Claws on the three middle digits are larger than those on the outer two digits.
Walruses use alternating strokes of the hind flippers to propel themselves in water.
Like sea lions, walruses can rotate their hind flippers under their pelvic girdle, enabling them to walk on all fours.

A walrus' head is square and broad, with conspicuous tusks and whiskers.
A walrus has about 400 to 700 vibrissae (whiskers) in 13 to 15 rows on its snout. Vibrissae are attached to muscles and are supplied with blood and nerves.
Most walruses have 18 teeth. A walrus' upper canine teeth are modified into long ivory tusks.
Both males and females have tusks. The tusks of males tend to be longer, straighter, and stouter than those of females.
Tusks erupt during a calf's first summer or fall.
Abrasion wears enamel from the crown.
The primary functions of the tusks are establishing social dominance and hauling out onto ice or rocky shores.
Tusks can grow to a length of 100 cm (39 in.) in males and 80 cm (31.5 in.) in females. Tusks grow for about 15 years, although they may continue to grow in males.
Eyes are small and located high and toward the sides of the head.
Ears, located just behind the eyes, are small inconspicuous openings with no external ear flaps.
Paired nostrils are located on the snout above the vibrissae. Nostrils are closed in the resting state.

Walruses do not have a distinct tail.

Skin and Hair.
A walrus' skin is thick and tough. It may reach a thickness of 2 to 4 cm (0.79-1.6 in.). It is thickest on the neck and shoulders of adult males, where it protects the animal against jabs by the tusks of other walruses.
The skin of males often has large nodules; these are absent in females. Since the nodules appear at the time of puberty, they are presumed by some researchers to be a secondary sex characteristic.
Hair is about 7 to 12 mm (0.3 to 0.5 in.) long over most of the body. It is shortest on the face and absent on the flippers.
Hair is densest on juveniles and becomes less dense with age.

An annual molt (hair-shedding) for most males takes place from June to August. Females may molt over a more prolonged period. Molting in walruses is gradual; individual hairs fall out and are replaced.
Calves shed a fine prenatal coat, called the lanugo, about two to three months before they are born. They shed a natal coat at about one to two months of age.

A walrus' hearing is thought to be sensitive. Eskimos imitating walrus sounds have obtained a response from walruses 1.6 km (1 mi.) away.

Eyes are small and located high and toward the sides of the head. The eyes can be rotated forward.
Researchers believe that walruses' eyesight is not as sharp as other pinnipeds'. Because walruses feed on sedentary bottom-dwelling animals, acute vision is not necessary.

A walrus' skin is thick and not particularly sensitive to touch.
Walruses seek out physical contact with other walruses.
Vibrissae (whiskers) are extremely sensitive tactile organs. A substantial nerve system transmits tactile information from the vibrissae to the brain.

Walruses prefer certain foods, but researchers do not know how acute the sense of taste is or how important it is in food preference.

The sense of smell in air is well-developed. It probably functions mainly in mother/calf recognition and for sensing approaching predators or other walruses while hauled out.

Normal swimming speed for walruses is about 7 kph (4.3 mph). They are capable of short bursts of up to 35 kph (21.7 mph).
Most propulsion comes from alternate strokes of the hind flippers. Foreflippers also work with the hind flippers for maneuvering.

Walruses generally breathe at the surface for about one minute after every five to eight minutes of subsurface activity. Walruses can remain submerged for as long as ten minutes.
A walrus' benthic prey usually inhabits waters no more than about 80 m (263 ft.) deep; a walrus generally dives no deeper than this. Deeper dives, however, have been documented. When the stomach contents of one individual were examined, researchers concluded that the walrus dove at least 91 m (299 ft.). Another observation confirmed a dive of 113 m (371 ft.) and a submersion time of 25 minutes.
All marine mammals have special physiological adaptations for diving. These adaptations enable a walrus to conserve oxygen while it is below water.
Walruses, like other mammals, slow their heartbeat while diving.
When diving, blood is shunted away from tissues tolerant of low oxygen levels toward organs where oxygen is needed, such as the heart and brain.
The muscle of walruses has a high content of the oxygen-binding protein myoglobin. Myoglobin transports and stores oxygen in muscle tissue.
Pharyngeal muscles prevent water from entering the trachea when the walrus opens its mouth below water.

A walrus breathes through its nostrils and through its mouth.

Heat loss in water is 27 times faster than in air at the same temperature.
A walrus' core body temperature is about 36.6 degree C (97.9 degree F). There is a heat gradient throughout the blubber to the skin. A walrus' skin stays about 1 to 3 degree C (1.8 to 5.4 degree F) warmer than the water.
From about -20 to +15 degree C (-4 to +59 degree F) a walrus' metabolism doesn't change, and the temperature doesn't appear to affect the walrus' behavior.
Walruses can withstand even cooler temperatures; they have been observed at -35 degree C (-31 degree F).
In cold water, blood is shunted inward as blood vessels in the skin constrict, reducing heat loss to the environment. The skin appears pale, almost white.
When warm, blood vessels in the skin dilate (expand), losing heat to the environment. The skin appears pink.
When air temperatures rise above about 15 degree C (59 degree F), walruses often stay in the water to stay cool.
Walruses deposit most of their body fat into a thick layer of blubber that lies just underneath the skin. This blubber layer insulates the walrus and streamlines its body. It also functions as an energy reserve. Blubber may be up to 10 cm (3.9 in.) thick. During the winter, blubber may account for one third of a walrus' total body mass.
Walruses seek out physical contact with other walruses. This helps walruses retain their body heat rather than lose it to the external environment. Physical contact is also indicative of their gregarious nature.

Social structure.
Walruses are among the most gregarious of animals. They exhibit social behavior all year and congregate by the hundreds. Walruses haul out in herds; they seldom haul out alone.
Males and females form separate herds.
Social dominance is well established in herds and subgroups. Dominance within herds is established by tusk size, body size, and aggressiveness. The largest walruses with the longest tusks are the most aggressive and threatening. Animals that are smaller or those with small or broken tusks have a lower social ranking.

Social behavior.
The role of tusks is primarily social. Walruses use them in dominance displays, and they are only secondarily used as weapons.
A male will fight if another male intrudes upon him during a courtship display. These fights often result in physical injury. The frequent scars and lacerations seen on the necks and shoulders of adult males after the breeding season are evidence of tusking.
Individuals frequently compete for the most favorable haul-out sites.
Vocalizations are an important part of a male's courtship display for females
Adult walruses occasionally trample walrus calves, but this is generally accidental rather than aggressive behavior.

Food preferences and resources.
Walruses prefer molluscs, mainly bivalves such as clams. They suck bivalve animals from the shells. Walruses also eat many other kinds of benthic invertebrates including worms, gastropods, cephalopods, crustaceans, sea cucumbers, and other soft-bodied animals. Walruses may occasionally prey on fishes such as polar cod.
Walruses may eat the remains of young seals when food is scarce.
There are some rare but habitual seal-eating walruses. Their diet consists mainly of ringed and bearded seals. These are usually male walruses, recognizable because they are usually larger than other males with powerful shoulder and chest muscles. Their skin may become grease-stained from the blubber of the seals.

Food intake.
Adult walruses eat about 4.2% to 6.2% of their total body weight per day. They eat less on their northward migration.
Observations of feedings indicate that walruses fill their stomachs twice daily.
Adults may eat as many as 3,000 to 6,000 clams at a single feeding.

Methods of collecting food.
Walruses usually forage within 80 m (262 ft.) of the surface. Most feeding probably takes place between 10 and 50 m (33-164 ft.).
Because visability is poor in deep and murky waters, walruses rely on their vibrissae to locate food.
A walrus moves its snout along the bottom, rooting through the sediment. Abrasion patterns of the tusks show that they are dragged through the sediment, but are not used to dig up prey.
Evidence shows that walruses may also take in mouthfuls of water and squirt powerful jets at the sea floor, excavating burrowing invertebrates such as clams.
A Walrus squirts a powerful jet of water at the sea floor to excavate burrowing invertebrates.
Walruses do not chew their food, but they do sometimes crush clam shells.
Soft-bodied invertebrates are usually not crushed or torn. A walrus sucks off the foot and the fleshy siphon of a clam and swallows it whole.
The cheek teeth do get worn, but this is probably from abrasion by minute particles of sand that walruses inadvertently take into their mouths and not from crushing clam shells.
Researchers have found numerous pebbles and small stones in the stomachs of walruses. They are thought to be ingested while feeding.

Sexual maturity.
Most male walruses are sexually mature at about eight to ten years. Successful reproduction, however, probably doesn't occur until 15 years of age, when the male attains full physical growth and is able to compete for females.
Most females are sexually mature at about five to six years. Successful reproduction probably begins at about ten years.

Mating activity.
Only a portion of the female population mates each year, as some are pregnant from the year before. Nonpregnant females may go into estrus some time between December and June, and most ovulate in February.
In the Pacific, female herds meet male herds as they move south into the central and south Bering Sea in January. Estrous females gather at traditional places separate from pregnant females.
Most mating probably occurs from December through March, when most sexually mature males produce viable sperm. Mating takes place off the pack ice, remote from shore; breeding locations are thus largely inaccessible for observation.
Each herd of estrous females is attended to by one or more large adult males. According to one study, the ratio of males to females averaged 1 to 23.
The males display visually and vocally from the water while the females rest. A display occurs both at and below the surface and lasts about two to three minutes. It includes teeth-clacking, clanging bell-like sounds, and whistles.
Bulls either maintain a distance of about 7 to 10 m (23-33 ft.) or fight violently with each other.
When displaying males are present, subadult males are scarce or absent. Those present remain on the fringes of the group and do not display.
Females leave the ice to join a male in the water, where copulation takes place.
After the mating season, mature bulls return to all-male herds.

Total gestation is 15 to 16 months.
Gestation includes a period of delayed implantation: when the fertilized egg divides into a hollow ball of cells one layer thick (blastocyst), it stops growing and remains free-floating in the uterus for four to five months. The blastocyst then implants on the uterine wall and continues to develop.
Delayed implantation ensures that the calf will be born when environmental conditions are optimal for its survival.

Birth seasons.
Calves are born mid-April to mid-June, on the northward migration.

Frequency of birth.
Most pregnancies are spaced at least two years apart for younger females. Older females probably bear calves less often.
A female generally gives birth to a single calf at a time. Twins are rare, although they have been reported.

Calves are usually born on the ice.
Calf at birth.
Newborn calves weigh about 45 to 75 kg (99-165 lb.) and are about 95 to 123 cm (3-4 ft.) long.
Calves are ashen gray to brown with dense, short soft fur. About two to three months before birth, the calf sheds a fine white layer of soft fetal hair called the lanugo.
Within days or weeks, the calf becomes more robust. Its fur turns reddish-brown to tawny within one to two weeks. Calves shed and replace their natal coat when they are one or two months old. This first molt is usually completed by August. Calves then molt annually.

Care of the Young.
Calves may nurse for as long as two years. Some may nurse even longer, if the mother does not have a new calf.
Walrus milk is about 30% fat, 5% to 10% protein, and 60% water, with traces of carbohydrates. The composition of milk remains relatively constant throughout the nursing period.
Nursing usually takes place in the water, but sometimes occurs on ice or land.
Milk is occasionally supplemented with solid pieces of food as early as six months of age. Most calves, however, rely primarily on milk for the first two years.
In a zoological environment, calves nurse about six to ten times per day. Orphaned calves are fed a formula of cream, ground fish and clams, proteins, milk replacer (Multi-milk), vitamins, and water. They consume up to 17.9 liters (300 oz.) per day. There is no information available on the frequency of nursing or the volume of milk consumed by a calf in the wild.
Cows with calves more than two days old tend to gather in herds separate from the bulls and other females. These "nursery herds" usually include 20 to 50 individuals but may include as many as 200 walruses.
A cow is extremely protective of her calf. She defends and protects her calf and may shelter it under her chest between her foreflippers. Calves often ride on their mothers' backs in the water.
There is some evidence that females may care for orphans, although it is unknown whether the female nurses the orphan.

Calf growth and development.
The calf grows about 10 to 15 cm (4-6 in.) in length each month. In a zoological environment, a calf gains 0.7 to 0.9 kg (1.5-2 lb.) per day. Growth rates gathered in zoological environments are considered to be indicative of the growth rates of free-living walruses. Males grow slightly faster than females.
By one month of age, calves are strong swimmers.
A pregnant, near-term cow and her calf from the previous pregnancy usually separate in late April, just before the new calf is born. Mother and calf stay together two years or longer if the mother doesn't produce another calf. Young males may stay an additional two or three years before joining an all-male herd. Females tend to stay with the same herd.

Sound Production.
Walruses have vocal cords.
Walruses produce sounds both above and below water.
Walruses are among the most vocal of the pinnipeds. They produce growls, grunts, barks, soft whistles, rasps, and clicks.
Male walruses produce bell-like sounds below water. These sounds are not produced by the vocal cords but originate from air sacs, which extend from the pharynx.
Calves bellow if disturbed.
Adults engaged in dominance conflicts may snort, cough, or roar.

Display Behavior.
Walruses communicate through auditory and visual displays.
During courtship, males display visually and vocally from the water. Stereotyped sequences of sounds occur both above and below water. Below-water sounds include clicks or knocks, bell-like sounds, and taps. Above-water sounds include teeth-clacking and whistles.
Males engage in tusk-threat displays to establish dominance.

Other Communication.
Walruses communicate through sound, sight, touch, and smell.
Tactual communication occurs through body contact.
Walruses haul out in herds in close contact with one another.
A mother shelters her calf under her chest between her foreflippers. A calf often rides on its mother's back in the water.
Adults engaged in dominance conflicts may strike each other with their tusks.
Courtship displays continue until a female physically contacts a displaying male in the water.

Walruses live about 16 to 30 years.
Pacific walrus natural mortality is estimated to be at least 3% annually. Natural mortality of other walrus populations is unknown. Additionally, human exploitation removes an estimated 4% to 8% of the walrus population annually.
Calf mortality was about 80% in 1980. That is, only 20% of newborn calves survived.

Aging studies.
As a walrus ages, it periodically produces growth layer groups of dental material. Age can be estimated by examining a sliced section of a cheek tooth and counting these layers.

Polar bears occasionally prey on young walruses and probably eat dead walruses. Walruses are not a main part of the polar bears' diet.
Killer whales may prey on walrus calves and injured adults.

Human impact.
Walruses have been hunted commercially for their meat, skin, and ivory tusks by traders from Norway, Russia, Great Britain, Greenland, Canada, and the United States.
Commercial walrus hunting has gone on since the 9th century.
Since the mid-1800s walrus populations have been severely depleted and allowed to recover three times. Researchers attribute this cycle of exploitation to a lack of communication and monitoring of harvests between the nations. Going largely unnoticed, many walrus populations of walruses were nearly wiped out before efforts were made to preserve them.
Most walruses are hunted at sea. Only about half of the walruses killed during hunting operations are recovered for harvest; the rest sink to the bottom of the sea.
Indigenous Arctic peoples of the U.S., Canada, Greenland, and Russia subsistence hunt walruses for food and other raw materials. This practice is an important part of their culture and tradition.
Human occupation in the form of weather stations, etc. have caused many walruses to abandon traditional haul-out areas.

Disease and parasitism.
Walruses are susceptible to a number of internal and external parasites, and to microbial infections of the skin and internal organs.
Intraspecific causes of death.
Many males die annually from injuries incurred while fighting during breeding seasons.
Calves may be accidentally crushed by adults.
Bans on commercial hunting.
Commercial walrus hunting was banned in Canada in 1931. A U.S. Department of Commerce regulation in 1937 and The Congressional Walrus Act of 1941 banned all U.S. commercial hunting, allowing only native subsistence hunting.

The U.S. Marine Mammal Protection Act (MMPA).
The U.S. Marine Mammal Protection Act (MMPA) of 1972 made it illegal to hunt or harass any marine mammal in U.S. waters.
The MMPA does allow for certain exceptions: native subsistence hunting; collecting or temporarily restraining marine mammals for research, education, and public display; and taking restricted numbers of marine mammals incidentally in the course of fishing operations.
The primary objective of the MMPA is to maintain the health and stability of the marine ecosystem and to obtain and maintain an optimum sustainable population of marine mammals.
According to the MMPA, all walruses in U.S. waters are under the jurisdiction of the U.S. Fish and Wildlife Service.

Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
In 1975, the Pacific walrus was placed on Appendix III of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Although Pacific walruses are not threatened or endangered, this classification assures the Pacific walrus some government protection.