Pacific
walruses are one of the largest pinnipeds. The head in both sexes is
characterized by a pair of enlarged upper canine teeth that project
downward as tusks, small eyes, a lack of external ears, dorsally situated
nostrils, and a squarish snout with hundreds of stiff mystacial vibrissae.
The head and body are covered with short, tawny hair but the flippers
are bare. Walruses are dark when they are young and become progressively
lighter with age. Immersion in cold water causes a restriction of blood
flow to the skin and a pale, almost white appearance. When warming out
of the water ("hauled out"), the skin again becomes perfused
with blood and a pink to red color results.
Distribution and Migration
The Pacific walrus mainly inhabits the shallow continental shelf waters
of the Bering and Chukchi seas. The distribution of Pacific walruses
varies markedly with the seasons. Virtually the entire population occupies
the pack ice in the Bering Sea in the winter months. Through the winter
they generally congregate in two areas, one immediately southwest of
St. Lawrence Island and the other in outer Bristol Bay. As the Bering
Sea pack ice begins to loosen in April, walruses begin to move northward
and their distribution becomes less clumped. By late April the distribution
extends from Bristol Bay northward to the Bering Strait. During the
summer months, as the pack ice continues to recede northward, most of
the population migrates into the Chukchi Sea. The largest concentrations
are found near the coasts, between 70 degrees North and Pt. Barrow in
the east and between Bering Strait and Wrangel Island in the west. Concentrations,
mainly of males, are also found on and near terrestrial haulouts in
the Bering Sea in Bristol Bay and the northern Gulf of Anadyr throughout
the summer. In October the pack ice develops rapidly in the Chukchi
Sea, and large herds begin to move southward. Many come ashore on haulouts
in the Bering Strait region. Depending on ice conditions, those haulout
sites continue to be occupied through November and into December, but
with the continuing development of ice, most of them move south of St.
Lawrence Island and the Chukchi Peninsula by early to mid-December.
Food and Habitat Requirements
Although capable of diving to deeper depths, Pacific walruses for the
most part are found in waters of 100 m or less, possibly because of
higher productivity of their benthic foods in the shallower water. Feeding
areas typically are composed of sediments of soft, fine sands; compacted
sediments apparently inhibit foraging. In some instances walruses forage
along rocky substrates. They use their sensitive vibrissae to locate
prey items in the sediments of the sea floor. With head down and vibrissae
in contact with the bottom, the walrus proceeds forward, propelling
itself by sculling with the hind flippers. Then they use their nose,
jets of water and suction to dislodge their prey from the sediments.
Bivalve molluscs (clams) are their most common food, however other
invertebrates such sea cucumbers, crabs, and segmented worms are frequently
found in their stomachs. Prey are manipulated by the lips and grasped
with the aid of roughly textured gums, rather than by the teeth. The
soft parts of mollusks are removed from the shells by suction and the
shells are then ejected. Occasionally, small mollusks less than 30 mm
in diameter are swallowed whole, shell and all, but from the larger
mollusks only the siphon or foot ordinarily is ingested. Invertebrates
without shells are swallowed whole without chewing. Walruses rarely
consume fish. They are frequently reported to prey on small seals such
as ringed and ribbon seals. The incidence of seal eating may vary with
location and population status. The frequency of walrus stomachs containing
seals generally is less than 10% but seems to have increased in recent
years.
Ice
Pack ice serves as a substrate for resting and giving birth, and walruses
require pack ice that will support their weight and allow ready access
to the water in which they forage. While walruses can break (with their
heads) ice up to 20 cm thick, they require ice thicknesses of 60 cm
or more to support their weight. Ice that rises too high out of the
water, such as multi-year floes, prevents walruses from coming out of
the water. Generally walruses occupy first-year ice with natural openings
such as leads and polynyas and are not found in areas of extensive,
unbroken ice. Thus, their concentrations in winter are in areas of divergent
ice flow or along the margins of persistent polynyas. In summer those
associating with ice are found along the southern margin of the Chukchi
pack ice, moving farther into the pack in stormy seas. Floe size and
topography appear to be important in the selection of haulout sites.
Terrestrial haulouts
Isolated sites such as islands, points, spits, and headlands are occupied
most frequently. A wide variety of substrates apparently are suitable,
but protection from strong winds and surf seems also to be important.
Social factors, learned behavior, and proximity to prey probably influence
the location of haulout sites but little is known about such factors.
In Alaska, major terrestrial haulouts are found in Bristol bay at Cape
Seniavin, Round Island, Cape Pierce, and Cape Newenham. Consistent seasonal
occupation of specific haulouts by some individuals suggests at least
some degree of site fidelity. Limited data from tagging and radio-tracking
studies suggest that site fidelity may be interrupted by human disturbances.
Growth and Reproduction
At birth calves of both sexes weigh anywhere from 100 to 150 lbs and
are approximately 4.5 feet in length. After the first few years of life,
the growth rate of females declines rapidly until a maximum body size
is reached by approximately 10 years of age. Adult females are generally
smaller than males, with an average weight of about 1900 lbs and an
average length of approximately 9 feet. Although females reach sexual
maturity at approximately 4-5 years of age they do not reach their full
reproductive potential until they are nine or ten years old.
Male walrus tend to grow faster and larger than females. After a secondary 
acceleration of growth, males reach a full adult body size at 15-16
years of age. The head of the male is larger and more block shaped;
and the tusks are stouter, straighter, and more elliptical in cross
section than those of females. The tusks are used in intra-specific
threat displays and fighting that is most severe in the case of breeding
males. Raised nodules on the skin of the neck and shoulders develop
only in sexually mature males. Adult males average over 2,700 lbs in
weight and 10.5 feet in length. Males tend to become fertile at 5-7
years of age but are likely unable to compete for mates until they reach
full physical maturity at approximately 15 years of age.
Pacific walrus breed in the winter between December and March. After
fertilization the ovum becomes a blastocyst and remains in a state of
suspended development for 3-4 months. Implantation occurs in June or
July and the fetus resumes development for approximately eleven months.
Calves are usually born in late April or May.
The walrus has the lowest reproductive rate of any pinniped species.
The 3-4 month delay before implantation of the embryo, followed by 
approximately
11 months of active gestation result in a reproductive cycle of more
than one year. A pregnancy which lasts through the next breeding season
lowers the minimum interval between successful births to 2 years. Most
pinnipeds mate within days or weeks of parturition. In contrast, walruses
give birth several months after the breeding season and do not have
a postpartum estrus. Furthermore, fertility appears to be reduced in
the breeding season following the birth of a calf. The factors affecting
the resumption of estrous cycles in walruses are unknown; however in
some mammals estrus is suppressed during lactation by elevated levels
of the pituitary hormone prolactin which is produced and maintained
in response to the suckling stimulus. Walruses nurse calves for more
than a year and ovulation may be suppressed until the calf is weaned.
In compensation for their low reproductive rate, walrus enjoy relatively
low rates of natural mortality. Walrus calves accompany their mother
from birth and are not weaned for 2 years or more. The prolonged lactation
period allows walrus calves to achieve an advanced developmental state
prior to weaning, which ultimately leaves them well equipped to forage
and escape predators.
Population
Size
The current size of the Pacific walrus population is unknown. The actual
size of the pre-exploitation population is unknown, but has been estimated
to have been between 200,000-250,000 animals. Cooperative aerial surveys
by the U.S. and the former Soviet Union (now Russia) were initiated
in 1975. The 1975 survey estimated the population size at 221,360. A
second joint census, conducted in 1980, estimated population size at
246,360. A third survey, conducted in 1985, produced a population estimate
of 234,020. The most recent aerial survey, flown in 1990, produced an
estimate of 201,039, however a considerable portion of the eastern Chukchi
Sea usually inhabited by walrus in more typical ice years was not surveyed
because ice was not present.
The estimates generated from these surveys should be viewed as conservative
population estimates that are not useful for detecting population trends.
Cooperative aerial surveys were suspended in 1995 due to budget limitations
and unresolved methodological problems.
Sensory Perception and Disturbance
The eyes are small and vision is not well developed in walruses. Tactile
perception via the mystacial vibrissae is well developed and important
in feeding. As in other pinnipeds, walrus have good directional hearing
capability underwater. Sensitivity to airborne sounds is lower than
to underwater sounds, but the degree of sensitivity loss is not clear.
Walruses
often flee haulouts en masse in response to the sight, sound, and especially
odors from humans and machines. The significance of such disturbance
to individuals and to populations is not well known, due to great variation
in the observed responses to disturbance and a lack of relevant data.
Walruses depend on hauling out to complete their molt and grow new
hair, to whelp, to nurse young, and probably just to rest. At those
times even temporary displacement from haulout areas may be detrimental
to the population. There is some evidence of haulouts being completely
abandoned as a result of prolonged disturbance but those cases must
be assessed carefully because evidence also exists for changes in walrus
distribution for reasons not fully understood.
Females with young are the most responsive to noise disturbance and
the potential for harm from disturbance probably is greatest when it
causes separation of females from their dependent young. Early abandonment,
especially in the first year of nursing, probably results in starvation
of the calf. In the first few days of the calf's life, the mother vigorously
maintains contact with the calf. As the calf grows older the responsibility
for maintaining the close association shifts to the calf, increasing
the potential for separation during disturbance.
Even temporary separations can be lethal, in that polar bears prey
upon calves and take advantage of even brief separations from the normally
attentive cow. Calves especially are vulnerable to disturbance on terrestrial
haulouts. Large numbers of calves have been trampled to death during
stampedes caused by human and natural disturbances of terrestrial haulouts.
The potential for mortalities during stampedes appears to be less in
the case of on-ice herds, which generally are smaller than herds on
land.
Natural Mortality
Walruses are preyed upon by polar bears, killer whales, and man. The
magnitude of natural mortality is unknown but is assumed to be low,
given the population's low productivity. Eskimo hunters from St. Lawrence
Island have described walruses becoming emaciated after becoming entrapped
in heavy ice. It is probable that in some instances those walruses starve
to death but no documentation of such events exists. Rock slides are
a hazard to walruses on terrestrial haulouts and occasionally result
in mortality.
Serious
injury and death can result from intra-specific interactions, mainly
involving strikes with tusks and trampling. Skin lacerations and subcutaneous
hemorrhages resulting from tusk strikes are common in both sexes and
all age-classes. The most serious wounds are observed on males during
the breeding season when they wound each other during vigorous fights
in the water. Trampling can result in abortion, injury, and death during
stampedes at crowded haulouts and has been observed at Wrangel Island
in the Chukchi Sea and the Punuk Islands in the Bering Sea.
