Ovis dalliDall's sheep

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Geographic Range

Dall's sheep occur in western Canada and the United States. They can be found throughout the mountain ranges of northeast, central and southern Alaska, as well as in the Yukon Territory, the northwest corner of British Columbia, and southwest of the Mackenzie River in the Northwest Territories (Valdez, 1982).

There are two subspecies, Ovis dalli dalli and Ovis dalli stonei. Ovis dalli dalli includes populations found in most of Alaska and the Yukon territories, extreme northwest British Columbia, and the Kenai Peninsula of Alaska. Ovis dalli stonei includes populations found in south-central Yukon and north-central British Columbia (Wilson and Ruff, 1999).

Habitat

Dall's sheep have broad habitat preferences in the arctic and sub-arctic regions but are largely alpine animals, living in high mountain ranges. Generally, they inhabit dry mountainous regions and select sub-alpine grass and low shrub communities (Bowyer and Leslie, 1992). They are dependent upon steep, rugged cliffs and outcrops that provide escape terrain from predators. They use nearby open grass and meadows for feeding. In winter they prefer areas with light snowfall and strong winds that remove snow and expose forage (Nichols and Bunnell, 1999)

Most populations occupy distinct summer and winter ranges, although some are sedentary. Migrations are correlated with snow depth, temperature and plant phenology. Most of the year is spent in the winter range in wind-swept areas that expose forage (Bowyer and Leslie, 1992).

Adult males can occupy six seasonal home ranges: pre-rutting, rutting, midwinter, late winter and spring, salt-lick, and summer (Bowyer and Leslie, 1992). Females usually have four ranges: winter, spring, lambing, and summer. Lambs inherit home ranges from older individuals and they return annually to these inherited ranges (Bowyer and Leslie, 1992).

There seems to be no competition with other ungulates in their ranges for food or space. Wolves (Canis lupus) prey on the sheep in regions where their ranges overlap and may decrease the populations severely if no other prey is available. Coyotes (Canis latrans), golden eagles (Aquila chrysaetos), lynx (Lynx canadensis), grizzly bears (Ursus arctos), black bears (Ursus americanus), and wolverines (Gulo gulo) are also predators. Deaths from accidental falls and avalanches are also common (Wilson and Ruff, 1999). Deep snow, low temperatures, high population density, disease, low-quality forage, and predation are primary sources of mortality, especially among lambs (Bowyer and Leslie, 1992).

Physical Description

Ovis dalli are the only species of thinhorn mountain sheep (Nichols and Bunnell, 1999). The horns are conspicious and sexually dimorphic. They can be either amber or almost transparent. Females have slender horns, while male horns are massive, flaring, and curled. Horns grow annually in both sexes, but, after the first 4-5 years, male horn growth increases greatly and can end up constituting 8-10% of their total body weight (Bowyer and Leslie, 1992). Horns grow in spring, summer, and into early fall when the growth slows down. Horn growth stops during the winter. This start and stop growth pattern results in a pattern of rings on the horns. These rings are called annuli, and can be used to determine age. As rams mature their horns form a circle. A full circle can be reached within about 8 years (Heimer, 1994).

Body size is also sexually dimorphic. Males are about 40% heavier than females, and continue to grow 2 years beyond female maturity at 4 years. Males can weigh between 73 to 113 kg, while females weigh between 46 to 50 kg. Body length also varies from 1.3 to 1.8 m in males and 1.32 to 1.62 m in females. Tail length is between 70 to 115 mm in males and between 70 to 90 mm in females (Wilson and Ruff, eds., 1999).

The pelage consists of a fine wool undercoat and stiff, long, and hollow guard hairs. The winter coat can be thicker than 5 cm (Bowyer and Leslie Jr., 1992). The subspecies, Ovis dalli dalli, has a pure white or creamy white pelage and tail, although variation is present. Some individuals have a black tail, others have slight grey patches often in the middle of the back. Ovis dalli stonei has a grey to black pelage. The inside of the ears are white, while the outsides are grey. The belly is white, as are the backs of the legs. They also have a white rump patch, and a black tail (Shackleton, 1999). A moult occurs from March to July, with mature males moulting before the females, young, and older individuals (Bowyer and Leslie Jr., 1992).

The skin of the face and rostrum is thickened, especially in males. The males also have a double layer of bone on their skulls which allows them to absorb heavy impacts suffered during battles between males (Bowyer and Leslie, 1992).

Dental formula is I 0/3, C 0/1, P 3/3, M 3/3, with 30 teeth total (Bowyer and Leslie, 1992).

  • Range mass
    46 to 113 kg
    101.32 to 248.90 lb

Reproduction

Rams can sire lambs at 18 months but do not usually mate successfully until they acheive social dominance and adult size at 5-7 years of age. Females are sexually mature at 30 months and have their first lamb by age 3 or 4. They produce lambs annually after that (Bowyer and Leslie, 1992).

Lambs are born in late May or early June. Ewes seek solitude and protection in the most rugged cliffs to bear their lamb. Ewes give birth and the mother-lamb pair remains in the cliffs until the lambs are strong enough to travel. Lambs begin to feed on vegetation within two weeks of birth, and are weaned after three to five months, usually by October (Heimer, 1994).

Gestation lasts about 175 days, after which a single lamb is born. Twins are rare (Bowyer and Leslie, 1992). The young weigh between 3-4 kg at birth. Females lick and paw the lambs soon after birth (Shackleton, 1999). Lambs can stand for the first time 15-32 minutes after birth, and are able to travel with their mother within 24 hours. The young grow rapidly, and can achieve 27-30 kg by 9 months of age (Bowyer and Leslie, 1992).

The annual rate of increase can be 11-18% in an unhunted population, but mortality of lambs can reach 40-50% by their first winter in populations nearing carrying capacity (Bowyer and Leslie, 1992)

  • Key Reproductive Features
  • gonochoric/gonochoristic/dioecious (sexes separate)
  • sexual
  • Range number of offspring
    1 to 2
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    5.7 months
  • Average gestation period
    173 days
    AnAge
  • Range weaning age
    4 to 5 months
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    684 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    639 days
    AnAge
  • Parental Investment
  • altricial
  • post-independence association with parents

Lifespan/Longevity

  • Average lifespan
    Status: captivity
    19.6 years
    AnAge

Behavior

Ovis dalli have a well developed social system. Ewes live in flocks with other ewes, lambs, yearlings and immature rams. Lambs are precocious and actively play among themselves. Nursery groups are sometimes formed in which one or two ewes watches over a group of lambs while the other ewes feed. Aggressive interactions between adult females are rare, but do occur, especially in cases of conflict over feeding or bedding sites. Older ewes typically win such confrontations because of their larger horns. Occasionally butting does occur, but not with horns clashing, as in conflict between rams . (Nichols and Bunell, 1999).

Adult rams live in bachelor bands that do not associate with ewes until the mating season in late November and early December (Heimer, 1994). Rams establish a dominance hierarchy in the summer, in which rank is determined by horn size. This ranking is not just for access to females, since there are no females present in the summer range, but also for social order. Established status allows rams to live close with each other in an ordered society with few physical injuries (Nichols and Bunnell, 1999) Males gather into groups and begin interacting by kicking one another with their forelegs, displaying their horns and sometimes jump-threat and clash with one another. Dominance mounting can also occur, and subordinate males sometimes rub the faces of dominants (Bowyer and Leslie, 1992) Usually dominance is settled without fighting, but if there is similar horn size, as when different bands meet, dominance is settled by a fight. These fights are often thunderous. The two males will back off 10 or 12 meters and then rush together, colliding headlong. The crash can be heard a kilometer away. Usually little harm results, and after several bouts the rams separate casually (Blood, 1999).

Dall's sheep are gregarious and have a polygynous mating system in which dominant males breed most often. As rut approaches, the males interact regularly with the females, interactions among males become more vigorous, and fighting for dominance occurs. Larger rams typically win fights and the dominance hierarchy affects breeding opportunities. These fights include aggressive broad-side displays, vigorous kicking, jump-threats, and horn clashes. Male interaction during the rut can become violent. Some males even throw competitors off cliffs (Bowyer and Leslie, 1999)

Males court females by nearing them in a low-stretch posture while flicking their tongues. Males can determine the state of estrus of the female by licking urine from the perianal region, while the female urinates, or from the ground after she had urinated (Bowyer and Leslie, 1992). Males kick the female with a foreleg during courtship to asses her willingness to mate. If the female is in estrus, the male will continue to court her and defend her from other males. Females can butt and rub against a male to elicit courtship (Bowyer and Leslie, 1992).

Copulation occurs when a receptive female stands for a mount rather than walking forward. Males guard females they have mated with for 2 to 3 days, which is probably the length of estrus. Males then cease guarding and look for additional mates. As the rut ends males become more solitary and less aggressive amongst each other. Younger males will attempt to court females as the more dominant males move on but females are seldom receptive (Bowyer and Leslie, 1992).

Communication and Perception

Food Habits

Dall's sheep are herbivorous, grazing primarily for grasses and sedges. They inhabit various habitats with many potential forage species. They can consume between 50-120 species of vegetation. In summer, food is abundant and a wide variety of plants is consumed. In the winter, diet is much more limited, nevertheless, 10-15 species are consumed year long.

They select grasses, sedges and forbs when available, but also ingest lichens and mosses in smaller quantities (Nichols and Bunnell, 1999). Wheatgrass, fescues, bluegrass, and sedges are important foods, while clover, peavine, lupines, pasture sage, dwarf willow, and cinquefoil are eaten when available (Blood, 1999).

In the winter, diet is influenced by accumulation of snow on the ranges, and consists mostly of dry, frozen grass, and sedge stems that are uncovered where snow is blown off, and more lichens and mosses than in other seasons (Bowyer and Leslie, 1992). Mineral licks of calcium phosphate or calcium magnesium concentrate are also important in the diet, especially in the spring and summer, to compensate for mineral deficiencies (Bowyer and Leslie, 1992)

Economic Importance for Humans: Positive

Native hunters hunt these sheep for subsistence. However, Dall's sheep are not as popular as other species, because they are difficult to hunt and do not provide as much meat as larger arctic species (e.g. caribou) (Shackelton, 1999).

  • Positive Impacts
  • food
  • body parts are source of valuable material

Economic Importance for Humans: Negative

There are no known negative effects of Dall's sheep.

Conservation Status

Federal, state, and provincial governmental agencies are responsible for the management of Dall's sheep populations. Human activities such as mineral exploration, road construction, and aircraft harassment disrupt populations of Dall's sheep (Bowyer and Leslie, 1992). In British Columbia range burning is used to slow population declines by improving the quality of forage in winter (Bowyer and Leslie, 1992). Most of their range remains remote and pristine and populations have remained relatively unaffected by humans . However, wild sheep face a precarious future because they are adapted to a limited habitat that is becoming increasingly fragmented (Nichols and Bunnell, 1999).

Populations are threatened by trophy harvest (especially adult rams), hunting in parks and reserves, and, to somel extent, subsistence hunting by native peoples (Bowyer and Leslie, 1992). Only two populations in Alaska exist for which subsistence hunting is allowed. Annual trophy harvest is restricted to mature rams. Most adult males are harvested by nonresidents who are required to pay special fees, hire guides and outfitters, and who are restricted to specific management areas (United States) or outfitter units (Canada) (Bowyer and Leslie, 1992).

In British Columbia, Ovis dalli dalli have been put on the Blue List of species at risk due to their low number in that province, approximately 500 sheep. Ovis dalli stonei have been downlisted from the Blue List to the Yellow List, due to improvement in population numbers (Shackleton, 1999).

Other Comments

Previously, three distinct subspecies were attributed to the name Ovis dalli, but this classification has been changed. The three subspecies included Ovis dalli dalli, Ovis dalli stonei, and Ovis dalli kenainesis, which was restricted to the Kenai Pennisula of Alaska. Studies, including comparisons of blood chemistry, suggest that this population is no different than Ovis dalli dalli (Nichols and Bunnell, 1999). Ovis dalli are known as thinhorn mountain sheep or Dall's sheep and include two subspecies, Ovis dalli dalli, Dall's sheep, and, Ovis dalli stonei, Stone's sheep. They are capable of interbreeding and produce offspring with intermediate coat pelage (Nichols and Bunnell, 1999).

Contributors

Agnes Gozdzik (author), University of Toronto.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

food

A substance that provides both nutrients and energy to a living thing.

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sexual ornamentation

one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

References

Blood, D. 1999. "Mountain Sheep" (On-line). Accessed (Date Unknown) at www.cws-csf.ec.gc.ca/hww-fap/mtnsheep/mtnsheep.html.

Bowyer, R., D. Leslie. 1992. Ovis dalli. Mammalian Species, 393: 1-7.

Heimer, W. 1994. "Dall Sheep" (On-line). Accessed (Date Unknown) at www.state.ak.us/local/akpages/FISH.GAME/notebook/biggame/dallshee.htm.

Shackleton, D. 1999. Hoofed Mammals of British Columbia. Vancouver: University of British Columbia Press.

Valder, R. 1999. The Wild Sheep of the World. New Mexico: Wild Sheep and Goat International.

Valdez, R., P. Krausman. 1999. Description, Distribution, and Abundance of Mountain Sheep. Pp. 3-22 in R Valdex, P Krausman, eds. Mountain Sheep of North America. Tuscon: University of Arizona Press.

Wilson, D., S. Ruff. 1999. The Smithsonian Book of North American Mammals. Washington and London: Smithsonian Institution Press.