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Ovis canadensisbighorn sheep

By Tanya Dewey and Liz Ballenger

Geographic Range

Ovis canadensis is found in the Rocky Mountains from southern Canada to Colorado, and as a desert subspecies (O. c. nelsoni) from Nevada and California to west Texas and south into Mexico. (Festa-Bianchet, 1999)

Habitat

Ovis canadensis canadensis inhabits alpine meadows, grassy mountain slopes and foothill country in proximity to rugged, rocky cliffs and bluffs. Bighorn sheep require drier slopes where the annual snowfall is less than about sixty inches a year, since they cannot paw through deep snow to feed. The winter range usually lies between 2,500-5,000 feet in elevation, while the summer range is between 6,000-8,500 feet. (Festa-Bianchet, 1999)

  • Range elevation
    800 to 2500 m
    2624.67 to 8202.10 ft

Physical Description

Males 119-127kg; females 53-91 kg. Rams typically measure 160-180 cm from head to tail, while ewes are approximately 150 cm. Bighorn sheep have double-layered skulls shored with struts of bone for battle protection. They also have a broad, massive tendon linking skull and spine to help the head pivot and recoil from blows. Horns may way as much as 14 kg, which is the weight of all the bones in a ram's body. The horns of a female are much smaller and only slightly curved. The horns of a ram can tell much about him such as his age, health, and fighting history. The desert subspecies, Ovis canadensis nelsoni, is somewhat smaller and has flatter, wider-spreading horns. The pelage of Ovis canadensis is smooth and composed of an outer coat of brittle guard hairs and short, grey, crimped fleece underfur. The summer coat is a rich, glossy brown but it becomes quite faded by late winter. (Festa-Bianchet, 1999)

  • Range mass
    53 to 127 kg
    116.74 to 279.74 lb
  • Range length
    150 to 180 cm
    59.06 to 70.87 in
  • Average basal metabolic rate
    114.674 W
    AnAge

Reproduction

Bighorn sheep are perhaps best known for the head-to-head combat between males. Horn size is a symbol of rank, and the mass of the horns (as much as 14 kg) is used to a male's best advantage as he smashes into an opponent at speeds of 20 miles per hour. Combat has been observed to last for as long as 25.5 hours (with approximately 5 clashes an hour) until one of the males conceded. Males do not defend territories but rather engage in battles over mating access to a particular female. Male dominance status is determined by age as well as horn size, and homosexual activity often occurs in groups of males with the dominant animal behaving like a courting male and the subordinate playing the role of an estrous female. Ewes are seasonally polyoestrous and will accept several rams, often frequently, when in oestrus. Because of intense competition between males for females and the dominance hierarchy based on age and size (including the size of the horns), males do not usually mate until they are seven years old. Younger males will mate sooner if dominant rams in their group are killed. (Festa-Bianchet, 1999)

Rutting season is in the autumn and early winter, and births take place in the spring. Mating for the desert bighorn, however, can last from July to December. Gestation lasts from 150-180 days, after which usually one, rarely two, young are born. Newborns are precocial and are able to follow their mothers at a good pace over the rocky terrain after the first week. Within a few weeks of birth, offspring form bands of their own, seeking out their mothers only to suckle occasionally. They are completely weaned by 4 to 6 months of age. Ovis canadensis females have been mated when 10 to 11 months old in captivity, but they generally do not breed until their second or third year in the wild. (Festa-Bianchet, 1999)

  • Breeding interval
    Bighorn sheep breed once yearly.
  • Breeding season
    Mating occurs from July to December, with most activity concentrated in the fall rut.
  • Range number of offspring
    1 to 2
  • Average number of offspring
    1.36
  • Average number of offspring
    1
    AnAge
  • Range gestation period
    5 to 6 months
  • Range weaning age
    4 to 6 months
  • Range time to independence
    4 to 6 months
  • Range age at sexual or reproductive maturity (female)
    1 (low) years
  • Average age at sexual or reproductive maturity (female)
    2-3 years
  • Range age at sexual or reproductive maturity (male)
    7 (high) years

Female bighorn sheep seek out protected areas to give birth to their lambs. They nurse the young for 4 to 5 months, with the lambs increasing from about 4 kg at birth to 25 to 35 kg at weaning (males generally weighing more). Lambs are capable of walking and following their mothers on precipitous terrain soon after birth. Males do not participate in parental care. (Festa-Bianchet, 1999)

  • Parental Investment
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
  • post-independence association with parents
  • extended period of juvenile learning

Lifespan/Longevity

Longevity depends on population status. In declining or stable populations, most sheep live over 10 years, with a maximum of 19 years. However in an expanding population with heavy reproduction, average life span is only 6 to 7 years. Females have been known to live up to 19 years and males to 14 but attaining these ages is rare. Even in areas where no hunting occurs, females rarely make it past 15 and males rarely live beyond 12. Juvenile mortality is variable and can be quite high, from 20 to 80%, averaging 5 to 30%. Between the ages of 2 and 6 there is relatively low mortality. (Festa-Bianchet, 1999)

  • Range lifespan
    Status: wild
    19 (high) years
  • Typical lifespan
    Status: wild
    6 to 15 years

Behavior

Although not as well built for climbing as mountain goats, bighorn sheep zigzag up and down cliff faces with amazing ease. They use ledges only 2 inches wide for footholds, and bounce from ledge to ledge over spans as wide as 20 feet. They can move over level ground at 30 miles per hour and scramble up mountain slopes at 15 mph. They also swim freely, despite their massive bulk and the weight of their horns.

Most populations undergo seasonal movements, generally using larger upland areas in the summer and concentrating in sheltered valleys during the winter.

Bighorn sheep are gregarious, sometimes gathering in herds of over 100 individuals, although small groups of 8 to 10 are more common. Mature males usually stay apart from females and young for most of the year in separate bachelor flocks. Young females generally remain in their mother's group (led by an older ewe), but males depart when two to four years old and join a group of rams. Young sheep of both sexes learn migratory paths and suitable habitats from adults in the group. (Festa-Bianchet, 1999)

Home Range

Home ranges are quite large, averaging nearly 17 square km.

Communication and Perception

Bighorn sheep are very alert and have remarkable eyesight that allows them to judge distances accurately in jumping and locating footholds. They often watch other animals moving at distances of up to a mile away. Bighorn sheep probably also use chemical cues, as do most mammals, to distinguish reproductive states and may use visual cues to assess dominance among males. Bighorn sheep are less vocal than domestic sheep. The lambs bleat, and ewes respond with a gutteral "ba." At other times of the year, adults utter throaty rumbles or "blow" in fright. During the rut, the rams frequently snort loudly. (Festa-Bianchet, 1999)

Food Habits

Bighorn sheep are mainly diurnal, feeding intermittently throughout the day. Ovis canadensis canadensis is largely a grazer, consuming grasses, sedges, and forbs, but it will take some browse when preferred food is scarce (especially in winter). Desert bighorns (O. c. nelsoni) eat a variety of desert plants and get most of their moisture from the vegetation, although they still visit water holes every several days. (Festa-Bianchet, 1999)

  • Plant Foods
  • leaves

Predation

The availability of escape territory in the form of rocky cliffs is important to bighorn sheep survival. If a sheep can reach a rocky outcrop or cliff, it is usually safe from the attack of wolves, coyotes, bears, Canada lynx, and mountain lions. Golden eagles (Aquila chrysaetos) may take some lambs. (Festa-Bianchet, 1999)

Ecosystem Roles

Bighorn sheep are important predators of grasses and shrubs in their native landscapes, they are also important sources of prey for large predators. Bighorn sheep are hosts for a number of parasites. Nematode lungworms, Protostrongylus stilesi and P. rushi, infect all bighorn sheep individuals and probably co-evolved with these sheep in North America. Most sheep do not experience any significant deleterious effects of lungworms. (Festa-Bianchet, 1999)

Commensal/Parasitic Species
  • Protostrongylus stilesi
  • P. rushi

Economic Importance for Humans: Positive

Native Americans and early settlers prized bighorn meat as the most palatable of American big-game species. Native Americans also used the horns to fashion large ceremonial spoons and handles for utensils. The horns have also been popular for many centuries as trophies. Bighorn sheep may serve as an attraction for ecotourism ventures in parts of western North America. (Festa-Bianchet, 1999)

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

Economic Importance for Humans: Negative

There are no known negative effects of bighorn sheep on humans.

Conservation Status

Several populations may be threatened with eventual extinction, bighorn numbers are only one-tenth the population that existed when western settlers first began exploiting the Rockies. Their main threats are unregulated or illegal hunting, introduced diseases, competition from livestock, and continual human encroachment on their habitat. The subspecies O. c. auduboni of the Black Hills and adjacent areas has already become extinct. Bighorn sheep are incompatible with domestic sheep because they are susceptible to diseases of domestic livestock, including pneumonia, which is periodically responsible for large die-offs in bighorn sheep populations. Hunting has been prohibited or controlled since the early 1900's, but much illegal poaching still occurs. Hunting for trophies is particularly damaging to the cohesiveness of bighorn groups because it eliminates the dominant, breeding males. Recovery of numbers has been slow for these animals and their future is threatened unless further conservation measures are implemented. California bighorn sheep (O. c. californicus) are considered endangered under the U.S. Endangered Species Act. (Festa-Bianchet, 1999)

Contributors

Tanya Dewey (author), Animal Diversity Web.

Liz Ballenger (author), University of Michigan-Ann Arbor.

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

acoustic

uses sound to communicate

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.

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

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

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

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.

folivore

an animal that mainly eats leaves.

food

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

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

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.

polygynous

having more than one female as a mate at one time

seasonal breeding

breeding is confined to a particular season

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

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Banfield, A.W.F. 1974. The Mammals of Canada. University of Toronto Press, Toronto.

Darymple, B.W. 1985. North American Big-Game Animals. Outdoor Life Books, New York.

Geist, V. 1979. Hoofed mammals. In: Wild Animals of North America. National Geographic Society, Washington, D.C.

Nowak, R.M. and J.L Paradiso. 1983. Walker's Mammals of the World. 4th edition. John Hopkins University Press, Baltimore, MD.

Festa-Bianchet, M. 1999. Bighorn sheep. Pp. 348-350 in D Wilson, S Ruff, eds. The Smithsonian Book of North American Mammals. Washington, D.C.: The Smithsonian Institution Press.

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