The Biogeography of Bighorn Sheep (Ovis canadensis)
 By Janelle Harraka, student in Geography 316

Source: Department of Fish & Wildlife

Thank you for visiting our site. This web pages was written by a student in Geography 316: Biogeography, and edited by the instructor, Barbara Holzman, PhD.  All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided.  Sources and references are cited at the end of each page.

Species Name:                                                                         Source: Department of Fish & Wildlife

Kingdom:  Animalia
Phylum:  Chordata
Class:  Mammalia
Order:  Artiodactyla
Family:  Bovidae
Genus:  Ovis
Species:  Ovis Canadensis

Description of Species:

          The bighorn sheep is known for its striking dramatically curved large horns.  They are hoofed mammals with smooth and stiff short brown fur in a range of shades from pale tan to dark brown.  Bighorn sheep are also recognized by a distinctive white patch on their rump and a contrasting short dark brown tail.  They stand approximately three feet tall at the shoulder (32” to 40”), and are 50” to 62” long.  Females grow to 140 pounds and males grow to 220 pounds.  Both rams and ewes have a life span of 15 years (US Department of Fish and Wildlife, 2002). 

Bighorn sheep have necks as broad as their shoulders to carry the weight of their 30 pound horns, but their head and snout are comparatively narrow and petite.  These namesake horns seem to dwarf the head of bighorn sheep, whose narrow ivory colored snout is suited for their diet of grasses, forbs and sedges.   On a male adult, the horns are 4 inches wide at the base, spiraling backward, down, and forward to create a curling flare with a spread of 33 inches (Sumner, 1980).  These formidable dark gray horns have growth rings, battle scars, and blunt tips pointing forward at either side of their mouths.  By the time a male reaches seven years of age, he has developed a full set of horns which then continue to grow for the remainder of his life in thickness instead of in length (Monson, 1980).  The rams’ horns are significantly larger than the females, and are used in frequent combative head-butting with other males during the mating season.  Females also grow horns and participate in head-butting, but their horns are not as large and never exceed a half circle in its spiraling shape, nor 12 inches in total length (Monson, 1980).

Diet and Food:

           Bighorn sheep are herbivores, feeding on grasses, sedges, and forbs.  Typical preferred plants include needlegrass, fluffgrass, downy chess, fescue, bluegrass, wheat grass, clover, cinquefoil, and phlox (Monson, 1980).  In the winter season, they eat more woody plants such as sage, rabbit brush, mountain mahogany, turpentine broom, and willow, switching to young, fresh growth in the spring.  They are able to receive most of their water needs through the vegetation they consume.  They can wait to visit a watering hole every three days in the desert or especially hot summer days, but prefer to water on a daily basis (US Dept. of Fish and Wildlife, 2002).  Bighorn sheep are diurnal, feeding in the daylight hours beginning at dawn.  From their bedding areas in the rocky, protected slopes, they seek food in low-lying shrublands and open meadows, avoiding dense or forested environments.  Open spaces that are bordered by precipitous rocky terrain is a requirement for feeding (Johnson, 2000).  This allows bighorn sheep to use their keen eyesight to keep watch for predators up to a mile away.  This ability for long range sighting allows the bighorn sheep advance warning of approaching danger, enabling a quick escape to the safety and cover of surrounding cliffs and steep, craggy slopes (Johnson, 2000).

Habitat description:

            Bighorn sheep live between the rocky slopes of mountainous terrain and the open meadows that feed them.  Migration occurs between elevations of 2,500-5000 feet in the winter, to elevations of 6000-8500 in the summertime, to minimize their exposure to extreme weather (Monson, 1980).  They are unable to paw through thick snow to uncover vegetation, and thus practice altitudinal migration, following the plants back up the mountain as they begin to grow in the spring.  In the summer and desert, if it becomes too hot or water holes are severely depleted, bighorn sheep will rest in the daytime shade and resume their feeding at night to conserve water, even though they are ordinarily diurnal animals (Wehausen, 2002). 

Typical environments where bighorn sheep have been found include alpine meadows, temperate foothill regions of coniferous and deciduous forests, low-lying scrubland, grasslands and deserts.  Bighorn sheep are extremely agile on precipitous slopes, and use these areas for lambing, bedding, mating, and escaping predators (Monson, 1980).  The more open areas used for feeding are only considered safe if flanked by steep rocky cliffs.  The bighorn sheeps’ muscular bodies and hard hooves allow deft maneuvering on these steep mountains, and they are known to race up the hillside at 15 miles per hour, jumping 20 feet across deep crevices, and using footholds of only 2 inches wide (Blood, 2000).  Such agility allows bighorn sheep to outrun their predators which have less-sure footing.

Distribution:

           The natural state of bighorn sheep populations is to follow a pattern of disjunct distribution across western North America.  This pattern of scattered populations is dispersed among mountain ranges, foothills and deserts.

            Bighorn sheep used to be plentiful in the western United States, but populations have declined since the westward expansion of human settlement and related introduction of exotic species by European settlers, especially domestic sheep.  Originally, bighorn sheep were found from Canada to Mexico, stretching from the Sierra Nevada Mountains to the Rocky Mountains, and as far south as the deserts of Mexico.  Today, bighorn sheep are a federally listed endangered species. A susceptibility to disease carried by domestic sheep is the leading cause of their decline (Singer, 2001).  Populations persist in fragmented groups, throughout western North America (Figure 3).

Figure 1. Historical range and current distribution of three subspecies of bighorn sheep in the south western United States (redrawn from Trefethen 1975 and Weaver 1985).   Source: http://biology.usgs.gov/s+t/imagefiles/r039f01.htm

            Bighorn sheep occupy many environments at varying latitudes, across many types of ecosystems, including cold coniferous and deciduous forests, alpine meadows, temperate grasslands adjacent to mountain ranges, and warmer, drier desert regions of Southern California, Utah, and Baja California, Mexico (McCutchen, 1995).  Bighorn sheep have adapted to many different climates. For example, they practice altitudinal migration between summer and winter to avoid extreme weather, and are able to meet water requirements through the moisture in their food during times of short water supply   (Blood, 2000).  These kinds of adaptations have allowed bighorn sheep to historically occupy a wide range of ecosystems, as long as there is open space in which to feed, and nearby rocky slopes in which to seek cover (Blood, 2000).

            Accounting for the persistence of Ovis canadensis through the disparate climates and environments of western North America, multiple sub-species of bighorn sheep have evolved. The populations of these subspecies roughly divide among various mountain regions and deserts (Figure 2): O.c. audobani were found in the midwestern regions of the United States and are now extinct, O.c. nelsoni is a desert bighorn sheep found at the convergence of southern California, Nevada, Utah, and Arizona (McCutchen 1995).  O.c. californiana is found in the southwest interior of British Columbia to the east side of the Coast and Cascade Mountain ranges, and south through California to the Sierra Nevada Mountains.  O.c. canadensis persists in the Rocky Mountains from Canada to Mexico (McCutchen 1995).  Today, a total of seven subspecies have populations in British Columbia, Arizona, California, Nevada, Colorado, Utah, New Mexico, Texas, Baja California, and northern Mexico (McCutchen 1995). 

Behavior and group dynamics:

            Bighorn sheep populations are typically subdivided by gender.  Ewes and lambs of both genders form a larger group, and their behavior as a herd is different than the behavior of rams (Ruckstuhl, 2001).  The herd of females, lambs and sub-adults remain in a cohesive group throughout a daily routine.  They traverse their habitat grazing and browsing as a group (Ruckstuhl, 2001).  While foraging, they remain close to the safety zone and protection of rocky slopes, and this choice results in a marked difference in the nutritional value of their diet when compared to rams of the same local population (Ruckstuhl, 2001). 

            Rams are known to travel between these female dominated groups alone or occasionally in small groups of two or three (Monson, 1980).  They travel a broader range and forage farther from rocky escape routes, exchanging an increased risk of predation for the increased diversity of vegetation (Ruckstuhl, 2001).  It has been shown that the rams’ willingness to range further from their bedding sites, results in a benefit of improved nutrition (Ruckstuhl, 2001).  Interestingly, sub-adult male bighorn sheep eventually break away from their female herd upon reaching a certain size, when they demand the extra nutrition typical of a ram’s diet.  Until they break away, their eating habits are dictated by the herd’s group choice, and they are confined to a limited feeding zone (Ruckstuhl, 2001). 

Mating habits:

Bighorn sheep are polygamous, and males may migrate separately among multiple subpopulations of females.  Ritual jousting matches between males is typical as the dominant male protects his group of females for his own, exclusive mating purposes (Monson, 1980). 

Rams and ewes reach sexual maturity at two years, and mating occurs in November and December.  Gestation is 5.5 to 6 months, and the lambing season is April to June (Monson, 1980).  Lambs are born into the same rocky, steep slopes as the herd uses for sleeping, and within a day can climb as well as adults (Wehausen, 2002).  Within a few weeks, lambs are found in their own young herds, seeking their mothers for occasional suckling.  Lambs are completely weaned by six months of age. Typically, females have one lamb per year, but if nutrition is poor then this timeframe shifts to every two years (Wehausen, 2002).  These jousting matches entail repeated head butting with their namesake horns, until the victor is established.

Competition and predation:

            Predators of the bighorn sheep include wolves, coyotes, mountain lions, bobcats, bears, gray foxes, and humans (Singer, 2000).  Natural competitors include deer and burros.  However, the more important competitor for today’s populations is domestic livestock and sheep.  Commercially herded sheep carry many diseases that are highly contagious and lethal to native bighorn sheep (Singer, 2001).  It has been shown that a buffer zone of 17 miles is the minimum distance needed between domestic and bighorn sheep to prevent infection and contraction of disease (Singer, 2001).  Figure 2 illustrates the change in historic habitat space versus the habitat used by today's shrinking populations.

 Figure 2. Historical range and current distribution of three subspecies of bighorn sheep in the south western United States (redrawn from Trefethen 1975 and Weaver 1985).   Source: http://biology.usgs.gov/s+t/imagefiles/r039f01.htm

            This issue introduces the larger competitive threat represented by humans.  Seton in 1929 “…estimated the pre-columbian numbers of all subspecies of bighorn sheep in North America (United States, Canada, Mexico) at 1.5 – 2 million,” (McCutchen, 1995).  However, by the 1880s, after decades of dramatically increased human settlement in the western United States, bighorn sheep populations had declined so rapidly that hunting restrictions were declared in California in 1873 and in Texas in 1903 (Monson, 1995).  Despite these early efforts for protection, populations continued to fall throughout the United States.  By 1960 estimates of total bighorn sheep populations were only 15,000 – 18,200 (see Figure 2).  Around this time, Arizona, California, Utah, Nevada, and Texas better enforced their restrictions on hunting, and began programs to monitor and reintroduce bighorn sheep in an effort to save them from extinction (McCutchen, 1980).  Table 1 shows a sample of how desert bighorn sheep populations increased as a result of these programs.

            Despite these efforts and the advances made with reintroduction programs, the bighorn sheep became a federally listed endangered species in 1998.  Much research has been focused on the success rate of various introduction programs, analyzing the transport method, the effect of lapsed time in captivity, and the minimum group size needed for long term reproductive success (Berger, 1999; Singer, 2000).

Table 1.  Status and trends of desert bighorn sheep in the United States 1960-93. Estimate for 1960 by Buechner (1960). Estimates for 1993 from state wildlife agency status reports presented to the Desert Bighorn Council, Moab, Utah, 1994. (McCutchen, 1995)

Evolution:

            Ovis canadensis, the bighorn sheep, is in the mammalia class, which includes approximately 5000 species in 26 orders.  During the Tertiary Period approximately 65 million years ago, following the extinction of the dinosaurs, mammals spread throughout the world through adaptive radiation (Figure 4). 

            The most primitive form of mammals first evolved approximately 230 million years ago in the middle Triassic Period, at a time when all continents were joined as one super continent called Pangaea (Simpson, 1962).  In the middle Jurassic period (170 millions years ago), Pangaea split into two landmasses; Laurasia in the northern hemisphere and Gondwana in the southern hemisphere, and the earliest forms of mammals were thus established on our present day continents, although not in the numbers or variety we see today (Simpson, 1962).  During the Triassic and Jurassic periods, dinosaurs had also colonized the continents, enjoying a cosmopolitan distribution.  Because of this fact, mammals did not speciate into larger numbers of morphologically distinct forms until after the dinosaurs went extinct (Simpson, 1962). After the KT border, the dinosaurs effectively vanquished their niche in the ecosystem.  Prior to this time, dinosaurs represented a substantial competition for resources to mammals.  After their demise, mammals were able to evolve through adaptive radiation and fill a variety of niches in all ecosystems (Halstead, 1978).

The modern day bighorn sheep is a member of the Artiodactyla order, a diverse order with 220 species organized into ten families.  Artiodactyls are hoofed mammals, and modern day examples include horses, antelope, goats, sheep, and pigs.  Artiodactyls appear in the fossil record during the Eocene epoch, approximately ten million years after the dinosaurs went extinct, or 56 million years ago (Halstead, 1978).  The artiodactyla order consists of mammals with modified multi-chamber stomachs which have evolved to efficiently digest cellulose, also called ruminants.  Ruminants are subdivided into three general evolutionary groups: the Traguloids, which are the earliest Artiodactyls, evolving in the Eocene epoch; the Cervoids, which evolved in the Oligocene epoch 38 million years ago and are the ancestors to deer and giraffes; and Bovoids, which evolved in the Miocene epoch 26 million years ago and are ancestors to antelopes, sheep and goats (Halstead, 1978).  As an order, the Artiodactyls include both fast and slow species which live in the open plains (the grasslands and savannas) of the world, and are present on all continents except Antarctica and Australia. 

Bighorn sheep belong to the Bovidae family.  Bovidae fossils appeared in the Miocene epoch, approximately 23-26 million years ago (Halstead, 1978).  Consisting of 137 modern day species in 45 genera, the Bovidae family can be classified as the “horn” family.  The bovidae family all have unbranched horns, ranging in form from short and spiky to long and spiraled.  Familiar members of the Bovidae family include gazelles, mountain goats, buffalo, yaks, cattle, and sheep.  Many different species of the Bovidae family are thought to have dispersed through the Americas during the Pleistocene epoch, 2.4 million years ago, taking advantage of lower sea levels and crossing the Bering Straight land bridge from Siberia to Alaska (Halstead, 1978).  This family is endemic to North and South America, Europe, Asia, and Africa.  Species with Bovidae ancestry are found in many different biomes, from arctic tunrdra to the tropics, but are most diverse in sub-tropical grasslands and savannas (Halstead, 1978). 

The Ovis genera includes six species (Figure 3).  There are three Eurasian forms and three American forms.  The Eurasian forms which evolved along with goats, are long-legged, and inhabit foothills where the upper rocky cliffs and steeper terrains are the domain of goats (Geist, 1971).  The American forms evolved in the absence of goats, and are comparatively short legged and broad chested, and inhabit rocky cliffs devoid of goat competition (Geist, 1971). 

A fossil record for Ovis canadensis appeared 100,000 years ago in North America (Monson, 1980).  Throughout glaciations and climate change, bighorn sheep have persisted in their current morphological form.  During the Wisconsin glaciation period, bighorn sheep survived south of the Cordilleran ice sheet, occupying western United States and northern Mexico habitats (Monson, 1980).  As the ice sheet melted 15,000 to 10,000 years ago, bighorn sheep gradually spread northward into the southern British Columbia and Alberta provinces of Canada. 

   
 
  Figure 3. Global distribution of Mountain sheep. Source: Radford University, October 2002,  http://www.runet.edu/~swoodwar/CLASSES/GEOG235/exercises/mtshpmap.html

Figure 4. Cladogram. Source: Antelopes, Deer, and Relatives, Yale University Press, New Haven and London. (2000)
 

Interesting links:

Sierra Nevada Bighorn Sheep Foundation:  http://www.sierrabighorn.org/newsletter.htm#top

Bighorn Sheep:  http://www.bighorn-sheep.com/

The National Bighorn Sheep Interpretive Center: http://www.bighorn.org/

Bighorn Sheep, Desert USA:  http://www.desertusa.com/big.html

British Columbia Adventure, Bighorn Sheep:  http://www.bcadventure.com/adventure/wilderness/animals/bighorn.htm

 Bibliography: 

Berger, Joel. (1999) “Intervention and Persistence in Small Populations of Bighorn Sheep.” Conservation Biology pp. 432-435, Volume 13, No.2.

Blood, Donald A. (2000) Presentation of: “Bighorn Sheep, British Columbia, Ecology, Conservation and Management,” by the British Columbia Ministry of Environment, Lands and Parks. [Online].  Available: http://wlapwww.gov.bc.ca/wld/documents/bighorn.pdf  [October 20, 2002]

Geist, Valerius. (1971) Mountain Sheep; A Study in Behavior and Evolution. University of Chicago Press, Chicago. 177pp.

Halstead, Lambert Beverly. (1978) The Evolution of Mammals. Eurobook Limited, Italy. 116 pp.

Johnson, Therese L., David M. Swift. (2000) “A Test of a Habitat Evaluation Procedure for Rocky Mountain Bighorn Sheep.” Restoration Ecology pp. 47-56, Volume 8, No. 45.

Krausman, Paul R.  (2000) “An Introduction to the Restoration of Bighorn Sheep.” Restoration Ecology pp.3-5, Volume 8, No. 45.

McCutchen, Henry E. (1995) Presentation of:  “A Report to the Nation on the Distribution, Abundance, and Health of U.S. Plants, Animals, and Ecosystems,” by the Department of the Interior, National Biological Service. [Online].  Available: http://biology.usgs.gov/s+t/index.htm [October 20, 2002]

Monson, Gale, Lowell Sumner. (1980) The Desert Bighorn, Its Life History, Ecology & Management. The University of Arizona Press, Arizona. 370 pp.

Ramey II, Rob Roy, Gordon Luikart, Francis J. Singer. (2000) “Genetic Bottlenecks Resulting from Restoration Efforts: The Case of Bighorn Sheep in Badlands National Park.” Restoration Ecology pp. 85-90, Volume 8, No. 45.

Rubin, Esther S., Walter M. Boyce, Chris J. Stermer, Steven G. Torres. (2002) “Bighorn sheep habitat use and selection near an urban environment.” Biological Conservation pp. 251-263, Volume 104, No. 2.

Ruckstuhl, Kathreen E., Marco Festa-Bianchet. (2001) “Group Choice by subadult Bighorn Rams: Trade-offs between Foraging Efficiency and Predator Avoidance.” Ethology pp. 161-172, Volume 107, March 2001.

Simpson, George Gaylord. (1962) Evolution and Geography. University of Oregon Press, Oregon. 64 pp.

Singer, Francis J., Christopher M. Papouchis, Kate K. Symonds. (2000) “Translocations as a Tool for Restoring Populations of Bighorn Sheep.” Restoration Ecology pp. 6-13, Volume 8, No. 45.

Singer, Francis J, Linda C. Zeigenfuss, Leslie Spicer. (2001) “Role of Patch Size, Disease, and Movement in Rapid Extinction of Bighorn Sheep.” Conservation Biology pp. 1347-1354, Volume 15, No. 5.

Tennesen, Michael. (2000) “Big Bucks for Bighorns.” Wildlife Conservation pp. 25-29, July/August 2000.

Thompson, Rock J., Vernon C. Bleich, Steven G. Torres, Gerald P. Mucahy. (2001) “Translocation Techniques for Mountain Sheep: Does the Method Matter?” The Southwestern Naturalist pp.87-93, Volume 46, No. 1.

Vrba, Elizabeth S. and George B. Schaller. (2000) Antelopes, Deer, and Relatives. Yale University Press, New Haven and London. 341 pp.

Wehausen, John D. (2002) “Natural History of Bighorn Sheep” by the Sierra Nevada Bighorn Sheep Foundation [Online]. Avaliable: http://www.sierrabighorn.org/index.htm [October 20, 2002]

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