by Kathleen Wallis, student in Geography 316  Spring 2005

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 and sources and references are cited at the end of each page.

Species Name:  Ovis canadensis sierrae 
                                                                                                                    photo courtesy of J.D. Wasserman  2005

Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Artiodactyla Family: Bovidae Subfamily: Caprinae Genus: Ovis Species: Ovis canadensis Subspecies: Ovis canadensis sierrae
	Sierra Nevada Bighorn Sheep

Description of the Species:
California Bighorn Sheep are a medium sided ungulate and like all bighorn sheep have large curved horns.  However as compared to other bighorn species the California and Sierra Nevada 
subspecies have relatively small  horns are more wide and flaring. Females have small narrow horns which rarely exceed 12 inches in length. Bighorn sheep have a generally stocky build.
Adult Sierra Nevada bighorn sheep stand about three feet tall at the shoulders. Males can weigh up to 220 pounds and females (ewes) can reach about 140 pounds (Buechner 1960). Coat 
color is variable from almost white to dark brown with a distinctive large white rump patch and white muzzle and a short dark tail. Bighorns have two adaptations which help them survive in 
their rocky, inhospitable environment. First is their agility on precarious slopes, which is their primary means of escaping predators (Whausen 1979). This is aided by their hooves that are 
hard on the outer edge, but spongy in the center which provides good traction for jumping and climbing. Second is their keen eyesight, which is their primary sense for detecting predators. 
Bighorns have extremely acute eyesight, which aids in jumping and gaining narrow mountain footholds. They often watch other animals moving at distances of up to a mile away. 

Natural History:

Habitat: The Sierra Nevada Mountain rage is located along the eastern boarder of California. The mountain peaks vary in elevation from 6000 to 8000 ft in the north, to 14,000 ft in the south. Most of the precipitation is in the form of snow which occurs from October through April (Wehausen 1980). Bighorn sheep exhibit a variety of seasonal movement patterns to meet varying needs for food, water and safety.  During the summer the Sierra Nevada bighorn sheep live in the alpine and sub-alpine zones from 10,000 ft to 14,000 ft. They prefer the open slopes where the terrain is rough and rocky with sparse vegetation. Here the topography also has steep slopes and canyons where they can escape to quickly if in danger. In winter they find cover on the
high windswept cliffs or migrate to the lower elevations down to 4800 ft in order to find forage on the sagebrush-steppe slopes (Wehausen 1980). Bighorns inhabit rocky, craggy cliffs because this is their strategy to avoid predators. Being able to escape into high rocky terrain is especially important for ewes when lambing and rearing young. Forests and thick brush is usually avoided, for in these areas they cannot see predators. Bighorns blend in so perfectly with the granite cliffs of their environment that they are seldom spotted by humans out in the wild.

Diet: Bighorns are primarily but they will browse on woody vegetation when available. They are opportunistic feeders selecting the most nutritious diet form what is available. They graze on
grasses, and browse on shoots, twigs and leaves of trees and shrubs and herbaceous plants depending on season and location (Wehausen 1980). The past success of the mountain sheep
may be due to their unique, effective digestive system, which allows them to exploit a hard, abrasive, dry forage of poor quality (Geist 1971). Mountain sheep are some of the most specialized 
grazers. Like all ruminates, sheep cultivate bacteria and protozoa in a specialized chamber in their digestive system. A mountain sheep has a four chamber stomach and its metabolic process
differs greatly from other mammals. Plants are shredded and ground by a sheep’s teeth to a fine consistency, which is mixed with water, salts and saliva and then dumped into a fermentation 
chamber called the rumen. Here it is broken down by microorganisms under anaerobic conditions (fermentation). The microorganisms digest cellulose to volatile fatty acids, and the plant 
proteins into amino acids. These acids pass directly from the rumen into the bloodstream. The undigested plant matter and microorganisms are then passed to the true stomach (the 
absomasum) where they are digested by the sheep’s own enzymes and provide high grade proteins, fats and vitamins. The digestive process does not stop here for after the material passes 
the small intestine, the remaining undigested plant material is dumped into a second fermentation vat and microorganisms go to work once more (Geist 1971).  What remains after that is
concentrated and pelleted in the large intestine and passes out. Through this multistage process about 90 percent of the cellulose is digested (Geist 1971). This highly specialized digestive 
system have enabled sheep to survive by being able to find food where others cannot. By being able to live on dry, dusty plants, sheep can exploit a reserve of poor forage and thrive where 
many other herbivores cannot. Plants consumed include varying mixtures of graminoids (grasses), browse (shoots, twigs, and leaves of trees and shrubs), and herbaceous plants depending 
on season and location (Wehausen 1980). In a study of the Mount Baxter and Mount Williamson subpopulations, Wehausen (1980) found that grass, mainly Stipa speciosa (perennial needle-
grass), is the primary diet item in winter. As spring green-up progresses, the bighorn sheep shift from grass to a more varied browse diet, which includes Ephedra viridis (Mormon tea), 
Eriogonum fasciculatum (California buckwheat), and Purshia species (bitterbrush) Wehausen 1980).

Reproduction: Sierra Nevada bighorn sheep are social creatures, and group size and composition varies with gender and from season to season. Males are usually relegated to bachelor
groups, except for when they join the females during breeding season. Males more then 2 years of age join the bachelor herd. Females and their young live in groups of 5 to 15 individuals but
the group size swells during mating season. Ewes usually spend their entire lives in the same band unto which they were born (Cowan and Geist 1971; Wehausen 1980). Breeding takes place
in the fall, usually in November (Cowan and Geist 1971). Single birth are the norm but twins occasionally occur (Wehausen 1980). As gestation is about 6 months lambing occurs from late April 
to early July, with most lambs born from May to June (Wehausen 1980, 1996). This timing corresponds to when the weather is warmest and when vegetation is most plentiful. Ewes with new-
born lambs live solitarily for a short period before joining nursery groups that average about six sheep. Lambs are agile and within a day or so climb almost as well as their mothers. Female 
lambs stay with ewes indefinitely and may attain sexual maturity during the second year of life. Male lambs, depending upon physical condition, may also attain sexual maturity during the second
 year of life (Cowan and Geist 1971). Average lifespan is 9 to 11 years for both sexes (Cowan and Geist 1971; Wehausen 1980).  

Social Group 
 

¨       Matrilineal (based on female associations) 

¨       Gregarious and faithful to natal home range. "Home range groups" or "ewe groups" tend to be related females and lambs (Geist 1971). 

¨       Young learn from older sheep their home range, escape terrain, water sources and lambing habitat. Familiarity with range is an advantage in evading predators.

¨       Rams are not as tied to a specific group - they tend to range, moving between ewe groups (Geist 1971).

¨       Rams with larger horns and body size dominate both males and females 
 (Hansen, 1980)(Shackleton,1985) 

Males do not defend territories but rather engage in battles over mating access to a particular female. Age as well as horn size determines male dominance status. The rams charge each other at about 20 miles per hour, and aim their foreheads while throwing full body weight at the opponent. Impact can be heard as far as a mile away. (Shackleton, 1999)

Evolution:
Bighorn sheep are of the family Bovidae which includes African and Asian antelope, the American pronghorn, the bison and sheep, goats and cattle. (Vaughan 1978). The history of mountain sheep begin somewhere in the early Pleistocene but we know very little about it, for the fossil record is poor (Geist, 1971). Mountain conditions are unfavorable for fossil formation and recurring glaciations have ground over the terrain inhabited by mountain sheep. (Geist 1971). True sheep (Ovis) first appear in the Villafranchian period of Europe
and Asia (Geist 1971) The Villafranchian was a long period of cool climates and minor glaciations about 2.5 million years ago, just before the Pleistocene. Mountain sheep were a  very successful species and during the Pleistocene they proliferated throughout the Northern Hemisphere to most  mountains of Europe, Africa, Asia and North American. (Geist 1971). They were so successful that  are 36 to 40 races of wild sheep living today (the numbers vary depending on which classification scheme you follow). They still have a distribution
unequaled by any living bovid (Geist 1979). Sheep remains in Alaska indicate that sheep crossed the Bering land bridge from Siberia during the Pleistocene and then spread through western North America (Geist 1971). Although the immediate ancestors of mountain sheep is not known, they appear to be offspring of the Rupicaprini, the goat-antelope subfamily.  Rupicaprids are short-horned, light skulled, hairy bovidids of small size whose most generalized representative is the serow (Capricornis) from Southeast Asia. The closest relative to sheep are true goats (Capra) as well as the “sheep-goats” (Ammotragus) from North Africa. Sheep are different from goats in that sheep possess a preorbital gland and inguinal gland at the groin whereas sheep do not. Also sheep have interdigital glands on all four feet (Geist 1971). In photos of bighorn sheep you can often see the dark tear shaped slit in front of the eye. Today wild sheep are grouped into two broad categories, Asiatic and American sheep. (Clark 1964). Wild sheep in North America have diverged into two existing species: thin horn sheep (Ovis dalli) that occupy Alaska and Northwestern Canada, and bighorn sheep (Ovis canadensis) which range from southern Canada to Mexico (Buechner 1960).
Several subspecies of bighorn sheep have been recognized on the basis of geography and differences in skull measurements (Cowan 1940; Buechner 1960). The six  recognized subspecies are Ovis c. canadensis (Rocky mountain bighorn),  Ovis c. californiana (California bighorn),O. c. nelsoni (Nelson bighorn)  O. c. mexicana (Mexicana bighorn), O. c. cremnobates (Peninsular bighorn) , O. c. weemsi (Weems bighorn) Their home ranges are shown in figure 1.

For more than a half century Sierra Nevada bighorn sheep were classified as California bighorn (Ovis canadensis california), which were distributed from north to south between British
Columbia and the southern Sierra Nevada and west to east, between the Cascade and Rocky Mountains (Geist 1971). However recent genetic and morphological research and reanalysis 
of historic data do not support this traditional classification of Sierra Nevada bighorns as Ovis canadensis california (Ramey 2000). For in recent years study of cranial measurements and 
analysis of mitochondrial DNA variation suggest bighorn sheep from the Sierra Nevada are a separate distinct subspecies (Moritz 1994). The Sierra Nevada Bighorn is distinct population 
segment that is geographically isolated and separate from other California bighorn sheep. There is no mixing of this population with other bighorn sheep, and this is supported by evaluation
of the population's genetic variability and morphometric analysis of skull and horn variation (Ramey 1993, 1995; Wehausen and Ramey 1993, 1994; The Sierra Nevada subspecies was 
therefore recently renamed Ovis canadensis sierrae. 

Figure 1:  Historical range of North American Bighorn Sheep (Hall 1952)

1)   Ovis c. canadensis (Rocky mountain bighorn) - British Columbia, Montana, Wyoming, Colorado, New Mexico, Idaho, Nevada, Utah, Oregon, Washington   2)   Ovis c. californiana (California bighorn) - British Columbia, Washington, Oregon, Nevada, California   3)   Ovis c. nelsoni (Nelson bighorn) - California, Nevada, Utah, Arizona,   4)  Ovis c. mexicana (Mexicana bighorn) - Arizona, New Mexico,  Mexico   5)   Ovis c. cremnobates (Peninsular bighorn) - Upper Baja   6)  Ovis c. weemsi (Weems bighorn) - Lower Baja

History of Population’s decline: 

Hunting:
It is thought that subpopulations of Sierra Nevada bighorn sheep started declining with the arrival of gold miners to the Sierra Nevada in the mid-1880s, and those losses continued through
the 1900s (Wehausen 1980). By 1900, about half of the Sierra Nevada bighorn sheep populations were lost, most likely because of introduction of diseases by domestic livestock, and illegal
hunting (Advisory Group 1997). Specific causes for the declines are unknown. Market hunting may have been a contributing factor as evidenced by menus from historic mining towns such
as Bodie, which included bighorn sheep. As well as being used for food (Wehausen 1980). Sierra Nevada bighorn were shot by sheep ranchers who considered wild sheep as competitors
for forage with domestic sheep (Geist 1971). By the 1970s, only 2 subpopulations of Sierra Nevada bighorn sheep, those near Mount Baxter and Mount Williamson in Inyo County, are known
to have survived (Wehausen 1979).  

Domestic Sheep:
The introduction of domestic sheep in Sierra Nevada 1860s and 1870s also contributed to the decline. Large numbers of domestic sheep were grazed seasonally in the Owens Valley and Sierra Nevada prior to the turn of the century (Wehausen 1988). The domestic sheep would use the same ranges as the wild sheep, occasionally coming into direct contact with them. Both domestic sheep and cattle can act as disease vectors. Scabies, most likely contracted from domestic sheep, caused a major decline of bighorn sheep in California in the 1870s to the 1890s and caused catastrophic die-offs in other parts of their range (Buechner 1960) By 1979, only 220 sheep were known to exist in the Mount Baxter subpopulation, and 30 in the Mount Williamson subpopulation (Wehausen 1979).

Disease:
Disease  is believed to have been the major contributing factor responsible for the precipitous decline of Sierra Nevada bighorn sheep starting in the late 1800s (Foreyt and Jessup1982). Bighorn sheep are host to a number of internal and external parasites, including ticks, lice, mites, tapeworms, roundworms, and lungworms. Most of the time, parasites are present in relatively low numbers and have little effect on individual sheep and populations, but with the surviving populations numbers so low for Sierra Nevada bighorns, any loss by disease is devastating. (Cowan and Geist 1971). Die-offs from pneumonia contracted from domestic sheep is another important cause of losses. In 1988, a strain of pneumonia, apparently contracted from domestic sheep, wiped out a reintroduced herd of bighorn sheep in Modoc County. Native bighorn sheep cannot tolerate strains of respiratory bacteria, such as Pasteurella species, carried normally by domestic sheep and close contact with domestic animals results in transmission of disease and subsequent deaths of the exposed animals (Foreyt and Jessup 1982). Bighorn sheep can also develop pneumonia independent of contact with domestic sheep. Lungworms of the genus Protostrongylus are often an important contributor to the pneumonia disease process in some situations (Wehausen 1999). Lungworms are carried by an intermediate host snail, which is ingested by a sheep as it is grazing. Lungworm often exists in a population, but usually doesn't cause a problem. However, if the sheep are stressed in some way, they may develop bacterial pneumonia, which is complicated by lungworm infestation. Bacterial pneumonia is usually a sign of weakness caused by some other agent such as a virus, parasite, poor nutrition, predation, human disturbance, or environmental or behavioral stress that lowers the animal's resistance to disease (Wehausen 1979).

Natural predators: 

The Bighorn sheep’s predators are coyote, bobcat, mountain lion, golden eagle and free-roaming domestic dogs. Loss to predators is usually considered insignificant except when preying on 
small populations like the Sierra Nevada Bighorn. The predator and prey interactions in healthy intact ecosystems typically maintain a balance, but this was not the case for the Sierra Nevada 
Bighorn whose fragmented populations had decreased to about 250 individuals by the 1980’s (Wehausen 1980). To add to the problem mountain in 1983 California banned the hunting of 
mountain lions. From 1907 to 1963, the state provided a bounty on mountain lions which kept the lion population relatively low for many years.  But a moratorium on mountain lion hunting began
 in 1972 and in the late 1980s, mountain lion predation of Sierra Nevada bighorn sheep increased throughout their range (Wehausen 1996) In 1990, a state-wide ballot initiative (Proposition 117) 
passed making the killing of mountain lions against the law, except if humans or their pets or livestock are threatened (Bentz, et al 1999). Between 1976 and 1988 mortalities from mountain lion 
predation represented 80 percent of all mortality of bighorn (Wehausen 1980). Due to the increased mountain lion population bighorns changed their winter habitat patternsand are now
wintering in higher elevation to avoid the mountain lions. Thus the sheep spend the winter months with less forage and leading to poorer condition and increased lamb mortality

because lambs were born later and died in higher elevations during the winter. 

Human encroachment: Human development impacts sheep through habitat loss, fragmentation, or other modification; impacts also extend into big horn sheep habitat beyond the urban edge. 
These include increased noise, predator attraction, and an increased number of humans and their pets that venture into sheep habitat. Numerous researchers, including the previous authors,
have documented altered big horn sheep behavior in response to human-related disturbance. In addition to development, a variety of other human activities, such as hiking, mountain biking, 
horseback riding, camping, hunting, livestock grazing, and use of aircraft and off-road-vehicles, have the potential to disrupt normal big horn sheep social behaviors. Big Horn sheep may also 
alter their use of essential resources resulting in physiological effects or abandon traditional habitat as a result of human disturbance (McQuivey 1978, MacArthur et al. 1979)
 

The following table best represents the total Sierra Nevada bighorn sheep population over various time periods. These totals represent the numbers of sheep emerging from winter in each of
these years, and best document the status of the population by incorporating winter mortality, especially of lambs born the previous year. These totals are not absolute values; numbers have
been rounded to the nearest five.

									    photo courtesy of J. DeForge 2003 

Table 1. Sierra Nevada Bighorn Sheep Population Numbers, by Year (Wehausen 1996) ------------------------------------------------------------------------      Year                    Number of  populations  Total number of sheep  ------------------------------------------------------------------------ 1978.............................................           2        250 1985.............................................           4        310 1995.............................................           5        100 1996.............................................           5        110 1997.............................................           5        130 1998.............................................           5        100 ------------------------------------------------------------------------

Encroachment from urban sprawl

Threats to survival                                                                                          

The continuing decline of the Sierra Nevada bighorn sheep has been attributed to a combination of the direct and indirect effects of the following factors:

• Unpredictable epizootics (rapidly spreading disease affecting a large number of animals throughout a wide range)
• Human disturbance. (Desert bighorn are highly intolerant of human activity)
• Competition from domestic sheep, goats, feral burros for forage, water, space
• Populations have a predominance of old indiviuals and insufficient lamb recruitment (relationship between production and survival)
• Habitat loss, degradation, and fragmentation by urban and commercial development
• Mountain lion, wolf predation coinciding with low population numbers
• Failure of populations to disperse into contiguous, suitable habitat (caution / fear)
• Loss of genetic variation is always a danger when dealing with small populations.
• Invasive plant species (e.g. tamarisk) compete with native plants bighorns feed on.
• Introduced Barbary sheep could take over bighorn habitat.
• Fire suppression has created taller, denser vegetation. Reduced visibility makes the bighorn more vulnerable.
   

Bibliography 

Benz, Carl 1999 Wildlife Service Federal Register: April 20, 1999 Volume 64, Number 75 Page 19300-19309.

Bucchner, H.K. 1960. “The bighorn sheep in the United States; its past, present and future”. Wildlife Monographs 4. 

Cowan, I. 1940. Distribution and Variation in the Native Sheep of North America. American Midland Naturalist 24:505-80.  

Clark, J.L. 1964. The Great Arc of the Wild Sheep. University of Oklahoma Press: 

Deforge, Jim. 2003 Photo "Encroachment from urban sprawl" [Online] Available: http://www.bighorninstitute.org (18 April, 2005).

Geist, V.1971. Mountain sheep: A Study in Behavior and Evolution. University of Chicago Press, Chicago 

Foreyt, W.J. and Jessup, D.A. 1982. Fatal Pneumonia of Bighorn Sheep Following Association with Domestic Sheep, Journal of Wildlife Diseases 18(2) pp 163-168. 

Hall, Raymond. 1952. Mammals of North America. New York Ronald Press. p 1070 Hansen, G. 1980. “Growth and Development of Bighorn Sheep”. Journal of Wildlife Management 29:397-91. 

McCarty, G. W., and J. A. Bailey. 1994. Habitat requirements of desert bighorn sheep. Colorado. Division. Of Wildland. Species. Rep. No. 69, Denver, 27pp. 

McQuivey, R. P. 1978. The desert bighorn of Nevada. Nevada Dep. of Fish and Game Biology . Bulletin. 6, Reno. 81pp. 

Mortiz, 1994. “The DNA Analysis of Bighorn Sheep.” Conservation Biology. 28 (1-2): 27-29.

Ramey, Rob Roy 2000. Cranial Morphometric and Evolutionary Relationshiep in the Northern Range of Ovis Canadensis. Journal of Mammalogy, 81(1): 145-161  

Region 1, U.S. Fish and Wildlife Service, [2003]“Draft recovery plan for the Sierra Nevada Bighorn Sheep” [Online] Available: http://purl.access.gpo.gov/GPO/LPS36593  (March 28, 2005).

Shackleton, D.M. (ed.) 1997. Wild Sheep and Goats and their relatives: Status Survey and Conservation Action Plan. IUCN/SSC Caprinae Specialist Group. IUCN, Gland,
       Switzerland and   Cambridge, UK.

Shackleton, David and Wikeem, Brian, 1999. “Natural history of Rocky Mountain and California bighorn sheep.” pp 78-80 Arizona Press. Tucson.

Vaughan, Terry A., 1978. “Mammalogy” Saunders College Publishing, Philadelphia Pa. Page 261.

Wehausen, J. D. 1979.”The Ecology of Sierra Nevada bighorn sheep” Conservation Biology 27(4):57-65

Wehausen, J. D. 1980.”Sierra Nevada bighorn sheep:  history and population ecology.” Ph.D. Dissertation University of Michigan, Ann Arbor.  240pp. 

Wehausen, J. D. 1996. Effects of mountain lion predation on bighorn sheep in the Sierra Nevada and Granite Mountains of California. Wildlife Society Bulletin 24:471-479

Wehausen, J.D. 2003.  Photo "Alert and Ellusive" Sierra Nature Notes, Volume 2, March 2003 [Online] www.yosemite.org/naturenotes/BighornSheep.htm

Wehausen, J.D. 2005. Photo "Sierra Nevada Bighorn Sheep" [Online] Available: www.sierrabighorn.org (May 01, 2005).

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