Sierra Nevada Bighorn Sheep

(Ovis Canadensis Sierrae)

By: Allyson Reines

(Fig. 1)

Description and Ecology

            The Sierra Nevada Bighorn Sheep is on of the rarest large mammals in North America. It is a subspecies of bighorn sheep unique to the Sierra Nevada mountains. The sheep are listed as a federally endangered species under the Endangered Species Act of 1973. Sierra Nevada Bighorn Sheep are easily distinguishable due to their large, curled, brown horns that grow throughout their lives. Both rams and ewes grow these horns, but the rams tend to have much larger horns that curve more. The Sierra Nevada Bighorn Sheep is similar in appearance to other desert associated bighorn sheep. In comparison to other desert bighorn sheep, the horns of the Sierra Nevada Bighorn are smaller in size, but far wider than those of the Desert Bighorn as you can see in Figure 2. As the sheep age, their horns become rough and scarred and will sometimes vary in color from yellowish-brown to dark brown. The Sierra Nevada Bighorn Sheep feature wide variations in their coat colors which range from white to dark brown with white patches on the belly, rump, back of legs, muzzle, and around the eyes as you can see in Figure 1 above. There is some seasonal change in their coat coloration due to the molting of the thick winter layer. On average, adult male sheep measure about 3 feet tall and weigh between 120-220 pounds while females weigh between 100-155 pounds.

            The Sierra Nevada Bighorn Sheep are equipped with special hooves that allow them to navigate the rocky terrain in the Sierra Nevadas. Their habitat ranges from the eastern base to the peaks of the Sierra Nevadas. Sierra Bighorns live in open areas where the land is rocky, has little vegetation, and has steep slopes and canyons. The sheep prefer this landscape because it provides them with great visibility and escape terrain, making it easier to avoid predators. Sierra Bighorns are herbivores that mainly consumer grasses, forbs, and shrubs.

Geographic and population changes

            In the past, Sierra Bighorns were spread out along peaks in the Sierra Nevadas from Sonora Pass in the north to Olancha Peak in the south. Before European settlers, there was believed to be more than 1,000 of these sheep in the Sierra Nevadas. By the beginning of the 20^th century, the Sierra Nevada Bighorn Sheep population had declined to approximately nine herds. This decline was mostly attributed to diseases contracted from domestic sheep, competition for forage with domestic livestock, predation, changes in habitat, and market hunting. By the late 1970s, Mt. Baxter and Mt. Williamson were the only two geographic areas where a total of 250 sheep could be found. From 1979 to 1988 bighorn were transported to other areas so that herds could reestablish in historic ranges. During the 1990s, the Sierra Bighorns experienced another population decline caused by 

                (Figure 3)                 mountain lion predation and drought, leaving only about 100 bighorns.

                                                                                                                        

Listing date and type of listing

            Sierra Bighorns have been listed under the California Endangered Species Act (CESA) since 1974, but were uplisted from endangered to threatened in 1999. In the same year, Sierra Bighorns were temporarily listed as endangered. Since 2000, the Sierra Nevada Bighorn Sheep has been listed as an endangered species.

Cause of listing and main threats to continued existence

            The Sierra Nevada Bighorn Sheep achieved its status as federally endangered because of a critically low population size and inefficient regulatory measures. The two main concerns were the negative effects of mountain lion predation and a major respiratory disease epidemic that was caused by contact with domestic sheep. Other factors that put Sierra Bighorns at risk are forest succession, genetic diversity, severe weather, climate change, and reduced geographic distribution. Issues involved with management of this species include losses to predation, environmental factors that limit use of lower elevation ranges, domestic sheep grazing, and unoccupied herd units.                                                               

Description of recovery plan

            The current Sierra Nevada Bighorn Sheep Recovery Program holds biannual population surveys of all herds and assesses nutritional status. The program also observes survival rates and patterns in habitat use in randomly selected individuals, compiles important data from GPS collars, and evaluates resource selection patterns. In addition, the program studies genetic variation across subspecies, approximates risk of disease transmission spread by domestic sheep and goats, looks at the effects of fire on bighorn habitat use, follows mountain lion movements, predation rates, and population size, carries out translocations, and models the effects of future management                (Figure 4)                    actions on bighorns. Since the implementation of the Recovery Program, the Sierra Bighorn population has grown from about 125 in 1999 to more than 600 in 2016. Thirteen out of sixteen herd units classified as satisfactory habitat are occupied with an additional unidentified herd unit occupied in the Cathedral Range.

What you can do

            The Sierra Nevada Bighorn Sheep Recovery Program benefits the most from public involvement and funding. Tracking these sheep throughout the rugged, large-scale Sierra Nevada terrain is a difficult job for the small amount of people that do it. Any time hikers see a group of sheep, they are encouraged to report their observations to the program. In the past, public sightings have discovered herds that the program didn’t even know were there. These hikers are a valuable resource for the program. Another option to help the Sierra Bighorn is to donate to their recovery program.

Related Articles and Resources

http://www.dfg.ca.gov/snbs/SheepFacts.html

http://sierrabighorn.org

https://news.nationalgeographic.com/2015/04/150415-bighorn-sheep-sierra-nevada-reintroduction-endangered-species-california/

https://www.sfchronicle.com/sports/article/Sierra-bighorns-on-ledge-of-breakthrough-11119446.php

https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=27634

Sources

Figure 1. Sierra Nevada Bighorn Sheep Foundation. sierrabighorn.org, http://sierrabighorn.org/natural-            history/. Accessed March 7, 2018.

Figure 2. California Department of Fish and Wildlife. CA.gov, http://www.dfg.ca.gov/snbs/SheepFacts.html. Accessed March 7, 2018.

Figure 3. California Department of Fish and Wildlife. CA.gov, http://www.dfg.ca.gov/SNBS/RecoveryHome.html. Accessed March 7, 2018.

Figure 4. High Country News. Hcn.org, https://www.hcn.org/issues/49.9/Wildlife-Services-  mountain-lion-killing. Accessed March 7, 2018.