Sierra Nevada Bighorn Sheep - Stephanie Smith

Sierra Nevada Bighorn Sheep

Ovis canadensis sierrae

The wild sheep ranks highest among the animal mountaineers of the Sierra. Possessed of keen sight and scent, and strong limbs, he dwells secure amid the loftiest summits, leaping unscathed from crag to crag, up and down the fronts of giddy precipices, crossing foaming torrents and slopes of frozen snow, exposed to the wildest storms, yet maintaining a brave, warm life, and developing from generation to generation in perfect strength and beauty.

— John Muir, 1894

 

Characteristics

Sierra Nevada Bighorn Sheep are typically 5 feet long and 2 ½ to 3 ¼ feet tall. Males, called rams, can weigh up to 220 lbs, and females, called ewes, can be up to 155 lbs. Rams have an average life span from 10 to 12 years, while ewes live anywhere from 12 to 20 years. The oldest ewe recorded lived to be 17 years old.

Both males and females have horns that continue to grow throughout their lifespan. Males have a large dominant display, while females have less prominant horns. One main difference between the Sierra bighorns and other species such as the Desert bighorns are the shape and curl of the horns. In comparison, the Sierra bighorns have more narrow horns that diverge more widely.

Fur color can range from dark brown to white, depending on the location and time of the year. Molting occurs in the late spring and afterwards results in a darker color. During winter, exposure to the sun reflecting off of winter slopes causes the fur to lighten considerably. 

The body is developed for climbing steep enbankments and scrambling across rocky slopes. The legs are short and stocky, and the rump and shoulders are highly muscular to enable success in this type of environment.

Geographic Range 

The Sierra Nevada Bighorn sheep live exclusively in the Sierra Nevada mountain range of California.

There are three species of Bighorn sheep in California.

Sierra Nevada Bighorn sheep herd locations.

 

Ecology

Ideal habitat is within alpine meadows or summit plateaus fed by springs, and surrounded by steep and rocky terrain. The open meadows provide ample grazing opportunities while the steep topography provides a quick escape from predators. The sheep favor open areas over heavy forest cover as they rely predominantly on their eyesight for predator detection. Limited seasonal migration allows sheep to overwinter at lower elevations where more vegetation is available for feeding. The bighorn sheep are herbivores, feeding primarily on grasses, herbacious plants, and shrubs, selectively favoring new growth.

Males establish a dominance hierarchy prior to the mating season. Dominance is established through visual displays, neck wrestling, kicking, and horn clashes. The most dominant males typically have the largest bodies and horns. 

Male horn display.

Population

Prior to European settlement, Bighorn sheep populations were estimated to be around 1,000 individuals at the least. However, unregulated hunting and spread of respiratory diseases from domestic animals decimated that number. In 1995 barely 100 individuals were counted in the Sierra Nevada range.

Listing Status

The Sierra Nevada Bighorn sheep was listed as an endangered species on January 3rd, 2000, under the Endangered Species Act. The species was given emergency listing in 1999 when there were only about 100 individuals in existence, and was formally added to the endangered species list the following year. It is currently still listed as an endangered species, although the recovery and population of the species is having major successes. In 2016, the population was estimated to be over 600 individuals.

Recovery

The most important actions needed in recovery for the sheep:

-Protect existing herds by maximizing population growth and managing predation.

-Augment small herds through translocation. Larger herb numbers can boost population more effectively and can better resist predators.

-Prevent contact between Sierra Nevada Bighorn Sheep and local domesticated sheep and goats to limit potential spread of disease.

-Reintroduce bighorn sheep to vacant herd units.

-Monitor genetic variation to ensure there is genetic variation and reduce inbreeding.

Females have a long life span and have above a 90% survival rate. Males are slightly lower, but also have near 90% survival rate. Lambs have a rate of 50% on average, which varies greatly. Lambs have a lower survival rate due to variable weather patterns and predation.

Lambs on a steep cliff.

Threats

The main threats to the population are diseases spread from domestic animals (sheep and goats), predation, low genetic diversity caused by inbreeding, and small population size in general (caused by varying natural factors such as unpredictable weather and other natural events). Extinction threatens the sheep populations if the main threats to survival are not properly managed or reversed.

What can you do?

Tracking devices have been fitted to many individual sheep in order to monitor and learn about behavior. Hikers and backpackers are encouraged to report any sightings in order to provide more information for scientists to work from. If you encounter a sheep, be careful not to disturb or spook it. Any negative disturbances can affect their fitness and survival.

Other resources?

More information can be found at the following sources:

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

- http://sierrabighorn.org/ 

- https://www.yosemiteconservancy.org/wildlife-habitat/protecting-sierra-nevada-bighorn-sheep