Sierra Nevada Bighorn Sheep Program - The Struggle to Survive

“Both ewes and rams made the frightful descent without evincing any extraordinary concern, hugging the rock closely, and controlling the velocity of their half falling, half leaping movements” —John Muir

Bighorn sheep are elusive mountaineers, often the only sign of their passing is the sound of rockfall from steep rocky precipices. Travel in this type of terrain requires training for both man and beast. In early summer, when food is abundant and days are long, groups of lambs can be seen honing their skills as mountaineers by playing games of “chicken”. The lambs appear to compete to see who can run farthest up steep, smooth rock slabs before their rubbery hooves loose grip.

Despite being engineered for this environment, random events sometimes get the best of bighorn sheep. Heavy winters produce frequent avalanches, and as the snow recedes rocks fall away from the defrosting cliffs. Sheep that winter in the alpine can suffer heavy losses to their numbers in just one winter. For small populations (most herds in the Sierra have about 30 females, but some have less than 15) two heavy winters in a row could be enough to push them to extinction.

Bighorn in some herds avoid the dangers of alpine winters by migrating to lower elevations. Here bighorn find ample snow-free forage, and by March or April, nutritious new plant growth. However, this luxury often comes with a price. When bighorn winter at low elevation they are often near large deer herds which attract predators like the mountain lion. In one winter a single mountain lion was documented to kill 11 Sierra bighorn. Heavy predation is another way that small herds can slip towards extinction.

Sierra Nevada Bighorn Sheep: View the world from a bighorn sheep's perspective as they move around in a video segment the Counting Sheep documentary, created by Green TV in 2004. Commentary by University of California White Mountain Research Station biologist John Wehausen. (1:49 min) Entire film(opens in new tab)
Avalanche victims
Avalanches sometimes claim bighorn after heavy snows
Mountain lion at bighorn kill
Mountain lions pose another hazard
Ewes traveling in snow
Travel may be difficult after a snow storm