Standing one metre tall and weighing more than most adult humans, the Sierra Nevada bighorn sheep is one of the most formidable-looking animals in North America.
Naturalist John Muir was so taken by the Sierra Nevada bighorn, that in his 1894 book The Mountains of California, he dedicated an entire chapter to it – The Wild Sheep – describing the creatures as the "bravest of all Sierra Mountaineers” and marvelling at their “perfect strength and beauty”. In his concluding words, Muir wrote: "Man is the most dangerous enemy of all, but even from him our brave mountain-dweller has little to fear in the remote solitudes of the High Sierra.”
These remarkable animals once covered an extensive area in California, stretching from Sonora Pass in the north to Olancha Peak in the south. Yet today, they are battling for survival, after centuries of hunting, disease and overgrazing by settlers.
Efforts to recover the species saw numbers rebound from just 100 in 1995 to 600 by 2015. However, according to new research by the American Geophysical Union, the increasing threat from avalanches (a result of climate change), as well as a rise in predation by mountain lions, has reversed much of that progress, almost halving the population to just 350 individuals.
Avalanches and bighorns
Sierra Nevada bighorn sheep are a distinct subgroup, with the remaining 350 individuals representing the entirety of the population. These sheep inhabit rugged, high-altitude terrain, feeding on grasses, herbs and shrubs.
While snow is a vital resource for the sheep, providing water and shaping their alpine ecosystem, it also poses a mortal danger. The record-breaking winter of 2022–2023 brought nearly 18 metres (60 feet) of snow to the Sierra Nevada. Avalanches that winter killed at least 73 bighorn sheep, with 12 perishing in a single incident, according to the California Department of Fish and Wildlife.
To understand how avalanches might affect bighorn sheep in a warming climate, Ned Bair, a researcher at the University of California, Santa Barbara, and his team ran simulations covering both current (1990–2020) and future (2050–2080) conditions.
They found that snow cover at lower elevations is likely to diminish as more precipitation falls as rain. However, higher elevations, where bighorn sheep reside, may see snow accumulations increase. Intensifying atmospheric rivers – weather systems responsible for the historic 2023 snowfall – could bring deeper snowpacks and more frequent avalanches.
“In dry winters, they don’t have enough forage. And in the big winters, they die of starvation and avalanches,” says Bair. “They choose extremely challenging environments to live in, which unfortunately increases their risk from avalanches."
Uncertain future
The California Department of Fish and Wildlife launched the Bighorn Sheep Recovery Program following the catastrophic winter of 2023. Yet it remains unclear how the project will address the growing threat of avalanches, says Bair, adding that recovering populations of predators such as wolves and mountain lions may add further pressure to the fragile bighorn sheep population.
“It’s a hard population to sustain,” Bair admits. “I worry about whether my children will have the chance to see a bighorn sheep in the wild when they’re my age.”
Bair's sentiments are shared by many of the people involved in the recovery of bighorn sheep in the High Sierra, and the future of this charismatic mountain dweller remains uncertain, making John Muir's words from the late 19th century even more poignant:
"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
Main image: biologists released bighorn sheep into Yosemite's Cathedral Range in spring 2015/Getty
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