Sierra Nevada Bighorn Sheep Description and Biology

There never has been a person who wrote more aptly and lovingly of Sierra Nevada bighorn sheep than John Muir. He devoted an entire chapter in his book The Mountains of California to “The Wild Sheep.” As he wandered the High Sierra in the 1870s, he composed accurate and beautifully written accounts of his many sightings of Sierra Nevada bighorns in their range that extended along the crest of the Sierra in a vast and largely undisturbed wilderness habitat.

The Sierra Nevada bighorn sheep living within Yosemite National Park’s boundaries today fit John Muir’s description of “wearing a thick overcoat of hair like that of the deer,” brownish-gray to white in color, with a white rump patch and a dark tail, and an overall appearance that looks “as if carefully tended with comb and brush.” Rams weigh up to 220 pounds and stand three feet tall at the shoulders. Their immense curved horns can weigh over 15 pounds, making necessary the stoutness of the ram’s neck for support. Ewes weigh up to 140 pounds and have gently curved spiked horns much smaller than the rams’.

 
Three bighorn sheep on a steep cliff
The bighorn sheep is a wilderness icon in Yosemite.

Steve Yeager

John Muir made the keen observation that the Sierra Nevada bighorn is a creature that is adapted to high alpine life like no other. The bighorn’s sure footedness in rocky elevations above 10,000 feet and atop solitary and almost inaccessible crags is made possible by extremely strong muscles and a unique foot adaptation. At the bottom of the foot is a rubber-like pad or cushion that grips both irregular and smooth surfaces. Muir was profoundly moved by the power and beauty of the bighorns’ movements and recorded that the wild sheep “did not make a single awkward step, or an unsuccessful effort of any kind.”

As the quintessential California naturalist, John Muir’s study of the Sierra Nevada bighorn sheep was purposeful and insightful. “While engaged in the work of exploring high regions where they delight to roam I have been greatly interested in studying their habits.” Muir correctly noted that rams and ewes flock together only during the mating season of November/December and are mostly apart the remainder of the year. During the rut (breeding season), bighorn rams compete for their right to mate with ewes. Dominance behavior includes kicking, biting, neck wrestling, and dramatic horn clashes as competitors face each other, rear up and pitch forward at rapid speeds, creating thunderous sounds.

Starting at two years old, ewes give birth to one lamb between late-April and mid-June. As an avid mountain climber, Muir had come upon “the beds of the ewes and lambs at an elevation of from 12,000 to 13,000 feet… such is the cradle of the little mountaineer, aloft in the very sky, rocked in storms, sleeping in thin, icy air… nourished by a strong, warm mother, defended from the talons of the eagle.” Within one day, the lambs are almost as strong and agile as their mothers in navigating their rocky birthplace. A lamb is independent by age one and a female lamb remains with her mother’s herd for the duration of her life. Rams live to be ten to twelve years old, and ewes live to be twelve to seventeen years old.

The foraging pattern of Sierra Nevada bighorn sheep is strongly influenced by their inherent fear of predators, their ability to inhabit inhospitable environments, and their unique adaptations, which all work together to provide sustenance and safety amidst the treeless slopes of the High Sierra. John Muir observed that both their resting places and their feeding grounds “were chosen with reference to sunshine and a wide outlook, and most of all to safety.” The complex four-part stomach of bighorn sheep allows them to feed on sparse grasses before retreating to a safe ledge where they chew the food twice to gain maximum nutrients and moisture. All the while, their wide-set eyes, situated well forward on their heads, are accurately scanning for predators.

Each summer, Sierra Nevada bighorn sheep travel among the highest elevation peaks, and drop to lower elevations during the winter, where the weather is less severe and food is more accessible. Despite John Muir’s observations that “highlanders have fewer enemies than lowlanders,” mountain lions pose the biggest predation threat to Sierra Nevada bighorn sheep and, at times, have caused significant reductions to their fragile numbers. Bighorns may become wary of mountain lions and remain high in the Sierra even during fierce winters in order to stay as safe as possible. During high snow years, when it is difficult for the bighorn to obtain food, they may suffer losses indirectly caused by mountain lion predation.

Golden eagles, who are among the fastest animals and who have huge hunting territories, pose a threat to Sierra Nevada bighorn lambs. Exhibiting astonishing speed (up to 200 miles per hour in dives) and maneuverability for their size, golden eagles can attack young lambs if they find them unprotected. Their hunting territories include mountains up to 12,000 feet in elevation, placing the raptors well within the preferred birth places of Sierra Nevada bighorn lambs. When the birth of every baby of this endangered species marks an important milestone toward recovery, the loss of even one lamb among the fragile herds of Sierra Nevada bighorn sheep represents a set-back.

At the conclusion of John Muir’s chapter on “The Wild Sheep” his last paragraph is particularly poignant to modern day readers of The Mountains of California. “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.” Despite his assurances to himself and his readers, even as the book was published in 1894, John Muir’s wild sheep had begun to vanish from the High Sierra due to unregulated hunting and disease introduced by domestic sheep. The Sierra Nevada bighorn would completely disappear from Yosemite National Park only twenty years later.

 
There could not have been a better deed to honor John Muir's legacy than the March, 2015, restoration of Sierra Nevada bighorn sheep to Yosemite National Park's Cathedral Range. The great naturalist made the first ascent of Cathedral Peak in 1869 during the time he was compiling his notes on the wild sheep in their untrammeled wilderness. John Muir's exultation and "appreciation of the self-reliance, strength, and noble individuality of Nature's sheep" was shared by the individuals who had worked so carefully and tenaciously to return the wild sheep to the Yosemite National Park wilderness.

 
A ranger watches a bighorn sheep run out of the cage used to transport the animal to a new location.
Biologists released bighorn sheep into the Cathedral Range in spring 2015.

Steven Bumgardner

 

Thanks to volunteer Bonnie Cassel for writing these bighorn sheep pages.

Last updated: August 3, 2017

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