Up Against Steep Odds

In a cluster of small mountain ranges in the California desert, bighorn sheep face plagues, drought, loss of land and even lions

02-01-1997 // Bob Holmes

Already the temperature is rising fast, at barely eight o'clock on a summer morning in Anza-Borrego Desert State Park. By afternoon, this part of southeastern California will climb well past 100 degrees. A small band of desert bighorn ewes and lambs lingers along the lower slope of a sunbaked canyon. One ewe approaches a barrel cactus and butts it with her horns, breaking it open, then eats the watery pulp within. Other ewes cluster around her, waiting their turn for the vital water source. Ten yards away, in the shade of a rock outcrop, the lambs lie quietly, watched over by a ewe.

This is the sort of scene being chronicled by biologists tracking small groups of desert bighorn scattered throughout the Peninsular mountain ranges, a broken rank of peaks that stretches from Palm Springs south through the Baja Peninsula. Currently recognized as comprising one of seven bighorn subspecies, these animals are commonly called the Peninsular bighorn sheep. In keeping with the almost biblical harshness of their habitat, they face hardships that could be straight from a Cecil B. DeMille epic: plagues, lions, drought and even the loss of the Promised Land.

These apocalyptic "Four Horsemen" have already reduced the number of Peninsula bighorn in California to 400, barely a third as many as two decades ago. As a result, conservationists have petitioned to list this population as threatened under the federal Endangered Species Act. As of this writing, the U.S. Fish and Wildlife Service has yet to rule on the petition.

Alarming situation: "Even in good years, the sheep usually lose 70 to 80 percent of their lambs in the first five or six months," says Mark Jorgensen, a resource ecologist at Anza-Borrego Desert State Park. "If you compound that with drought, or disturbance of lambing areas or diseases, then you have a pretty alarming situation."

Bighorn sheep may evoke images of snowy peaks and alpine meadows, not desert cliffs. But in fact, both settings provide the animals with their habitat needs: open, treeless expanses where predators are easy to spot, as well as steep rocky heights to which the bighorn can flee for safety. The sheep nimbly bound up and down incredibly sheer cliffs, often dropping 10 feet between contact points and keeping their footing using a unique double-shelled hoof that spreads over sharp rocks and a soft, cushiony pad in the middle for traction on slick surfaces. They can also follow 2-inch-wide trails across steep cliffs.

Desert bighorn live in the mountain ranges of the Southwest--including the arid regions below the lower treeline where the climate is too parched for forests to survive. The animals feed on desert grasses and shrubs, plus whatever other vegetation they can find--even the fiercely spiny cholla cactus. "I've seen them gingerly nip the new growth off the end, but with barrel cactus, they will butt the cactus and smash the spines," says Esther Rubin, a graduate student at the University of California, Davis, who has studied the Peninsular bighorn for four years. "Once they get it open, they eat it out like a big salad bowl."

Stout horns: The barrels' water content can reach as high as 93 percent, and desert bighorn depend so much on the widely distributed cacti that older animals often have horns that are badly worn or even broken from barrel-bashing. That may be one reason desert bighorn have stouter horns than their high-elevation cousins.

All bighorn use their horns for the same main purpose, however: elaborately stylized fights to establish pecking orders within herds. Usually, the largest horned ram is dominant, with other males constantly challenging each other. When two rams charge one another, they can reach a combined speed of 45 mph, and the crack of their horns meeting can be heard more than a mile away. Evolution has given bighorn double-layered skulls, cross-connected with bone and thick facial hide, enabling the animal to withstand tremendous forces.

The difference in horns is one of several features that, to a trained eye, distinguish desert bighorn from Rocky Mountain bighorn. Desert bighorn also tend to be smaller, with rams weighing just 150 to 200 pounds--100 to 150 pounds less than Rocky Mountain bighorn rams. At present, scientists assign Peninsular bighorn to their own subspecies, Ovis canadensis cremnobates, distinct from all other desert bighorn.

Hot debate: That designation has been a matter of hot debate among scientists. Measurements of skull size and shape reported more than half a century ago suggested the animals are quite distinct. Other, more recent analysis, however, has not found the Peninsular bighorn unique. "But that doesn't mean that they aren't locally adapted and worthy of preservation," says California's Department of Fish and Game wildlife veterinarian David Jessup. "I think there are more threats to Peninsular bighorn than other desert bighorn populations, and thus I think they're worthy of more protection." And that is the argument being used in the pending petition for listing the animals, not the fact that they may or not be a subspecies.

Because Peninsular bighorn and other desert sheep inhabit the lower fringes of isolated mountain ranges, their populations are small and scattered, and they often come into direct contact with human activity--two key warning signs of extinction risk. Within the last century, desert bighorn sheep have vanished from at least 14 mountain ranges in California. Both Peninsular bighorn and the California desert bighorn already are listed as threatened by the State of California.

Medical mystery: For the Peninsular bighorn, trouble developed quickly. As recently as the late 1970s, biologists estimated that the Peninsular ranges contained thriving populations totaling about 1,200 bighorn. But by 1977, something was clearly going wrong. "We saw lamb after lamb dying," says Jim DeForge, director of the Bighorn Institute, a private research institute in Palm Desert, California. DeForge often saw lambs with coughs and runny noses, and several sick or dead lambs he examined showed clear signs of pneumonia. By the end of the summer, more than 90 percent of that year's lambs had died. The poor lamb survival, less than half what was needed to replace old animals that died, continued to the end of the 1980s.

All the evidence pointed to lung infections as the culprit. For unknown reasons, the lungs are the weak link in a sheep's constitution, just as the heart and blood vessels are for humans. And pneumonia was known to be a major threat to other bighorn populations, especially where they mingle with domestic sheep and catch diseases to which they are unaccustomed. Bighorn sheep seem especially sensitive to diseases carried by their domestic relatives, although other wildlife species can also pick up hand-me-down diseases. Bison, for example, probably acquired brucellosis from cattle a century ago.

The Peninsular ranges had no sheep ranching for many years, but they have had cattle. And DeForge's blood samples from Peninsular bighorn held antibodies against several cattle-borne respiratory viruses. "Sure enough," says DeForge, "virus was a silent killer, stripping the lining of the trachea and predisposing the animals to bacterial pneumonia."

Surprise improvement: In 1992, biologists launched a research program to study the Peninsular bighorn. Wildlife veterinarian Walter Boyce at the University of California, Davis, and his colleagues fitted more than 100 of the animals with radio collars. To the scientists' surprise, they found the epidemic was apparently over. Lambs were healthier and much more likely to survive. "You can tell by their behavior that they're feeling perkier," says graduate student Rubin. The only exception to the clean bill of health is a small band of bighorn in the Santa Rosa Mountains, where 90 percent of lambs still die before their first birthday.

But if diseases weren't killing the bighorn any more, something else was. By tracking down the carcasses of 45 radio-collared bighorn over the past three years, researchers learned that 70 percent were killed by mountain lions. "That really surprised us," says Rubin. "It had never been documented that they eat sheep at this rate." Lions kill not just lambs and old animals, but also ewes in the prime of their reproductive lives. No one knows whether this level of predation is new in the history of lions and lambs. The radio collars may have helped biologists discover an existing phenomenon, although years of field observations never found lion kills to be common.

On the other hand, lion populations appear to be on the rise, and people living in southeastern California have reported that lions have been venturing down into the desert more often than in the past. "By every indirect measure we have, it appears that the numbers of mountain lions are increasing, they're spreading into habitats with more bighorn sheep and they're using more bighorn sheep," says Jessup. But whether this represents an unprecedented glut of lions or merely a return to normal after decades of rarity is anyone's guess.

Lion food: Normal or not, mountain lions now eat enough sheep that most Peninsular bighorn populations have not bounced back from the disease outbreak of the 1980s. That means the bighorn are more vulnerable to drought or disease. "When a combination of factors team up together, that's when a population is likely to get into real trouble," says wildlife veterinarian Boyce.

Human intrusion, too, plays a role. Much of the Peninsular herd's habitat lies within the Anza-Borrego park. But houses, highways and golf courses are very much a problem at the north end of the Peninsular ranges. "People look up and see these desert mountains and say, ÔOh, that's the sheep habitat,'"says Boyce. "But the truth is that the sheep used to come down a lot further than they can and do now. As we continue to take over these areas, that's going to make it more and more difficult for the sheep to make it in the long run." Even in the Anza-Borrego park, off-road vehicles and hikers can cause pregnant ewes to shy away from some canyons. Adds Rubin, "If that's their only good lambing area, it could have quite an effect."

Not everyone agrees that the low numbers of the Peninsular bighorn are cause for special concern, however. "It's only low compared to what was believed to be the population in the late 1970s," says John Wehausen, a bighorn biologist at the University of California's White Mountain Research Station in Bishop. He also notes that other desert bighorn populations in California are doing fine at similarly low numbers. "We may just be seeing part of a long-term cycle. At what point we should be really concerned, I don't know."

Easier lives: Despite such uncertainties, biologists are doing what they can to make life easier for the bighorn. During the 1980s, Anza-Borrego park ecologist Jorgensen used helicopters and net guns to remove 117 feral cattle--and their potential diseases--from the park. He and his colleagues also are working to remove introduced salt cedars, which have thirsty roots that dry up precious water holes. "Personally, I don't care what we call the Peninsular bighorn," says Jorgensen. "I think for their own sake they have a right to survive like they have for thousands of years. It would be a sad day to go to Anza-Borrego Desert State Park and say that bighorn used to be here, back in the 1990s, but they're not here any more."

California journalist and former biologist Bob Holmes last wrote about prairie dog behavior, in the June-July 1996 issue.

Victory For Rocky Mountain Bighorn

Along the Snake River between Oregon and Idaho, wildlife managers have been working for years to reestablish Rocky Mountain bighorn in the upper part of Hell's Canyon National Recreation Area. At least 200 bighorn have been transplanted into the area since 1971. But repeated outbreaks of bacterial pneumonia have taken a toll, and numbers dwindled from more than 160 animals in the early 1980s to barely two dozen in 1996.

Domestic sheep, which also graze in the canyon and carry a bacterium that causes pneumonia in bighorn, are the most likely source of the disease. Microbiologist Bill Foreyt of Washington State University reported in 1989 that when bighorn mingled with domestic sheep in experiments, the bighorn died within days of nose-to-nose contact with the domestics. "We know now that you do not try to maintain wild sheep where there are domestic sheep allotments," says Patrick Matthews, a biologist with Oregon's Department of Fish and Wildlife.

For several years, biologists tried to keep the two species apart, even destroying bighorn that wandered into domestic herds. In 1994, however, the U.S. Forest Service, which administers the area, announced the domestic sheep would have to go. The sheep industry appealed the decision, and the Forest Service backed down. But after pressure from groups including NWF and the Idaho Wildlife Federation, an NWF affiliate, the Forest Service decided ranchers must move most domestic sheep out of the area. And last April, the Federal District Court in Portland upheld the decision, ordering almost all domestic sheep herds to leave the area by October 1996.

That's a big switch for managers who once had to resort to destroying bighorn that wandered into domestic sheep herds in order to control disease. Says Pete Frost, attorney with NWF's Western Natural Resource Center, in Portland, "The court said that if there's a conflict between bighorn and domestic sheep, bighorn have priority."

Join today and get a 1 year subscription to National Wildlife magazine
     Flickr Icon           Find NWF on Facebook.           Follow NWF on Twitter.           YouTube Icon    
Connecting...
Certify your yard today!