Desert bighorn sheep

Overhead, desert bighorn sheep cross U.S. Highway 93. The sheep aren’t headed to Las Vegas, like other travelers on the highway near Hoover Dam on the Arizona-Nevada border. The bighorn are using man-made wildlife bridges to access food and water and to meet up with sheep on the other side of the road.

Three wildlife bridges were built for the desert bighorn sheep in 2010, when 15 miles of U.S. 93 was widened along with the construction of the 2-mile Hoover Dam bypass. The overpasses, made of concrete and secured to ridgelines, are similar to most highway overpasses, except the bighorn bridges have dirt surfaces planted with native grasses. Funnel fences safely steer the sheep toward the three bridges, two of which are 50-feet-wide and the third is 100-feet-wide. Parapet fencing prevents the sheep from jumping off the bridges onto the road.

Desert bighorn sheep on overhead wildlife bridges

Planning for the Hoover Dam Bypass and the upgrade of U. S. 93 from a two-lane to a four-lane highway began in the 1990s, but was fast-tracked after September 11, 2001, when Hoover Dam was closed indefinitely to commercial trucks to protect it from terrorist attack. The additional widening of U.S. 93 was intended to improve safety for motorists and was targeted for completion at the same time the bypass was opened.

Before the upgrade, U.S. 93 was a slow-moving, two-lane road that crossed the dam and zigzagged up through Black Canyon. The bighorn sheep managed to cross the road, without too much incident, into other parts of their habitat. About 11 sheep were struck and killed by vehicles each year. The new four-lane divided highway was going to create a bigger obstacle.

Local and long-distance migration of bighorn sheep is critical to their survival and the future of naturally fragmented populations. The widening of U.S. 93 into a four-lane highway void of wildlife crossings would prevent the dispersal of sheep and might lead to a decline in the population, and cause an increased number of bighorn deaths on the road.

Desert bighorn sheep

Desert bighorn sheep congregate in small, fragmented populations. These subpopulations interconnect when young rams roam from one population to another and form a metapopulation—a population of populations.

Young rams stay with their mother’s herd until the juveniles are about 2 years old, when they usually join a band of rams. Some of these rams disperse short and long distances looking for mates, spreading the gene pool, and boosting the population of the new herd. The dominant ram in a herd breeds with the majority of the females and prevents other rams from mating.

Smaller, more isolated bands of bighorn sheep are replenished with animals, usually males, from more abundant populations when rams disperse. Man-made barriers, however, impede their movement. Roads, fences, canals, railroads, housing developments, and agriculture block natural migration corridors and inhibit the movement of bighorn from one subpopulation to another. When sheep can’t mingle with other subpopulations, the stability of the metapopulation is weakened and the sheep are more susceptible to decline in numbers.

Metapopulations are vital to the continued success of many species of animals, but for bighorn a metapopulation is essential, said Paul Krausman, a professor of wildlife conservation at the University of Montana. Keeping subpopulations of sheep connected to each other is key to establishing and maintaining a healthy, self-sustaining metapopulation of sheep. Bighorn are a fragile species that depend on connectivity with other populations to survive, Krausman said.

Getting Across

U.S. 93 cuts through Arizona’s Black Mountains, which is the preferred habitat for the largest surviving population of desert bighorn sheep in the world. The Black Mountain sheep travel back and forth along ridges that run perpendicular to the highway, shifting from Mount Wilson on the east side of the highway to Black Canyon on the west side, along the Colorado River. The ridges provide high-quality forage and good visibility for spotting predators. The bighorn use both areas to connect with other sheep and to reach lambing grounds and breeding areas on both sides of the highway. The Colorado River provides a prime source of forage and the sheep’s primary source of water.

The Black Mountain sheep are an important source for restoring desert bighorn sheep in other parts of Arizona and the Southwest. Hundreds of transplanted sheep have come from the Black Mountains.

Providing safe passage for these sheep became an integral part of the widening of U.S. 93. Initially, the Arizona Department of Transportation set up temporary concrete tunnels to see if the sheep would use them to get across. The sheep avoided the culverts. In addition, surveillance footage revealed that bighorn sheep weren’t regularly using the three underpasses built for them to cross State Route 68.

Research studies indicate that sheep climb to higher elevations when avoiding predators or other perceived threats. As a result of information gathered on sheep behavior, the Arizona Game and Fish Department, the Arizona Department of Transportation, and the Federal Highway Administration agreed that constructing overpasses rather than underpasses was the best way to provide the sheep safe passage. Overpasses are also less expensive.

A two-year study began in 2004. With federal funding Arizona Game and Fish captured 36 sheep and fitted them with radio collars to see where the sheep crossed U.S. 93. Based on the results, the bridges were later constructed at mileposts 3.3, 5.2, and 12.2 at a cost of $4.8 million provided by both federal and state highway dollars. (The radio collars were designed to fall off after two years.)

Desert bighorn sheep crossing an overhead wildlife bridge

The overpasses and funnel-fencing designed for bighorn sheep on U.S. 93 were completed in early 2011. To date, video cameras on the overpasses have recorded more than 5,000 bighorn sheep crossings. In contrast, a study of the three underpasses on State Route 68 documented just 32 crossings in a two-year period.

Nature’s Way

A large number of sheep don’t need to migrate from one subpopulation to another to increase the genetic components of a population, Krausman said. “Even one animal migrating each generation will improve the genetic pool.”

If genetic diversity diminishes, inbreeding occurs, and sheep become more vulnerable to outside threats. Disease or drought, for example, can wipe out an isolated herd with fewer than 50 animals. A localized catastrophe such as drought “could go on so long that the animals are unable to replace themselves and they simply die out,” said John Hervert, the wildlife program manager for Arizona Game and Fish in Yuma.

Before man-made barriers fractured bighorn habitat, one population would naturally rebuild or replace another population that had declined, Krausman said. When those areas are cut off from each other, however, the natural process to refresh a population is eliminated.

Desert bighorn sheep

For reasons yet to be determined, hoofed animals, such as bighorn sheep, can withstand inbreeding better than a lot of other species, said Jim Heffelfinger, the regional game specialist in Tucson for Arizona Game and Fish. Deer, elk, pronghorn, and bighorn herds with only a few animals can grow into large, healthy populations without the downsides of inbreeding, such as an increased vulnerability to disease and a higher percentage of juvenile deaths, Heffelfinger said. Problems related to inbreeding may appear much later, but so far, few issues have occurred.

Helping Hands

Before humans created barriers, bighorn sheep simply walked across valleys when they were looking for mates, food, water, or whatever else drew the sheep to an adjacent mountain range. Highways not only create barriers that fragment wildlife habitat and block connectivity between populations, but they also destroy and degrade home ranges. At least 20 percent of wildlife habitat in the U.S. has been lost to or altered by highway construction, according to studies conducted in 1998 and 2000 on the effects of highways on wildlife habitat. Collisions with vehicles on roads also reduce wildlife populations. Between 500,000 and 700,000 deer are killed each year on U.S. highways, according to other studies.

Since the overpasses and fencing were completed on U.S. 93 in 2011, the number of sheep and vehicle collisions has declined by 81 percent. Only three sheep-vehicle collisions were recorded from 2012 thru 2014, according to Justin White, the roadside resources manager for the Arizona Department of Transportation’s Environmental Planning Group. The number of ewes, lambs, and rams that use the overpasses has continued to grow since completion of the project. “Sometimes it takes a little while for the animals to learn that it is safe to cross,” White said.

Another way to help bighorn herds thrive and overcome the effects of man-made barriers is to provide the sheep with another form of transportation. The Arizona Game and Fish Department has been moving sheep from one area to another since the 1950s. Ninety percent of the translocation projects have occurred since 1979.

“Translocation is the most obvious tool for maintaining metapopulation dynamics,” Heffelfinger said. Current projects frequently move 20 or 30 sheep from a large, healthy population and transplant them into a declining herd. Arizona Game and Fish has done more than 100 translocations of bighorn sheep, relocating about 2,000 individual sheep.

Desert bighorn sheep

In the future, genetic refreshment is another management tool that may be implemented more often than translocation to restore small, isolated populations of sheep. Wildlife managers relocate two or three sheep, as if individuals were migrating from one population to another. This technique “more closely mimics the natural movement indicative of a metapopulation,” Heffelfinger said.

Arizona Game and Fish works with land management agencies and local municipalities, such as Pima County, to preserve the natural corridors that provide connectivity between populations of desert bighorn sheep. The Pima County Wildlife Connectivity Assessment developed a series of maps showing the most probable wildlife corridors across highways and in undeveloped parts of Pima County, Heffelfinger said, in order “to avoid future development and preserve those sensitive areas for wildlife.”

Risky Business

Desert bighorn sheep

When bighorn sheep migrate through traditional corridors, they can come in contact with domestic goats and sheep that harbor diseases. These diseases don’t manifest in the domestic animals, but they can kill off an entire population of desert bighorn sheep. During dispersal the desert bighorn sheep intermingle with the domestic animals and can catch a disease, much like people contract viruses from nose-to-nose contact. The infected animals then continue to migrate and carry the virus to the population of sheep they join.

Wildlife management agencies work to maintain distances between domestic sheep and goats and the wild sheep, but keeping the animals separate is a challenge when bighorn rams are making long-distance journeys. Ranchers are encouraged to keep their sheep away from migration corridors used by sheep. In some parts of Arizona where domestic sheep are herded across the landscape, ranchers are encouraged to transport their sheep in trucks to avoid contact with desert bighorn.

Solar panels present another obstacle for the safe passage of sheep, said wildlife biologist Paul Kraussman. Green energy is a valuable resource, but miles-long walls of solar panels can prevent sheep from moving from one mountain range to another.


Bighorn sheep form small herds of about 100 animals. They are habitat specialists, seeking terrain where precipitous rock formations tower over canyons and washes. These sparsely vegetated areas can’t support large populations of sheep.

Desert bighorn sheep

Nevertheless, at one time desert bighorn sheep flourished in the southwestern United States and northern Mexico, where herds formed throughout a vast habitat free of domestic livestock. European settlers described seeing many bighorn in the early 1800s.

By the early 1900s the desert bighorn had disappeared from 95 percent of its historical range. Unregulated hunting, disease, and competition with domestic livestock were the primary causes for the decline. Today about 20,000 bighorn eke out an existence in fragmented environments. As human development grows and roads expand, the opportunities, for migration between populations, also shrink.

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The sheep, especially the males, “have a propensity to move tremendous distances” and can overcome intimidating barriers such as barren desert landscape, canals, or residential development to reach a new population, said Heffelfinger, from Arizona Game and Fish. After radio collars were used to track their movement, it became evident that sheep are “real pioneers” and travel long distances of more than 100 miles between populations. “We saw movement between populations that you wouldn’t have expected to be connected,” Heffelfinger said.

Desert bighorn sheep

For example, in the late 1980s, when wild sheep still occupied the Santa Catalina Mountains near Tucson, a collared ram released in the Galiuro Mountains across the San Pedro River showed up with the other sheep on Pusch Ridge. The ram had traveled about 80 miles through dense forest, riparian habitat, and desert—areas incompatible with sheep. Another ram from the Superstition Mountains near Mesa, Arizona, made a 100-mile trek to the Catalinas. Despite evidence of rams making long treks between herds the decline of available corridors has become a greater focus for management of the species.

In the past, the management of bighorn sheep focused on preserving the habitat in mountain ranges where bighorn lived without considering the significance of the valleys in between, Krausman said. Today, wildlife managers recognize the importance of the natural migration corridors connecting herds.


A metapopulation of bighorn sheep between Yuma and Tucson, Arizona, has recently been on the move, connecting naturally with other subpopulations. The sheep are crossing Interstate 8 (an east-west thoroughfare), heading north and sometimes back south again, either by running between cars or using underpasses built to drain rainwater. Bighorn are also crossing State Route 85, a north-south highway west of Tucson.

Route 85 is still a two-lane road, but like U.S. 93, it may be upgraded to four lanes, which would potentially cut off the migration of sheep between subpopulations east and west of the highway, said Hervert, from the Yuma office of the Arizona Game and Fish Department.

The existing metapopulations of sheep from Tucson to Yuma roam through nearly two million acres within the reach of several mountain ranges. “These sheep still move back and forth between mountain ranges relatively uninhibited,” Hervert said.

Not all populations of bighorn are doing as well as those in western Arizona. Certain herds of bighorn sheep are thriving, while others have declined. “It’s a mixed bag,” Hervert said.

Desert bighorn sheep

For the most part, however, managers in the Southwest are optimistic about the bighorns’ future. Ongoing active management primarily through translocations helps to sustain large metapopulations of sheep as well as small, isolated herds without a natural connection to other subpopulations.

Surprisingly, bighorn sheep are expanding their range along the most southern part of the Colorado River, in the Laguna Mountains near Yuma. The sheep have never occupied this area, Hervert said. The animals found the new habitat themselves, and a recent survey indicates that 30 to 35 sheep make up the new herd.

More to Come

Desert bighorn sheep

In 2014 the Arizona Department of Transportation began widening seven miles of Oracle Road (State Route 77) near Catalina, Arizona. Among various improvements the project includes adding one lane of traffic in both directions, better pathways for pedestrians, raised medians, retaining walls, noise-softening walls, and two wildlife crossings—an underpass for smaller animals and an overpass for larger animals such as bighorn sheep. The latter is the first wildlife overpass in southern Arizona.

More and more, wildlife crossings are becoming part of highway upgrades. Safety for motorists is often the primary incentive for keeping animals off the roads, White said. The animal crossings prevent collisions between vehicles and animals.

The animal crossings over and under Oracle Road were included in the upgrade of the road for a different reason, White said. Although some accidents with wildlife occurred along Oracle, the wildlife passages were included primarily to connect landscapes for wildlife, White said.

About four and a half miles of fencing will line Oracle Road to funnel wildlife toward the 150-foot-wide overpass and 50-foot-wide underpass, where wildlife have crossed Oracle in the past. Ten built-up breaks in the fencing, or jump-out structures, for larger animals, along with ramps for smaller animals, will also be installed in case deer, mountain lion, bighorn, javelina, or even a desert tortoise breeches the fence and gets trapped on the road.

The Pima Association of Governments Regional Transportation Authority is footing the bill of $6.8 million for the crossings as well as the other wildlife-related elements on Oracle Road. The funds come from taxpayers’ money designated for wildlife crossings obtained from a 2006 voter-approved one-half cent transaction privilege excise tax implemented in Pima County. The entire road project, which will cost $33.9 million, is scheduled to be completed in spring 2016.

A Large-Scale Project

Future road projects in Arizona might further block connectivity between bighorn sheep populations. A new federal highway, Interstate 11, is in the early stages of development. If approved, I-11 will extend from Nogales, Arizona, to Las Vegas, Nevada, using some existing roadways as part of the link running from north to south through Arizona.

If I-11 jeopardizes sheep corridors, the environmental impact statement for the new roadway will address maintaining connectivity between metapopulations of bighorn, White said. The environmental impact statement will cost $15 million.

The Arizona Department of Transportation doesn’t make road improvements specifically for wildlife, White said, but when a road project is approved, an opportunity to consider the needs of wildlife often becomes part of the plan. The primary goal of the Arizona Department of Transportation is always safety first.