Desert Bighorn Sheep Ovis canadensis nelsoni
The Desert Bighorn Sheep
Desert bighorn sheep (Ovis canadensis nelsoni), a subspecies of bighorn sheep, are native to the deserts of the United States southwestern and intermountain regions, with their natural range extending down in northwestern Mexico. The trinomial (third part of the scientific name) honors American naturalist Edward William Nelson (1855–1934), who studied the sheep as part of his work for the U.S. Dept. of Agriculture from 1891-1929, including early explorations of Death Valley and remote areas in northern Mexico. The characteristics and behavior of the desert bighorn sheep generally follow those of other bighorn sheep, except for adaptation to the lack of water in the desert.
Desert bighorn sheep are stocky animals, similar in size to mule deer. Weights of mature rams range from 115-300 lbs., though most will not exceed 250 lbs., while ewes are smaller. They have unique concave, elastic hooves that enable them to climb the steep, rocky terrain with great speed and agility. They rely on their keen eyesight to detect their most common predator, the mountain lion, as well as coyotes and bobcats, and use their climbing ability to escape. Both genders develop horns, with horn growth continuing throughout life. Older rams have beautiful curling horns that can measure over three feet long, with bases that exceed a foot in circumference. Ewes’ horns are much smaller and lighter. It is not uncommon for older rams to rub, or broom, their horns, which improves their field of view. A ram’s horns are used both for fighting and dominance, and both rams and ewes use their horns as tools to break open cactus, which they consume.
Desert bighorn sheep typically live for 10–20 years, though females rarely make it past 15 years and rams rarely live more than 12 years. Juvenile mortality is variable and can be quite high, ranging from 5 percent to 30 percent. Sheep between 2-6 years old have low mortality. They live in separate ram and ewe bands most of the year, coming together during the breeding season, which occurs July-October. Their diet typically consists of a variety of grasses, as well as sedges, forbs, and cactus. Like other bighorns, rams charge head-on and crash their horns together to determine herd dominance and breeding rights.
The range of the desert bighorn includes remote desert areas in the Mojave, Sonoran, Great Basin, and Chihuahuan deserts, as well as the Colorado Plateau. Because of their fragility, several protected areas have been created for them, including the Anza-Borrego State Park, Joshua Tree National Monument, Death Valley National Park, Kofa National Wildlife Refuge, Cabreza Prieta National Wildlife Refuge, San Andreas National Wildlife Refuge, Zion National Park, and Mojave National Reserve. Prehistoric populations are unknown, but believed to be strong up until the American Southwest became colonized by European settlers beginning in the 16th century, and by the 1960s populations had dwindled down to an estimated 6700-8100 sheep. Declines were attributed to overhunting, loss of watering areas and critical habitat to human encroachment, and diseases contracted from domestic livestock, particularly domestic sheep. Today populations are stable to slightly increasing, as the included table illustrates.
The desert bighorn has become well adapted to living in the harsh, ever-changing climate found in the deserts of the southwest, where temperature fluctuations can be large and water scarce. Unlike most mammals, their body temperature can safely fluctuate several degrees. During the heat of the day, they often rest in the shade of trees and caves. In areas or during times when groundwater is scarce or unavailable, body moisture can be sustained from food and what little rainwater might be collected in temporary shallow rock pools. In lean times they have the ability to lose up to 30 percent of their body weight and still survive. In fact, they can survive in areas where it is too dry for many of their predators to survive.
As is the case with the Rocky Mountain Bighorn, all state governments jealously protect their bighorn populations by severely restricting the number of available hunting permits to a handful each year, with the vast majority of tags issued either through the state’s tag draw system, with a few tags sold by various conservation organizations at auction each year for big money, with the funds put back into sheep conservation efforts. In most states, drawing a Desert Bighorn tag is a “once in a lifetime” tag. Hunters might accumulate bonus or preference points for several decades before even having a chance to draw the most coveted of all North American big game hunting tags. The other option is to purchase a guided hunt in old Mexico costing tens of thousands of dollars. Desert Bighorns are also placed in high-fence ranch areas in Mexico, where “hunts” are sold for big money. Rams taken under these conditions are not able to be entered into the record books of either the Boone & Crockett or Pope & Young clubs, however, who deem such hunts as not following the tenets of fair chase.
State Population Estimates
- Bighorn sheep groups protect themselves from predators by facing different directions, allowing them to keep watch on their surroundings.
- Desert bighorn sheep can go for weeks at a time without drinking water. However, during summer, bighorn will often choose to drink daily when possible, although they can go without drinking water for 3 days in 100-degree temperatures. When water is available, they may consume nearly 20 percent of their body weight in a comparatively short period of time.
- After eight years of growth, the horns of an adult ram may weigh more than 30 pounds.