At their peak, North American bighorn sheep numbers were estimated at 2 million. Desert populations have since fallen to about 20,000 and Rocky Mountain populations are at about 45,000. Arizona’s bighorn population, consisting of both desert and Rocky Mountain races, is estimated at 6,000 animals. The causes for this decline, which occurred primarily between 1850 and 1900, were competition with livestock for food and water and exposure to livestock associated parasites and diseases.
Desert bighorns show considerable differentiation between the sexes. Adult males, rams, weigh between 160 and 200 pounds with a maximum weight of 225 pounds. Adult females, ewes, range from 75 to 130 pounds and average 110 pounds.
Rocky Mountain bighorn rams can weigh up to 340 pounds; the ewes are much smaller. A full grown male may stand over 3 feet tall at the shoulder.
For both desert and Rocky Mountain bighorn, the biggest visual difference between the sexes is the horns. Ewe horns are generally 10 to 13 inches long with a circumference of 5 to 6 inches. Ram horns may measure 30 to 40 inches along the outside curl with a basal circumference of 13 to 15 inches. The horn core is honeycombed with chambers, or sinuses, which reduce the weight of the skull.
Newborn bighorn lambs weigh 8 to 10 lbs. The young are active within minutes after birth. The young have dark eyes and fuzzy, dark-grey hair. As they mature, their eyes take on the characteristic golden or amber color. After several months, they take on adult coloration; dark brown in their northern range and pale buff in the southern. Color accents are a white muzzle, rump patch, eye rings, and edging on the rear legs, with a black tail. Bighorn sheep have a life expectancy of 10 to 12 years, but can reach 17 or older.
Usually one, rarely two lambs will be born. Young rams stay with their mothers until two years of age. They then leave the nursery herds and join in the bachelor herds. Except during the breeding season, and sometimes during the spring when early vegetation sprouts, bighorn adults separate according to sex.
Totally protected by the territorial legislature in 1893, bighorn sheep were not legal game in Arizona until 1953, when it was determined that the limited hunting of trophy desert bighorn rams might be the only way to save these animals. Two limited desert bighorn sheep hunts of 20 permits each were authorized, and 20 desert bighorn were taken. Since then, permit numbers, the number of units open to hunting, the number of rams taken, and hunt success have gradually increased. In 1984, Arizona began offering Rocky Mountain as well as desert bighorn sheep hunts. Between 80 and 100 hunt permits are authorized each year, mostly desert bighorns, with hunt success ranging between 90 and 95 percent.
Physiological and behavioral sexual maturity varies. While rams as young as six months may be capable of breeding, they don't, due to the dominance of older rams. In nature, ewes probably aren't bred until they are two and a half years old and rams don't breed until three and a half. The breeding season extends from early June through October. Peak rutting activity occurs in August. The gestation period is 179 days.
Bighorn sheep are diurnal animals. Females, with lambs and yearlings, usually travel further then rams because of there being more mouths to feed. Bighorn usually occur in small groups, but have been seen in herds of 50 or more.
Rocky Mountain bighorn sheep are very social animals, generally separated into two groups. Mature rams stay in one group while the ewes, lambs, and young rams congregate separately (rams stay with the "nursery group" until 2-3 years of age). The groups join during the rut (mid November through late December) and sometimes for a short period in the spring.
Ram bands have a social hierarchy established by body and horn size. Dramatic head-butting occurs between mature rams to determine leadership and dominance, but once the hierarchy is established, rams live in the same group with little further conflict (normal life span is 10 to 12 years.
In the wild, grasses are important to the bighorn. Bighorn also feed heavily on jojoba. Pincushion and saguaro cactus provide moisture. Preferred plant species vary with habitat quality, locality, and species availability.
Mountain lions, golden eagles, bobcats, and coyotes have all been implicated as predators.
Breeding Period: September-November
Young Appear: March-April or later
Average Number of Young: 1-2
Distribution: 90-4,500 ft, desert ranges of southern and western Arizona
Habitat: Desert mountain ledges and grassy basins
Food Preference: Fluff grass, catsclaw, ocotillo spurges, buckwheat, mescal, janusia, slim triodia, Indian wheat, filaree, and weeds in season
Range: Extremely variable
Live Weight: M-250lbs.; F-140lbs.
Predators: Eagles, coyote, bobcat and mountain lion