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.
Rocky Mountain Bighorn Viewing Areas
Look for Rocky Mountain bighorn at the following locations: