Desert and Rocky Mountain Bighorn Sheep – Arizona

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.

On January 18, 1939, over 1,500,000 acres (6,100 km²) were set aside and a civilian conservation corps side camp was set up to develop high mountain waterholes for the sheep. The desert bighorn sheep is now the official mascot for the Arizona Boy Scouts.

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.

Desert bighorn sheep are social, forming herds of eight to 10 individuals; sometimes herds of 100 are observed.

Rams battle to determine the dominant animal, which then gains possession of the ewes. Facing each other, rams charge head-on from distances of 20 ft (6.1 m) or more, crashing their massive horns together with tremendous impact, until one or the other ceases. Once the hierarchy is established, rams live in the same group with little further conflict.
Bighorn sheep have a life expectancy of 10 to 12 years, but can reach 17 or older.