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.