The Desert Bighorn Sheep (Ovis canadensis nelsoni) is a subspecies of Bighorn Sheep that occurs in the desert Southwest regions of the United States and in the northern regions of Mexico. The trinomial of this species commemorates the American naturalist Edward William Nelson.The characteristics and behavior of Desert Bighorn Sheep generally follow those of other Bighorn Sheep, except for adaptation to the lack of water in the desert: bighorn sheep can go for extended periods of time without drinking water.
Populations of the Desert Bighorn Sheep declined drastically with European colonization of the American Southwest beginning in the 1500s. As of 2004, Desert Bighorn Sheep numbers are extremely low, although the overall population trend has increased since 1960. These declines were followed by a period of population stabilization that was ascribed to conservation measures.
Both sexes develop horns soon after birth, with horn growth continuing more or less throughout life. Older rams have impressive sets of curling horns measuring over three feet long with more than one foot of circumference at the base. The ewes' horns are much smaller and lighter and do not tend to curl. The head and horns of an adult ram may weigh more than 30 pounds. Annual growth rings indicate the animal's age. Both rams and ewes use their horns as tools to break open cactus, which they consume, and for fighting.
Southern desert bighorn sheep are typically found in small scattered bands adapted to a desert mountain environment with little or no permanent water. Some of the bighorn may go without visiting water for weeks or months, sustaining their body moisture from food and from rainwater collected in temporary rock pools. They may have the ability to lose up to 30 percent of their body weight and still survive. After drinking water, they quickly recover from their dehydrated condition. Wildlife ecologists are just beginning to study the importance of this adaptive strategy, which has allowed these small bands to survive in areas too dry for many of their predators.
Bighorns live in separate ram and ewe bands most of the year. They gather during the breeding season (usually July-October), but breeding may occur anytime in the desert due to suitable climatic conditions. Gestation lasts about 6 months, and the lambs are usually born in late winter.
|Population estimate by year
In 1939, after intense lobbying by Frederick Russell Burnham and the Arizona Boy Scouts, President Franklin D. Roosevelt signed a proclamation that established two desert areas in southwestern Arizona to help preserve the Desert Bighorn Sheep: Cabeza Prieta National Wildlife Refuge and the Kofa National Wildlife Refuge In 1941, the San Andres National Wildlife Refuge in New Mexico was added.
Desert Bighorn Sheep population trends have been upward since the 1960s when their population was estimated at 6,700-8,100. The upward trend was caused by conservation measures, including habitat preservation. In 1980 Desert Bighorn Sheep populations were estimated at 8,415-9,040. A state-by-state survey was conducted a few years later and estimated the overall U.S. Desert Bighorn Sheep population at 15,980. The 1993 estimate of the population is 18,965-19,040. The results of the state-by-state survey are shown to the right.