Definitions

Desert Bighorn Sheep

Desert Bighorn Sheep

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.

Characteristics

Desert bighorn are stocky, heavy-bodied sheep, similar in size to mule deer. Weights of mature rams range from 125 to 200 pounds (55 to 90 kg), while ewes are somewhat smaller. Due to their unique padded hooves, bighorn are able to climb the steep, rocky terrain of the desert mountains with speed and agility. Bighorn rely on their keen eyesight to detect potential predators such as mountain lions, coyotes, and bobcats, and they use their climbing ability to escape.

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.

The typical diet of a desert bighorn sheep is mainly grasses, sedges and forbs.

Desert adaptations

The desert bighorn has become well adapted to living in the desert heat and cold and, unlike most mammals, their body temperature can safely fluctuate several degrees. During the heat of the day, bighorn often rest in the shade of trees and caves.

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.

Social life

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 feet or more, crashing their massive horns together with tremendous impact, until one or the other ceases.

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.

Conservation status and trends

  Population estimate by year

State 1960 1993
Arizona 3,000-3,500 6,000
California 2,140-2,450 4,300-4,325
Colorado 0 475
Nevada 1,500-2,000 5,294
New Mexico 400-500 295
Texas 25 401
Utah Remnant 2,200-2,250
Total 7,065-8,475 18,965-19,040

The number of desert bighorn in North America in pristine times is unknown but most likely was in the tens of thousands.. Seton estimated the pre-Columbian numbers of all subspecies of bighorn sheep in North America at 1.5-2 million. By 1960, however, the overall bighorn population in the United States, including desert bighorns, had dwindled to 15,000-18,200. Buechner documented major declines from the 1850s to the early 1900s. These declines were attributed to excessive hunting; competition and diseases from domestic livestock, particularly domestic sheep; usurpation of watering areas and critical range by human activities; and human-induced habitat changes.

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.

References

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