Desert Bighorn Sheep


Desert Bighorn Sheep
Desert Bighorn Sheep
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Bovidae
Subfamily: Caprinae
Genus: Ovis
Species: O. canadensis
Subspecies: O. c. nelsoni
Trinomial name
Ovis canadensis nelsoni
Merriam, 1897

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 (1855–1934). 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 16th century. 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.[2]

Contents

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.[3]

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.[3]

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

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.[3]

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.[3]

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 (6.1 m) or more, crashing their massive horns together with tremendous impact, until one or the other ceases.[3]

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.[3]

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.[2] Seton estimated the pre-Columbian numbers of all subspecies of bighorn sheep in North America at 1.5-2 million.[2][4] By 1960, however, the overall bighorn population in the United States, including desert bighorns, had dwindled to 15,000-18,200.[2] Buechner documented major declines from the 1850s to the early 20th century. 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.[2][5]

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[6][7] In 1941, the San Andres National Wildlife Refuge in New Mexico was added.[8]

Desert Bighorn Sheep populations have trended 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.

In Southern California, by 1998, only 280 individuals of the Peninsular Bighorn Sheep population remained, and that population was added to the list of the United States' most imperiled species.[9] Populations in three southern counties had suffered greatly from disease, development, and predation. As of 2008, about 800 peninsular bighorns are believed to populate the desert backcountry from the U.S.-Mexico border to the San Jacinto Mountains, with known populations in Anza-Borrego Desert State Park. These gains, combined with Bush Administration policies, prompted the U.S. Fish and Wildlife Service to propose a reduction in protected sheep habitat by more than 50 percent, from 844,897 acres (3,419.18 km2) to 384,410 acres (1,555.7 km2).[10]

The results of the state-by-state survey are shown to the right.[2]

References

  1. ^ Caprinae Specialist Group (1996). Ovis canadensis ssp. nelsoni. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 11 May 2006.
  2. ^ a b c d e f McCutchen, H.E. (1995). "Desert Bighorn Sheep". In Stohlgren, T.J. (PDF). The Interior West. In: Our Living Resources: A report to the nation on the distribution, abundance, and health of U.S. plants, animals, and ecosystems. Washington, D.C.: U.S. Geological Survey. Archived from the original on 2007-10-31. http://web.archive.org/web/20071031053605/http://www.cs.indiana.edu/sudoc/image_30000078909581/30000078909581/olrdocs/interior.pdf. 
  3. ^ a b c d e f "Desert Bighorn Sheep of Cabeza Prieta NWR". Cabeza Prieta National Wildlife Refuge. U.S. Fish and Wildlife Service. http://www.fws.gov/southwest/REFUGES/arizona/cabighrn.html. Retrieved 2007-08-14. 
  4. ^ Seton, E.T. (1929). Lives of game animals. New York: Doubleday. Vol 3. 
  5. ^ Buechner, H.K. (1960). The bighorn sheep in the United States: its past, present, and future. Wildlife Monograph 4. 
  6. ^ Edward H. Saxton (March 1978). "Saving the Desert Bighorns". Desert Magazine 41 (3). http://www.scribd.com/doc/2404512/197803-Desert-Magazine-1978-March. Retrieved 2008-04-27. 
  7. ^ van Wyk, Peter (2003). Burnham: King of Scouts. Trafford Publishing. ISBN 1-4120-0901-4. 
  8. ^ "San Andres National Wildlife Refuge home page". United States Fish and Wildlife Service. http://www.fws.gov/southwest/refuges/newmex/sanandres/index.html. Retrieved 2009-01-18. 
  9. ^ "Endangered Peninsular Bighorn Sheep". Bighorn Institute. http://www.bighorninstitute.org/endangered.htm. Retrieved 2009-01-18. 
  10. ^ Lee, Mike (March 23, 2008). "Bighorns facing smaller habitat - Federal agency wants to reduce protected area by more than 50%". San Diego Union Tribune. http://www.signonsandiego.com/uniontrib/20080323/news_1n23sheep.html. 

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