Desert Bighorn Sheep
Encyclopedia
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.
Population
s 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.
. 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, coyote
s, and bobcat
s, 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 grass
es, sedge
s and forb
s.
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.
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.
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.
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 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.
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 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. 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 844897 acres (3,419.2 km²) to 384410 acres (1,555.7 km²).
The results of the state-by-state survey are shown to the right.
Bighorn Sheep
The bighorn sheep is a species of sheep in North America named for its large horns. These horns can weigh up to , while the sheep themselves weigh up to . Recent genetic testing indicates that there are three distinct subspecies of Ovis canadensis, one of which is endangered: Ovis canadensis sierrae...
that occurs in the desert Southwest regions of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and in the northern regions of Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
. The trinomial of this species commemorates the American naturalist Edward William Nelson
Edward William Nelson
Edward William Nelson was an American naturalist and ethnologist. He was born in Manchester, New Hampshire. In 1871 together with his family, he became homeless due to the Chicago Fire....
(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.
Population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
s of the Desert Bighorn Sheep declined drastically with European colonization of the American Southwest
European colonization of the Americas
The start of the European colonization of the Americas is typically dated to 1492. The first Europeans to reach the Americas were the Vikings during the 11th century, who established several colonies in Greenland and one short-lived settlement in present day Newfoundland...
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
Conservation biology
Conservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...
measures.
Characteristics
Desert bighorn are stocky, heavy-bodied sheep, similar in size to mule deerMule Deer
The mule deer is a deer indigenous to western North America. The Mule Deer gets its name from its large mule-like ears. There are believed to be several subspecies, including the black-tailed 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, coyote
Coyote
The coyote , also known as the American jackal or the prairie wolf, is a species of canine found throughout North and Central America, ranging from Panama in the south, north through Mexico, the United States and Canada...
s, and bobcat
Bobcat
The bobcat is a North American mammal of the cat family Felidae, appearing during the Irvingtonian stage of around 1.8 million years ago . With twelve recognized subspecies, it ranges from southern Canada to northern Mexico, including most of the continental United States...
s, and they use their climbing ability to escape.
Both sexes develop horns
Horn (anatomy)
A horn is a pointed projection of the skin on the head of various animals, consisting of a covering of horn surrounding a core of living bone. True horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae and Bovidae...
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 grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
es, sedge
Cyperaceae
Cyperaceae are a family of monocotyledonous graminoid flowering plants known as sedges, which superficially resemble grasses or rushes. The family is large, with some 5,500 species described in about 109 genera. These species are widely distributed, with the centers of diversity for the group...
s and forb
Forb
A forb is a herbaceous flowering plant that is not a graminoid . The term is used in biology and in vegetation ecology, especially in relation to grasslands and understory.-Etymology:...
s.
Desert adaptations
The desert bighorn has become well adapted to living in the desertDesert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
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
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. 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
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
condition. Wildlife ecologists are just beginning to study the importance of this adaptive strategy
Behavioral ecology
Behavioral ecology, or ethoecology, is the study of the ecological and evolutionary basis for animal behavior, and the roles of behavior in enabling an animal to adapt to its environment...
, which has allowed these small bands to survive in areas too dry for many of their predators.
Social life
Rams battle to determine the dominantDominance hierarchy
A dominance hierarchy is the organization of individuals in a group that occurs when competition for resources leads to aggression...
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.
Bighorns live in separate ram and ewe bands most of the year. They gather during the breeding season
Breeding season
The breeding season is the most suitable season, usually with favourable conditions and abundant food and water, for breeding among some wild animals and birds . Species with a breeding season have naturally evolved to have sexual intercourse during a certain time of year in order to achieve the...
(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 20th century. These declines were attributed to excessive hunting
Hunting
Hunting is the practice of pursuing any living thing, usually wildlife, for food, recreation, or trade. In present-day use, the term refers to lawful hunting, as distinguished from poaching, which is the killing, trapping or capture of the hunted species contrary to applicable law...
; competition and diseases from domestic livestock
Livestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
, particularly domestic sheep
Domestic sheep
Sheep are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order Artiodactyla, the even-toed ungulates. Although the name "sheep" applies to many species in the genus Ovis, in everyday usage it almost always refers to Ovis aries...
; usurpation of watering areas and critical range by human activities; and human-induced habitat changes.
In 1939, after intense lobbying by Frederick Russell Burnham
Frederick Russell Burnham
Frederick Russell Burnham, DSO was an American scout and world traveling adventurer known for his service to the British Army in colonial Africa and for teaching woodcraft to Robert Baden-Powell, thus becoming one of the inspirations for the founding of the international Scouting Movement.Burnham...
and the Arizona Boy Scouts
Scouting in Arizona
Scouting in Arizona has a long history, from the 1910s to the present day, serving thousands of youth in programs that suit the environment in which they live.-The founding of the Boy Scouts in Arizona:...
, President Franklin D. Roosevelt
Franklin D. Roosevelt
Franklin Delano Roosevelt , also known by his initials, FDR, was the 32nd President of the United States and a central figure in world events during the mid-20th century, leading the United States during a time of worldwide economic crisis and world war...
signed a proclamation that established two desert areas in southwestern Arizona to help preserve the Desert Bighorn Sheep: Cabeza Prieta National Wildlife Refuge
Cabeza Prieta National Wildlife Refuge
The Cabeza Prieta National Wildlife Refuge is located in the Sonoran Desert in southwestern Arizona in the United States. The refuge, established in 1939 to protect Desert Bighorn Sheep, is located along of the U.S.-Mexico border, and covers 860,010 acres — larger than the land area of the state...
and the Kofa National Wildlife Refuge
Kofa National Wildlife Refuge
The Kofa National Wildlife Refuge is located northeast of Yuma, Arizona, southeast of Quartzsite, Arizona, in the southwestern United States. The refuge, established in 1939 to protect Desert Bighorn Sheep, encompasses over of the Yuma Desert region of the Sonoran Desert...
In 1941, the San Andres National Wildlife Refuge
San Andres National Wildlife Refuge
The San Andres National Wildlife Refuge is located in the southern San Andres Mountains of southcentral New Mexico, USA. The mountain range, which lies within the northernmost extension of the Chihuahuan Desert, rises to an elevation of 8,229 feet at San Andres peak...
in New Mexico was added.
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
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
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. 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
San Jacinto Mountains
The San Jacinto Mountains are a mountain range east of Los Angeles in southern California in the United States. The mountains are named for Saint Hyacinth . The Pacific Crest Trail runs along the spine of the range.The range extends for approximately from the San Bernardino Mountains southeast to...
, with known populations in Anza-Borrego Desert State Park
Anza-Borrego Desert State Park
Anza-Borrego Desert State Park is a state park located within the Colorado Desert of Southern California. The park takes its name from 18th century, Spanish explorer Juan Bautista de Anza and borrego, the Spanish word for bighorn sheep...
. 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 844897 acres (3,419.2 km²) to 384410 acres (1,555.7 km²).
The results of the state-by-state survey are shown to the right.
External links
- Desert Bighorn Council - Scientific organization for biologists working with Desert Bighorn Sheep
- Arizona Desert Bighorn Sheep - Includes free video about desert bighorn sheep in Arizona
- Desert Bighorn Sheep Facts California Department of Fish and GameCalifornia Department of Fish and GameThe California Department of Fish and Game is a department within the government of California, falling under its parent California Natural Resources Agency. The Department of Fish and Game manages and protects the state's diverse fish, wildlife, plant resources, and native habitats...