Desert Bighorn Sheep (Ovis canadensis
nelsoni) is a subspecies of Bighorn
Sheep that occurs in the desert Southwest regions of the
States and in the northern regions of Mexico.
trinomial of this species commemorates the American naturalist
Edward William Nelson
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.
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
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
, and they use their climbing
ability to escape.
Both sexes develop horns
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
The desert bighorn has become well adapted to living in the
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
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
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
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
Arizona Boy Scouts
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
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
propose a reduction in protected sheep habitat by more than 50
percent, from 844,897 acres to 384,410.
The results of the state-by-state survey are shown to the