BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Ovis canadensis | Bighorn Sheep 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Breeding season - Bighorn sheep are polygamous.  Ewes are monesterous.
Rams of most subspecies rut in November and December.  However, desert
bighorn sheep may rut for up to 9 months, with rutting reaching a peak
in August and September [6].

Age at sexual maturity - The age at which ewes attain sexual maturity is
quite variable and is dependent mainly on their physical condition [6].
Most bighorn sheep become mature at 2.5 years of age.  Large-bodied rams
may reach sexual maturity within 18 months, but smaller rams may take as
long as 36 months.  Very old ewes generally do not breed [6].

Gestation and lambing - The gestation period is 5.5 to 6 months.  The
majority of ewes give birth to one lamb per year.  Lambing of northern
bighorn sheep occurs between late April and late June, with most lambs
born before the end of May.  Desert bighorn sheep ewes give birth
throughout the year; however, the peak is from January to April [6].

Development of lambs - Bighorn sheep lambs are precocious and within a
day or so climb almost as well as their mothers.  Within 2 weeks lambs
can eat grass.  They are weaned between 1 and 7 months.  By their second
spring bighorn sheep are totally independent of their mother.  Ewes
reach their adult weight by 4 to 5 years of age, while rams do not
achieve maximum weight until they are 6 or 7 years old [6].

Life span - Mortality is high for bighorn sheep 1 to 2 years of age,
drops to a relatively low rate for 2 to 8 years of age, then increases
to a maximum for those older than 8 to 9 years.  Bighorn sheep that live
past 8 or 9 years may live to 15 to 17 years of age, but 10 to 12
years is more common [6].

Bighorn sheep are territorial.  By 4 years of age, individuals have
established home ranges that are utilized throughout their life span [6].


PREFERRED HABITAT : 
Bighorn sheep inhabit remote mountain and desert regions. They are
restricted to semiopen, precipitous terrain with rocky slopes, ridges,
and cliffs or rugged canyons [6,26].  Forage, water, and escape terrain
are the most important components of bighorn sheep habitat [26].

Winter range - Generally, bighorn sheep have two distinct, separate
summer and winter ranges [6].  Most of the year is spent on the winter
range, where the elevation is typically below 10,826 feet (3,300 m).
The aspect is usually south or southwest.  Rams often venture onto the
more open slopes, although rugged terrain is always nearby.  Desert
bighorn sheep rarely stray far from the base of a mountain and usually
are found on eastern aspects, where they use dry gullies.  During
severe weather, if snow becomes unusually deep or crusted, bighorn sheep
move to slightly higher elevations where wind and sunshine have cleared
the more exposed slopes and ridges [6].

Spring range - The spring range is generally characterized by the same
parameters as the winter range. However, bighorn sheep begin to respond
to local greenups along streambanks and valleys.  Bighorn sheep use
areas around saltlicks heavily in the spring.  Preferred lambing range
is in the most precipitous, inaccessible cliffs near forage, and
generally has a dry, southern exposure [6].

Summer range - In the summer, bighorn sheep are mostly found grazing on
grassland meadows and plateaus above timber.  In early summer south and
southwestern exposures are most frequently utilized; however, in the
case of the desert bighorn sheep the eastern aspect is preferred.  By
late summer the more northerly exposures are preferred [6].  Snow
accumulation seems to be the principal factor that triggers bighorn
sheep to move from summer to winter ranges [26].

Water - Bighorn sheep obtain water from dew, streams, lakes, springs,
ponds, catchment tanks, troughs, guzzlers, and developed seeps or
springs [26].  Alkaline water is not suitable.  Bighorn sheep spend most
of their time within 1 mile (1.6 km) of water but have been located as
far as 2 miles (3.2 km) from water.  Water sources more than 0.3 mile
(0.5 km) from escape terrain or surrounded by tall dense vegetation are
avoided by bighorn sheep [26]. Desert bighorn sheep primarily utilize
ephemeral water sources.  They may drink every day if water is nearby,
but may go without water for up to 14 days in the dry season.  Since
water is one of the major limiting factors of desert bighorn sheep,
management agencies have installed cisterns and other water developments
in critical areas [6].


COVER REQUIREMENTS : 
Escape terrain is an important habitat requirement for bighorn sheep.
Cliffs, rock rims, rock outcroppings, and bluffs with sparse cover of
trees or shrubs typify escape habitat, which provides both thermal and
hiding cover.  While bighorn sheep are not always found in precipitous
mountain areas, ewes and lambs rely on these areas for escape cover,
especially during the lambing period [6,26,27].

Visibility is another important habitat component for bighorn sheep.  It
allows for predator detection, visual communication, and efficient
foraging [4].  Bighorn sheep tend to forage in open areas with low
vegetation such as grasslands, shrublands, or mixes of these.  They
avoid foraging on mild slopes with shrub or canopy cover in excess of 25
percent and shrubs 2 feet (60 cm) or higher. On steep slopes they have
been noted to travel through or bed in dense brush [26].


FOOD HABITS : 
Bighorn sheep primarily graze grasses and forbs, but eat other
vegetation depending on availability [6].  They prefer green forage and
move up- or downslope or to different aspects for more palatable forage.
Forage areas that provide a variety of aspects are preferable because
they provide green forage for longer periods [26].

Bighorn sheep eat sedges and a variety of grasses including bluegrasses
(Poa spp.), wheatgrasses, bromes, and fescues.  Browse species include
sagebrush, willow (Salix spp.), rabbitbrush, curlleaf mountain-mahogany
(Cercocarpus ledifolius), winterfat (Kraschnennikovia lanata),
bitterbrush, and green ephedra (Ephedra spp.).  Forbs include phlox
(Phlox spp.), cinquefoil (Potentilla spp.), twinflower (Linnaea
borealis), and clover (Trifolium spp.) [6,23].

Because of the dry climate, browse is the dominant food of the desert
bighorn sheep and includes desert holly (Atriplex hymenelytra),
honeysweet (Tidestromia oblongifolia), brittlebush or encelia (Encelia
spp.), hairy mountain-mahogany (C. breviforus), Wright silktassel
(Garrya wrightii), desert mallow (Sphaeralcea ambigua), Russian-thistle
(Salsola kali), false mesquite (Calliandra eriophylla), goatnut
(Simmondsia chinensis), white ratany (Krameria canescens), bursage
(Hyptis emoryi), mesquite (Prosopis glandulosa), catclaw (Acacia
greggii), ironwood (Olneya tesota), paloverde (Cercidium spp.),
pincushion (Mammillaria spp.), and saguaro (Carnegiea gigantea).  Dry
grasses are eaten throughout the year and are an important food reserve,
especially near waterholes [6].


PREDATORS : 
Golden eagles (Aquila chrysaetos) occasionally threaten bighorn sheep
lambs, but are rarely successful in taking one.  Bighorn sheep are an
incidental food item in the diet of grizzly or black bears (Ursus
arctos, U. americanus) and wolverines (Gulo gulo), and are generally
eaten only as carrion.  Wolves (Canis lupus), coyotes (C. latrans),
mountain lions (Felis concolor), and bobcats (Lynx rufus) are other
predators of bighorn sheep [6,26].  The number of bighorn sheep taken by
predators is usually of little consequence to healthy populations.
Predators are most effective when locations of escape terrain or water
limit sheep movement and allow predators to concentrate hunting efforts
[26].
  
Bighorn sheep are hunted by humans.  Hunting has traditionally been for
rams only and is further restricted by a 3/4 or full horn curl policy.
In the last few years most states and provinces have adopted more
stringent horn curl regulations.  While the overall trend has been for
more restrictive hunting seasons, in some cases local situations have
dictated either sex or 1/2 curl ram seasons [6].


MANAGEMENT CONSIDERATIONS : 
Bighorn sheep are very susceptible to diseases.  Incidence of lungworm
infestation approaches 100 percent in some herds, although the level of
individual infection varies depending upon sheep and domestic livestock
densities, range conditions, climate, season, and age.  Desert bighorn
sheep appear to have lighter infestations, possibly due to climate or
low density.  A significant correlation exists between the intensity of
the lungworm infestation and the amount of precipitation in the spring
of the previous year.  In Washington state both wild and captive bighorn
sheep have been successfully treated with the experimental drug
albendazole.  Further research is needed to determine the feasibility of
treating remote populations [6].

The future of bighorn sheep depends on the preservation and improvement
of critical native ranges.  Bighorn sheep are poor competitors with
other wild and domestic ungulates, and their range is diminishing.
The effect of domestic livestock grazing on bighorn sheep is
controversial and depends on the proximity and population size of
competing species.  Domestic livestock have been reported to have little
deleterious effect if they do not graze on critical bighorn sheep winter
ranges.  Nevertheless, extensive competition by livestock, especially on
public lands, persists and is one of the reasons for the decline in
density of bighorn sheep populations [6].  Elk (Cervus elaphus) and deer
(Odocoileus virginianus and O. hemionus) can also be serious competitors
with bighorn sheep on marginal habitat [6,18].

Human activities on bighorn sheep range are the most widespread threat
to bighorn sheep [4].  These activies reduce the number of bighorn sheep
by decreasing habitat, causing bighorn sheep to reduce or terminate
their use of prime habitat, stop migration, or split from large herds
into smaller herds [4,26].  Human activities responsible for declines in
sheep use of an area include hiking and backpacking, snow skiing, water
skiing, fishing, motorbiking, four-wheel-drive vehicle use, construction
and use of roads, urban development, and recreational development. When
bighorn sheep are pushed from prime to marginal habitat, mortality
usually increases and productivity decreases.  Some herds have adapted
to human activity [26].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Ovis canadensis | Bighorn Sheep