BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Antilocapra americana | Pronghorn Antelope 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Minimum breeding age:  Males breed at 16 months of age [76].  Females
generally breed as 2-year-olds [28], although breeding of 1-year-old
females has been reported [18,75].  Once sexual maturity is reached,
pronghorn apparently breed for the rest of their lives [53].

Breeding season:  Pronghorn breed from late summer [76] to fall [3,42];
rutting season lasts for 2 to 3 weeks [67].  Mating systems are
described in Maher [47] and Byers and Kitchen [19].

Gestation period:  averages 252 days [76]

Fawning period:  May to June [3,76]; does deliver a single fawn at first
birth and twins thereafter [76]

Fawn development:  Fawns walk within hours of birth but are generally
inactive for the first few days of life; they run by their fifth day
[76].  Fawns under 3 weeks of age spend up to 90 percent of their time
lying in seclusion; newborns are generally active only for a brief
period when their mothers return to the fawning grounds to nurse them
[31].  Fawns graze by 3 weeks of age and are completely weaned by fall
[3].

Fawn survivorship:  Survivorship probably varies greatly by habitat; in
the Trans-Pecos region of Texas, mortality was 90 percent (18 of 20
radio-collared fawns), with all deaths attributed to predation [21].
Fawn survivorship of 50 percent has been reported in favorable habitat
in Arizona [46].

Life span:  Pronghorn seldom live more than 9 years in the wild [36],
but a few wild does have been aged at 16 years [70].

Movement/migration:  Pronghorn movement is usually in response to
changing environmental conditions such as drought, blizzards, or new
food sources.  Some cold-climate populations migrate from one
seasonal-use area to another, using the same routes each year.  Migrating
populations may travel up to 200 miles (320 km) or more to leave areas
of deep snow [59].  Southern herds show localized movement but seldom
migrate [7].


PREFERRED HABITAT : 
Pronghorn typically inhabit low, rolling, expansive lands with less than
30 percent slope.  Kindschy and others [41] reported that less than 5
percent slope was ideal for pronghorn.  Temperature is not limiting:
Pronghorn occur in cold continental climates and in warm deserts.  They
reach highest densities on ranges with annual precipitation rates of 10
to 15 inches (254-381 mm); populations in areas with greater or lesser
amounts of precipitation have lower survival rates.  Pronghorn are found
from sea level in Mexico to alpine meadows reaching 11,000 feet (3,353
m) elevation in Oregon and Wyoming.  Greatest densities in the Great
Basin occur between 4,000 to 6,000 feet (1,220-1,830 m) elevation [7].
The following characterisitics were common to preferred pronghorn ranges
in the Great Basin [41]:

        * ground cover averaging 50 percent live vegetation;
        * range composition of 40 to 60 percent grasses, 10 to 30 percent
          forbs, and 5 to 20 percent browse;
        * a variety of plant species including 5 to 10 grass species, 20
          to 40 forb species, and 5 to 10 shrub species;
        * succulent plants, available in spring and wet summers;
        * low vegetation structure averaging 15 to 24 inches (38-61 cm)
          in height

Most of these preferred habitat characteristics would probably also
apply to pronghorn habitat east of the Continental Divide [15].

Pronghorn require readily accessible water.  Sundstrom [66] reported
that 95 percent of observed pronghorn in Wyoming's Red Desert were
within 3 miles (6 km) of water.  Otherwise suitable pronghorn habitat in
Oregon has remained unoccupied year-round due to lack of water in summer
[4].  Pronghorn prefer water in pH range from 6.5 to 8.5 [41].  They
cannot tolerate strongly alkaline water.  In the Red Desert of Wyoming,
pronghorn did not drink water above pH 9.25 [56].

Pronghorn habitat requirements are described in detail in Allen and
others [2], Autenrieth [6], Kindschy and others [41], and Yoakum [76].
Allen and others [2] provide a pronghorn habitat index suitablility
model applicable to the Great Basin and the Great Plains.


COVER REQUIREMENTS : 
Reaching top speeds of 50 mph (80 km/hr), pronghorn are North America's
fastest mammal.  Pronghorn rely on keen eyesight, vigilant watch, and
rapid flight to avoid predation [35].  Pronghorn therefore require open
cover, either grassland or grassland interspersed with low shrubs, that
provides long-range visibility.  Prenslow and others [56] never observed
pronghorn in areas where views were restricted by terrain or vegetation
for more than a few minutes at a time.  Pronghorn typically occupy areas
where vegetation is at a mean height of 15 inches (37.5 cm), even if
more suitable forage is available on sites with taller vegetation [15].

Adult pronghorn may use low shrubs for bedding cover [1].

Does seek areas with greater than average shrub cover and height for
fawning [5].  Fawns under 3 weeks old tend to stay in their birthing
area [1,31], using the tallest vegetation in the area for cover [6,56].
On the shortgrass prairie of Colorado, habitat diversity provided by
silver sagebrush (Artemisia cana), small depressions, and stands of
grasses and forbs 9.8 inches (25 cm) or more in height contributed to
above-average fawn survival.  Eighty-eight percent of fawns captured
were located in washouts, tall grass, or near large rocks [56].


FOOD HABITS : 
Foods utilized by pronghorn vary seasonally depending upon
availability, palatability, and succulence of vegetation [39].  Over a
year's time, pronghorn consume nearly all available plant species, with
a preference for succulent forage [79].  Forbs are preferentially
selected when available.  Pronghorn select the most succulent,
high-protein browse or grasses available when forbs are scarce [11].

Pronghorn food habits vary throughout their range.  The average
pronghorn diet on the shortgrass prairie of Colorado was 43 percent
forbs, 40 percent browse, 11 percent cacti, and 6 percent grasses [39].
Cole and Wilkins [23] found similar dietary patterns on
grama-needlegrass-wheatgrass (Boutelous spp.-Stipa spp.-Agropyron spp.
(as described by Kuchler [43])) types in central Montana.  In central
Wyoming, however, pronghorn annual diet averaged 5 percent forbs, 3
percent graminoids, and over 90 percent browse [61].

In winter, shrubs are high in protein relative to other forage, and
shrubs comprise the majority of the pronghorn diet [10].  Browse was the
most heavily utilized pronghorn winter food in Alberta even though its
availability was extremely limited [51].  In Utah browse comprised over
90 percent of the pronghorn diet, with black sagebrush (Artemisia nova)
selected most often.  Other important browse species were winterfat
(Krascheninnikovia lanata), Brickellia spp., and green rabbitbrush
(Chrysothamnus viscidiflorus) [9].  Sagebrush (Artemisia spp.),
rabbitbrush (Chrysothamnus spp.), and bitterbrush (Purshia tridentata)
are important pronghorn browse throughout the Great Basin [77].  Big
sagebrush (A. tridentata), bitterbrush, and saltbush (Atriplex spp.) are
important in Montana [9].  When vegetation is mostly covered with snow,
pronghorn seek windswept areas and graze lichens [62,69].

Pronghorn consume primarily forbs in spring, summer, and fall [14].
Forbs consistently selected throughout pronghorn's range include yellow
sweet clover (Melilotus officinalis) and alfalfa (Medicago sativa).
Scarlet globemallow (Sphaeralcea coccinea) is highly preferred on the
Great Plains, and Louisiana sagewort (Artemisia ludoviciana) is
consistently selected in the Intermountain region [20,45].  Lists of
other forbs often used by pronghorn in the Great Basin [76], the
Intermountain region [14], and shortgrass prairies of Saskatchewan [27]
and Wind Cave National Park, South Dakota [45], are available.

In summer pronghorn supplement their forb diet with browse and green
grasses.  Spring is the only time of year when grasses are heavily
grazed [11,39], but grasses are also utilized during other periods of
green-up [9].  The high protein content of early spring grasses may be
particularly beneficial at a time when other forage is of low quality
[74].  Pronghorn in Utah were not observed to use dry, mature grasses at
any time [11].  In fall pronghorn consume forbs and browse [11,39].

Except for alfalfa and wheat (Triticum aestivum), pronghorn do not
usually consume agricultural crops [27,41].  Hepworth [35] reported use
of winter wheat in Nebraska only when browse was unavailable.  Alfalfa,
however, may be grazed year-round [7].


PREDATORS : 
Coyote (Canis latrans), domestic dog (C. familiaris), bobcat (Felis
rufus), mountain lion (F. concolor), and golden eagle (Aquila
shrysaetos) are important pronghorn predators [52,58,76].  Humans also
hunt and poach pronghorn [52].  Coyote, bobcat, and golden eagle prey
mostly on fawns, especially newborns [34,76].

The importance of predation as a limiting factor for pronghorn
population increases varies with habitat quality.  Studies of predation
on pronghorn showed that fawns on the shortgrass prairie of Alberta had
high survival rates [8], while survivorship of fawns on desert
shrublands of Nevada had was low.  Populations in habitat of marginal
quality (i.e., where dense and/or tall shrubs predominate) are likely to
experience high fawn mortality from predation [50].


MANAGEMENT CONSIDERATIONS : 
Range:  It has been suggested that pronghorn thrive on vegetation in a
subclimax condition with a mixture of forbs, grasses, and browse [7,41].
Subclimax conditions were historically created by fires and seasonal
grazing patterns of pronghorn in association with bison (Bison bison),
elk (Cervus elaphus), and/or deer (Odocoileus spp.).  Pronghorn and
bison in particular may have been symbiotic:  Bison grazed grasses
heavily which in turn stimulated growth of forbs and browse used by
pronghorn [41].

Management practices promoting pronghorn including grazing systems,
range rehabilitation methods such as site prepartion and seeding
rates/ratios, and specifications for building water storage facilities
are discussed in detail in Authenrieth [7], Kindschy and others [41],
O'Gara and Yoakum [54], and Yoakum [76,77].

Pronghorn and bison, cattle, or horses have little dietary overlap on a
yearlong basis except on overgrazed ranges [29,49,60,73].  Pronghorn can
benefit livestock ranges by eating forbs such as paperflower
(Psilostrophe spp.) and groundsel (Senecio spp.) that are poisonous to
livestock [55].  In the Great Basin, diets of mule deer (O. hemionus)
and pronghorn overlapped moderately in winter and only slightly in other
seasons [73].  Diets of domestic sheep and pronghorn, however, have
considerable overlap, and domestic sheep often outcompete pronghorn for
forage.  Moderate domestic sheep use of winter range in the Great Basin
of Utah significantly lowered pronghorn use of the range [22].

Heavy cattle stocking can be detrimental to pronghorn by converting
shrub-grassland to shrubland, which renders the area unusable to
pronhorn.  Due to cattle grazing, historic pronghorn range in portions
of California, Nevada, and Oregon no longer meet pronghorn needs.  Heavy
cattle grazing in Texas forced pronghorn to a diet heavy in poisonous
plants, resulting in direct pronghorn mortality and reproductive losses
[81].

Cattle use of traditional or potential pronghorn fawning grounds
during fawning season has been shown to displace does to less suitable
birthing areas.  This usually results in higher fawn mortality due to
predation [81].

Livestock fences, especially those designed to retain domestic sheep,
can severely restrict pronghorn movement and lower pronghorn numbers.
Pronghorn herds are especially vulnerable when movement from depleted
ranges or to water is restricted [55].  Specifications for building
fences (including sheep fences) that allow pronghorn passage are
available [7,14,76].

Reintroduction:  Pronghorn have successfully been reintroduced in
sagebrush steppe of Mono County, California [33], and in desert
grassland of Arizona [16].  

A reintroduction strategy of releasing small groups of pronghorn over
several years may be more successful than one large release of animals
[33].  Britt [16] reported that in Arizona, pronghorn establishment was
successful only after three releases of 126 pronghorn over 10 years.
Pronghorn from the original translocation may have served as a nucleus
of experienced animals that provided social stimuli and established
behaviors for new animals that followed.  

Genetic considerations for pronghorn reintroduction programs are
provided in Lee and others [44].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Antilocapra americana | Pronghorn Antelope