Pronghorn Management Guide - 2006

I. INTRODUCTION: PRONGHORN HABITATS AND LIFE HISTORY

Habitat Requirements

Habitat requirements for pronghorns in intermountain, plains, shrub-steppe, valley, and semi-desert grassland communities have been investigated and summarized by Sundstrom et al. (1973), Ockenfels et al. (1994), and Yoakum (2004)(Figures 1-). Similar criteria are just now being developed for desert-dwelling pronghorn (Figure 10).

Figure 1. In Idaho, certain pronghorn herds occupy intermountain grasslands at the foot of high mountain ranges. These areas are spring to autumn rangelands containing mixed communities of low-growing shrubs, grasses, and forbs. Aerial photo courtesy of the Edson Fichter collection.

Landscape Physiography: Although pronghorn typically use sites having slopes of less than 10%, these animals can and do occupy steeper terrain; slopes greater than 20% are generally avoided, however. Rugged landscapes also effect survival because mountain lions (Puma concolor) and other predators are often found in such sites (Ockenfels 1994b). Pronghorn typically use low rolling, expansive terrain. The area required depends on both habitat quality, quantity and, in some areas, the provision of seasonal movement and/or both habitat quality and quantity in migration corridors to avoid deep snow. Summer and winter rangelands are usually differentiated on the basis of snow accumulation, the availability of seasonal forage, and sources of drinking water.

Figure 2. Many pronghorn populations maintain small herds with a territorial male and harem of 6 to 12 females during the autumn breeding season. This scene depicts these conditions on the Palous intermountain grassland of the National Bison Range in Montana. Photo by Jim D. Yoakum.

Topography and Elevations: Pronghorn inhabit open, gentle landscapes, characterized by hills, ridges and draws. Substrates may be clay, gravel, or sand with dunes up to 6 feet (2m) in height. Elevations range from near sea level to an altitude of 11,000 feet (3,353 m). Animals in Mexico occur close to the seacoast while small herds in Oregon and Wyoming use alpine meadows. Greatest pronghorn densities, however, occur between altitudes of 4,000 and 6,000 feet (1,300 and 1,900 m); (Yoakum 2004).

Natural Barriers: Natural obstacles can curtail movements and exclude the occupancy of otherwise suitable habitats. Natural barriers include abrupt escarpments or mountain ranges, deep canyons, thick copses of shrubs or trees, and densely wooded areas. For example, steep-walled canyons effectively separate pronghorn populations into distinct herds in central and northern Arizona (Ockenfels et al. 1994, Ockenfels et al.1997). Einarsen (1948) cited two examples of such barriers in Oregon, the Columbia River and a forested region, where pronghorn did not move into suitable but isolated habitats nearby. A few trees, especially in open areas subject to high temperatures may be desirable for shade, however.

Figure 3. Prior to the early 1800s, millions of pronghorn and bison grazed the central grassland prairies of North America from Canada south through the western United States to northern Mexico. They still do but in much less numbers as depicted in this recent scene taken in eastern Wyoming. Photo by Robb Hitchcock.

Figure 4. One of the largest ecosystems with the highest densities of historic and extant numbers of pronghorn, is the short grass prairie. Portrayed here, is a small herd of pronghorn that live year-long on the prairies of southern Alberta, Canada. Photo by Dave Simpson.

Figure 5. Pronghorn occupy portions of the tall grass prairie. When tall grasses are heavily grazed by bison or cattle, or burned by wild fires, then preferred grasses and forbs grow profusely and are readily consumed by pronghorn. A small herd pictured here lives on the Wind Cave National Park in South Dakota. Photo by Jim D. Yoakum.

Climatic Limitations: Highest pronghorn densities occur in ecosystems where mean annual precipitation ranges from 8 to 15 inches (20-38 cm). Populations in precipitation belts above or below these parameters have lower survival rates and densities (Yoakum 2004, Sundstrom et al. 1973).

Precipitation and Water: Winter precipitation appears to be more important to pronghorn recruitment and survival than summer precipitation. Studies in the Southwest have shown that pronghorn populations require a minimum of two inches (5 cm) of precipitation during the period October through March for herd maintenance (Brown et al. 2000). No mean maximum precipitation amount have been documented, but probably ranges between 20 and 30 inches (60-76 cm) (Büechner 1950).

Dew and the water content of cacti and forage plants may be sufficient to provide necessary water requirements for adult survival, but may not meet lactation needs (Fox 1997). Reports by Sundstrom et al. (1973) and Yoakum (2004) indicate that herds occupying rangelands with abundant drinking water every 1-3 miles had densities compared to areas with scant drinking water (Hervert et al. (1997) and Cancino et al. (1998)).

Figure 6. Thousands of pronghorn historically foraged the Central Valley of California when the Spanish arrived in the 1770s. Much of the native forage has been replaced with exotic annual plants. Small isolated pronghorn herds to day forage for grasses, forbs and shrubs on the Carrizo Plain National Monument and surrounding habitats in southern California. Photo by Peter Knapp.

In Texas, droughts were associated with decreased pronghorn vitality and fertility (Hailey1979). Baker (1953a) found that pronghorn in Wyoming sometimes died while trying to get through fences to reach water. Pronghorn will drink from most facilities designed to water livestock, but these facilities should remain functional and usable by pronghorn throughout the spring, summer and autumn on northern rangelands and year-round in southern habitats.

The quantity of water consumed by pronghorn varies with body size, sex, health, lactation demands, and physical activity, and the succulence of the forage, as well as humidity and ambient temperatures. Water use decreases with lower temperatures and the availability of snow, succulent forage, and the amount of dew or rain. Conversely, water use increases with drier atmospheric conditions, lack of snow, dry forage, and higher temperatures. In Wyoming, pronghorn were stressed when snow or free water was not available during winter (Cook et al.1984, Guenzel et al. 1982).

Based on studies of laboratory animals, summer water requirements for an adult pronghorn in Wyoming were 0.95 gal/day/100 lb animal (3.6 l/day/45 kg) (Whisler 1984). However, the water needs for free-roaming pronghorn accustomed to drinking water may be greater. Measurements of pronghorn water consumption were conducted in a Wyoming field study (Sundstrom 1968).

Daily consumption rates per adult pronghorn varied from 0.09 gal/day (0.34 l/day) in May to 1.19 gal/day (4.5 l/day) in August. Water in quantities of approximately 1/4 of summer consumption rates should be provided to pronghorn during winters when free water (including snow) is unavailable to herds accustomed to drinking in Wyoming (Sundstrom 1968).

A close relationship was observed between pronghorn distribution and water locations in Wyoming's Red Desert; 95% of 12,465 pronghorn surveyed from the air were within 4 miles (6.4 km) of a water source (Sundstrom 1968). Most pronghorn observations in Arizona and New Mexico are usually within two miles (3.2 km) of water (Ockenfels et al. 1994, Clemente et al. 1995). Occasionally, adult males are seen farther from water, and pronghorn in the Sonoran Desert have been seen 40 miles (24 km) from water (J. Hervert pers. com).

Benson (1956) considered the advent of water developments in Saskatchewan to be associated with the dispersion of pronghorn populations. In Oregon (Anonymous 1961), it was speculated that although suitable forage was available for pronghorn, the limiting factor was adequate drinking water in late summer. Beale and Smith (1970) suggested that water developments might encourage a greater distribution of pronghorn where natural water sources were limited, particularly during dry seasons or drought years. Water developments may also increase competition with livestock and elk into formerly unused habitats, however.

Minimum distance to water may be as important as maximum distance. Pronghorn in Arizona avoided the first 400 yards (400 m) from water sources, possibly to reduce the threat of predation (Ockenfels et al. 1992, Ockenfels et al. 1994). If an area is well watered, distance to water may vary little with the season (Ockenfels et al. 1994). In southern New Mexico, pronghorn ranged farthest from developed water in summer, when precipitation was the highest, thereby reducing reliance on stock tanks and other artificial water sources (Clemente et al. 1995).

Snow: When snow depths exceed 10-12 inches (25-30 cm), pronghorn frequently have difficulty obtaining forage. Prolonged periods of deep snow are especially detrimental when combined with such factors as inadequate forage, low temperatures, and snow crusting due to alternate freezing and thawing temperatures. Although wind increases chill factor, a complete absence of wind precludes bare patches of ground and interferes with foraging. Fences and other obstacles to movement may be especially detrimental as such times (Sundstrom 1969, Riddle and Oakley 1973, Hailey 1979). The severe winters of 1964-65 and 1968-69 resulted in high losses of pronghorn in Montana, the Dakotas, and even Arizona, often because the animals could not reach areas with adequate food (Compton 1970, McKenzie 1970, West 1970). Wishart (1970) reported that severe winter weather in Alberta caused prolonged emigration, starvation, and increased predation as well as depressed reproduction the following year. Even in southern regions, pronghorn are not immune to winter mortality. Many pronghorn trapped by drift fences froze to death during blizzards in the 1880s, and settlers killed 1,500 trapped by a drift fence in Texas in 1882 (Haley 1949).

Low temperatures seldom are a major limitation unless combined with deep, crusted snow. Freezing temperatures and precipitation during fawning may cause mortality to newborns. The effects of high temperatures, while poorly documented, nonetheless play a role in water loss, forage availability, and physiological functions. (Brown et al. 2006)

Figure 7. Semi-desert grasslands in the state of Chihuahua sustained the highest number of pronghorn in Mexico; however, total numbers have greatly decreased during the last 100 years. They are classified as an endangered species by the federal government. Photo by Patrick Robles.

Vegetation: Ground cover in grasslands occupied by pronghorn averages 60-80% living vegetation (mostly grasses and forbs) with 20-40% being without vegetation. In shrub-steppe and semi-desert grassland habitats, the percentages are 50% or more living vegetation and less than 50% bare ground, rock, litter, etc (Yoakum 2004). Habitats used by desert pronghorn possess less than 50% ground cover (Brown 1994).

Plant Composition and Structure: Generally, composition of vegetation is 5-15% grasses, 5-10% forbs, and 10-35% shrubs on shrub-steppes; in grasslands, the typical composition is 50-80% grasses, 10-20% forbs, and less than 5% shrubs (Yoakum 2004). In semi-desert grasslands in Arizona, grass cover averaged 15%, shrub cover averaged approximately 10%, and forbs cover fluctuated between two and 10% (Ockenfels et al. 1994). Desert lands used by pronghorn may have <10% shrub cover with annual grasses and forbs composing less than two percent of the ground cover. The use of semi-desert and desert habitats with tree cover is usually low, but increases during hot, dry periods when pronghorn use scattered trees or other structural cover for shade (Ockenfels 1994)

Figure 8. Semi-desert grassland communities formerly hosted moderate to large numbers of pronghorn but because of fire prevention and the invasion of woody shrubs, reduced grass cover, and a loss of nutritious forbs, many of these areas now support few if any animals. The trees are Juniperus monosprma, a grassland invading juniper. Photo by Richard Ockenfels.

Low vegetative structure, averaging 10-18 inches (25-46 cm), is preferred. Vegetation over 25 inches (63 cm) is typically avoided, and that taller than 30 inches (76 cm) is infrequently used. Pronghorn may use areas having high shrubs while traveling to or from preferred habitats. However, reduced visibility or decreased mobility due to tall vegetation, are important factors in pronghorn survival (Goldsmith 1990).

Pronghorn in the Southwest often use savannas if canopy cover averages less than 20% and other vegetation is less than 24 inches (61 cm) (Ockenfels et al. 1994). Other special southwest habitats include dunes in the Vizcáino subdivision of the Sonoran Desert and "cholla forests" in northwest Sonora and southwest Arizona (Cancino et al. 1995, Hervert et al. 1998).

Figure 9. Great Basin shrub-steppes and desert scrub are biotic communities supporting limited numbers of pronghorn. These areas produce a wide variety and quantity of endemic and exotic forbs that consumed in large quantities. Shrubs are typically abundant and are browsed throughout the year, often becoming the key survival forage during critical winters. Photo by Tom Pojar

Figure 10. Sonoran Desert-scrub community inhabited by pronghorn. Photo by David E. Brown.

Plant height and density are synergistic factors affecting pronghorn. Plants, including grasses, more than 2.5 feet (0.75m) tall are detrimental to pronghorn and dense stands of such plants preclude the animal's presence. Conversely, shrubs and other plants less than 18 in. (0.5m) tall are often advantageous to pronghorn, especially if the shrub cover is less than 40%. Trees and tall shrubs >2.5 feet tall should comprise less than 5% of the cover, and a density of such plants >15 % may exclude the permanent presence of pronghorn. As a consequence, pronghorn are usually lacking from dense big sagebrush (Artemisia tridentata), greasewood (Sarcobatus vermiculatus), one-seed juniper (Juniperus monosperma), and other tall shrub communities, just as this animal has always shunned tall-grass prairies (Yoakum 2004). Tree density in most pronghorn habitats in Arizona are typically less than 2/acre (5/ha) (Alexander and Ockenfels 1994).

Plant Diversity: Within shrub-steppes occupied by pronghorn, the number of plant species averages 5-10 grasses, 10-70 forbs, and 5-10 shrubs (Yoakum 2004). On grasslands the averages are 10-20 grasses, 20-60 forbs, and 5-10 shrubs. Although semi-desert grassland habitats exhibit similar diversity, most desert habitats possess less than five species of grass, five forbs, and one or two shrubs with some of the grasses and forbs being annuals. As a consequence, plant species richness may vary by month, with the greatest variety usually being in spring (Ockenfels et al. 1994).

Trees are often absent or scarce in grasslands and shrub-steppes When present, species richness is usually two or less except in the Sonoran Desert where up to five species of trees may be present (Brown 1994).

Open landscapes supporting a patterning of vegetative types (meadows, forbs patches, riparian areas, dunes, etc.) are preferred in contrast to monotypic vegetative communities (Yoakum 1957, Sundstrom et al. 1973). Pronghorn also forage and often congregate in areas of recent wildfires as these "burns" typically produce new grass growth and a flush of succulent forbs (Deming 1963, Yoakum 1980, Courtney 1989).

Key vegetative components are those areas necessary to sustain a pronghorn population during critical periods (e.g., severe winters, droughts, etc.). The use of such areas may or may not be seasonal, and often depends on environmental conditions. Vegetative requirements for pronghorn vary widely in relation to land management practices, geographic location, climate, soils, and habitat types. Examples of key rangelands used by pronghorn include: spring feeding areas (Becker 1972), winter range (Compton 1970, McKenzie 1970, West 1970, Taylor 1975); seasonal movement routes, areas having water (Sundstrom 1968, Beale and Holmgren 1975), and fawning areas (Einarsen 1948, Pyrah 1974, Autenrieth 1976).

Life History

Behavior: Due to the ease of observations, pronghorn behavior has been studied extensively. Northern pronghorn tend to winter in large herds, with animals of both sexes and all ages feeding and bedding in close association with minimal social conflicts. However, the sexes may remain segregated in areas experiencing mild winters (O'Gara 2004).

Behavioral adaptations during winter to conserve energy include: establishing hierarchies at feeding craters pawed in the snow; selection of microhabitats with lower wind velocities and less or softer snow, reducing daily travel, traveling single file, and lying down during days having hard snow and low temperatures. During periods of high winds, pronghorn may lay down in compact groups with their heads curled back along their bodies (Bruns 1969).

Does typically isolate themselves prior to giving birth. By mid-summer, does are typically found in doe-fawn groups, with non-territorial bucks interacting with others in bachelor herds (Prenzlow et al. 1968, Kitchen 1974, Autenrieth and Fichter 1975). Mature bucks are often territorial or strive to maintain a harem at this time.

The presence of mature, territorial bucks ensures that bachelor bucks do not compete with pregnant and lactating does for forage on the best rangelands (Gilbert 1973). The most rigorous bucks do most of the breeding in either a territorial or harem breeding strategy (Byers 1997). Unlike some other ungulates, younger bucks, < 5 years of age, possess the largest horns (Brown et al. 2002).

Movements: Pronghorn change locations due to drought, blizzards, disturbance, and forage and/or water availability. Pronghorn migration is defined as traditional movement from one seasonal-use area to another, following approximately the same route year after year. According to Einarsen (1948) and Yoakum (1978), most pronghorn exhibit seasonal movements and relatively few populations now participate in traditional migrations.

The timing and length of seasonal movements are generally in response to changes in climatic and vegetative conditions. In the northern parts of their range, pronghorn sometimes move up to 200 miles (320 km) in response to deep snow or to reach available winter forage (Riddle 1990). During dry seasons, southern pronghorn may move some distance in search of forage and water (Büechner 1950, Hailey 1979).

Reproduction: Although pronghorn fawns occasionally breed (Wright and Dow 1962, Mitchell 1967), does usually breed the first time when 16-17 months of age. The gestation period, averages 252 days, and is long compared to similar-sized ruminants (Hepworth and Blunt 1966). Most northern pronghorn breed during a short period from mid-September to early October (O'Gara 1968), but animals in more southern areas may breed from July through October (Lehman and Davis 1942, Büechner 1950, Hervert pers. com.). The mean number of fetuses per doe in 6 studies involving 209 does was 1.94 (O'Gara 2004b.). Mid-summer fawn to doe ratios (f:d) often are used as an index of recruitment. Fawn production rates in Montana were rated as follows (Trueblood 1971):

Fawns:100 Does Rating

20-39Very poor

40-59Poor

60-79 Fair

80-99Good

100+Excellent

Care should be exercised in interpolating these ratios for other habitats, especially areas with lower carrying capacities such as semi-desert grassland and desert ecosystems. Ellis (1970) gathered information on f:d ratios and reported means of 80-100:100 in the Great Plains and 30-50:100 in the Intermountain West. Fawn to doe ratios in the arid Southwest average less. In Arizona, hunt management guidelines indicate permit decreases when f:d ratios fall below 30:100 and indicate permit increases when fawn: doe ratios rise above 40:100.

Diet Selection: Pronghorn food habits vary greatly due to these ungulates occupying diverse vegetative communities in three biomes--grasslands, shrub-steppes, and deserts. Pronghorn are selective, opportunistic foragers, feeding on grasses, forbs, shrubs, and trees, depending on plant palatability and availability. More than 200 diet selection studies have been conducted during the past half- century (Yoakum 2004d); 21 of these provided data on forage classes available as well as the percent composition taken each season. Preference ratings, calculated from these data, were 4.7 for forbs, 1.5 for shrubs, and 0.2 for grasses (Fig. 5). Thus, management decisions favoring an abundance of palatable forbs throughout the year are desirable (Yoakum 2004d).

Grasses are grazed most intensively during first "Green-up" when shoots are 2-3 inches (5-8 cm) tall and highly nutritious. They will consume dry grass, but not in quantities. Shorter and fine-textures bunch grasses are preferred over large coarse bunch grasses. Annual and perennial forbs are grazed throughout the year when available. During the winter in the northern Great Basin shrub-steppes, Hansen et al. (2001) reported pronghorn consumed large quantities of perennial forbs during a mild winter with little snow covering small herbaceous plants.

Figure 11. Comparison of vegetative composition, diet composition, and preference ratings for pronghorn year-long food habits in grassland and shrub-steppe (Yoakum 2004).

Forbes may well be the most important forage class to pronghorn densities, for they provide greater amounts of nutrients than grasses or shrubs during late winter and early spring (Ellis 1970, Smith and Beale 1980, Hervert et al. 2000). This is the time of year that most (southern herds may differ) parturient does are carrying two fetuses that grow rapidly during the third trimester. Hence, Pyrah (1987) referred to herbaceous plants as "production plants". Early summer is by far the most important season for consumption of forbs, as the doe must obtain sufficient quality forage for her to nurse two fawns and at the same time obtain enough nutrients to keep healthy.

Shrubs, like forbs, are often consumed each month of the year, however, they are browsed in greater quantities during autumn and winter. Shrubs are "survival food" because they often are available during deep snow depths and during years of droughts. Shrubs often have greater concentrations of nutrients during the autumn-winter than spring-summer. Some of the lesser preferred shrubs are consumed in greater quantities during severe winters and droughts because the total for nutritional forage is scarce.

While conducting studies of pronghorn for more than 20 years on the grasslands of the National Bison Ranch in Montana, Byers (2003) concluded that rangelands with an abundance of succulent, nutritious forbs in the autumn-winter sustained heavier, larger fawns and had high survival rates during summer following parturition.

Pronghorn mortality due to inadequate quality and quantity of nutritious forge is often not detected in the Field. Then too, malnutrition may be a secondary mortality factor that is credited as an agent for killing animals: (e.g., severe winters, droughts, entrapment in fenced pastures, predation, and others (Martinka 1967, Compton 1970, Ellis 1970, Riddles and Oakley 1973, Barrett 1974, Pyrah 1978, etc.)

Adult pronghorns require an average of 2.5-3.0 pounds (1.1-1.4 kg) of air-dry forage per day, and these animals consume less than 1% of the forage produced on western rangelands (Wagner 1978). The amount of forage required by young animals is unknown. Pronghorn seldom suffer ill effect from eating normally toxic plants and, at times, apparently relish them (Yoakum and O'Gara 1990).

Although pronghorn often visit salt and mineral blocks, their mineral requirements and use remains unstudied (O'Gara and Yoakum 2004).

Parasites and Diseases: Extensive epizootics controlling pronghorn populations are uncommon. However, 33 species of roundworms, 21 genera of bacteria, 14 viral diseases, 8 species of protozoa, 5 species of tapeworms, and 4 species of ticks, one fluke, and a louse fly have been reported in or on pronghorn (Lance and Pojar 1984, O'Gara and Yoakum 2004a). The impact of most of these agents on free-ranging populations is unknown. Bluetongue, is the most serious disease affecting pronghorn; at least 3,200 pronghorn died from this disease in eastern Wyoming during 1976, and another 300 known deaths occurred in northeastern Wyoming during 1984 (Thorne et al. 1988). How important bluetongue might have been in early pronghorn population declines is unknown. Bever (1950) reported the loss of 30-60% of pronghorn fawns in South Dakota, primarily due to parasitic worms. Bever (1957) stated that, with proper management of domestic livestock, no parasite-caused mortality was found, but that rangelands overgrazed by domestic sheep resulted in high parasite loads in pronghorn in South Dakota.

Figure 12. Territorial bucks' scent-mark vegetation to warn other bucks entering the area. Territoriality ensures that bachelors generally do not harass or compete for forage with pregnant or lactating does on rangelands in good ecological condition. Bucks may have territorial or harem breeding habits, or both, during different years in the same location. Photo by Jim D. Yoakum

Longevity: Pronghorn seldom live more than 9 years under natural conditions (Einarsen 1948, Hepworth 1965), but Trainer et al. (1983) reported collected does living as long as 16 years as determined by tooth sectioning. In their northern range, pronghorn are susceptible to frequent winter-kill (Einarsen 1948). Winter-kill is rare in more southern ranges, but can be catastrophic when it does occur (White 1969). Advancing age and disease also reduces survival. Hunting is generally strictly regulated and most of the harvest is males (O'Gara and Morrison 2004). As a result, male survival is often less than that of females. Predation is not considered a primary factor in adult survival. Four major fawn predators are common throughout most of the pronghorn range: coyotes, bobcats, golden eagles, and domestic dogs. Mountain lions are a concern in rugged terrain in the Southwest (Ockenfels 1994a, b). Other mortality factors include poaching, crippling, road kills, toxic poisoning (Hailey et al. 1966), fence entanglement, parturition complications (Canon and Byrant 1992), starvation, drought, and accidents (Einarsen 1948).

Adult survival rates vary by gender and by area. Hunting management strategies, weather, density of cover, and ruggedness of terrain are significant factors affecting survival rates. Annual survival rates may be as low as 0.55 (Mitchell 1980) or approach 1.00 (Canon and Bryant 1992, Ockenfels 1994a, and O'Gara 2004a, 2004c).

Go to the next section:  II. MANAGEMENT RECOMMENDATIONS

• Population Surveys
• Harvest Management
• Population Models and Estimates

 

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