Pronghorn

(Antilocapridae)

Class Mammalia

Order Artiodactyla

Suborder Ruminantia

Family Antilocapridae

Thumbnail description
Small ruminant characterized by large head, elongated nose, large eyes and ears, heavily built body, long and slim but powerful limbs, and short tail

Size
Total length: 52.1–58.8 in (132.3–149.4 cm); shoulder height: 32.7–37 in (83.1–94 cm); for extant species: 87–129 lb (40–59 kg)

Number of genera, species
1 genus; 1 species

Habitat
Grassland, steppe, and desert plains

Conservation status
Not threatened

Distribution
Southeastern Alberta and southwestern Saskatchewan in Canada; in the 18 westernmost states (except Washington) of the United States; and in the states of Baja California Sur, Chihuahua, Coahuila, and Sonora, Mexico

Evolution and systematics

Two subfamilies, with five genera of Merycodontinae and 12 of Antilocaprinae, generally are recognized but researchers have not successfully divided them into species. Five subspecies have been named because of minor differences in color, size, and form. Mitochondrial DNA analyses since the early 1990s, plus intergrade zones among named subspecies, support the concept of clines within a wide-ranging species more than that of subspecies. Currently the subfamily Atilocapridae contains just one genus with one species.

The taxonomy for this species is Antilocapra americana (Ord, 1815), plains and highlands of the Missouri River, United States.

Physical characteristics

Extant antilocaprids (as were extinct antilocaprids) are long-legged runners. Merycodonts and early antilocaprins were small for ruminants: about 20 in (51 cm) high at the shoulder and weighing 26–44 lb (12–20 kg). Larger antilocaprins varied from that size to the size of pronghorn.

The pronghorn head, which varies from creamy white on the sides to various shades of brown on top, is marked in the male with brownish black patches that start just below the ears and extend downward 3–4 in (7.6–10.2 cm). Blackish markings sometimes occupy the entire face below a line connecting the horns. The throat is elegantly marked with a white crescent above a white shield; the point of the shield usually joins the white underparts at the base of the neck. Erectile hairs of the mane on the back of the neck are russet, tipped with black. A tan strip connects or nearly connects the upper surface of the short tail, about 4 in (10.2 cm) long, with the colored back. Rusty brown to tan hairs cover the back, most of the neck, and the outer sides of the limbs. White hair extends well up the sides of the body, forming a rectangular area between shoulder and hip. White hairs of twin rump patches are about 3 in (7.6 cm) long and almost gleam when erected, fanning beyond the normal contours of the body. The stalks of hairs are pithy with large central air cells that provide excellent

protection from wind and cold, and a pronghorn can regulate the insulating properties of its pelage by erecting or flattening hair.

The pronghorn is a plastic species with a variable number of chromosomes. Occasional individuals have extra horns, teats, or ribs. Females may or may not have horns, and some does have a different type of uterus than those possessed by most. The prongs that give pronghorn their name are unique, as is the annual shedding of horn sheaths. Normally, all adult bucks, and about 70% of adult does, have horns. The horns of does are small. In relation to its size, the pronghorn has the largest eyes of any North American ungulate. Pronghorn feet are long and slim and lack dewclaws common to most ruminants.

Distribution

A map of pronghorn distribution was developed in 1999, based on reports received from provincial and state wildlife agencies in Canada and the United States. Locations in Mexico were taken from a variety of publications. In 1909, E. T. Seton produced an outer delineation of what he called "primitive" range. That line agrees quite well with present knowledge concerning former distribution of the species. However, a good deal of the land within the outer delineation (high mountains, heavy forest, tall shrublands, etc.) was not pronghorn habitat.

Habitat

This small ruminant is a selective feeder adapted to succulent, high-protein vegetation. Thus, under most conditions, it does compete much with more generalized feeders such as bison (Bison bison) or cattle (Bos taurus). Excellent eyesight and great speed adapt the pronghorn to a "see and flee" existence on plains with short vegetation.

Pronghorn are most numerous in short-grass prairies where they can reach shrubs for forage when the ground is covered with snow. The next most important habitat is steppe, preferably with vegetation ranging 5–30 in (13–76 cm) in height. Deserts support less than 1% of the population.

Pronghorn are associated most frequently with treeless, undulating, or flat terrain. Herds range from near sea level to 11,000 ft (3,353 m) above sea level, but the greatest concentrations occupy landscapes 3,000–8,000 ft (914–2,438 m) above sea level.

Behavior

Pronghorn herd together for protection (especially of fawns) from predators, seasonal movements, and to feed on the best available forage during winters. Because the animals can maintain visual contact over considerable distances and come together quickly when threatened, pronghorn bands are well organized even when widely scattered. Erection of gleaming white rump patches and alarm snorts quickly alert the scattered animals.

Pronghorn produce a variety of vocalizations and mechanical sounds. Both sexes snort or blow through their noses

(sounding much like cha-oo) when they note something strange in their habitat. Fawns produce soft, mew-like bleats, which help does locate their concealed offspring. Distressed fawns and sometimes adults emit loud bleats. Does emit low grunts or clicking sounds when approaching their hidden fawns or when pursued by bucks. The buck roar sounds like a loud stomach growl and probably is caused by exhaling air. It is used principally during chases of either bucks or does. The snort-wheeze is a strictly male vocalization (a typical cha-oo, followed by a series of shorter bursts of sound descending in pitch and volume). Courting males make three distinct sounds: one is vocal, while the other two result from movements of the mouth. The pitch of the whine starts high and then smoothly decreases. If a doe moves away, the whine grades into the roar. During courtship approaches, bucks lip smack or tongue flick, both of which create a low, staccato, sucking sound.

Marking by territorial pronghorn bucks is accomplished during a linked sniff-paw-urinate-defecate (SPUD) sequence. Postural changes are extreme and each element of the sequence is performed in exaggerated form. A buck performing the SPUD precedes urination by pawing and, sometimes, by flehmen (lip curling). He then moves each front foot ahead to take an extended stance, urinates on the pawed spot, then moves his hind feet ahead, squats, and defecates. The feces fall on or close to the urine spot.

Nearly equal numbers of studies have described pronghorn bucks as territorial (defending a breeding area) and as having a harem-type mating system in which dominant bucks controlled and defended does without regard to specific locations. Breeding systems obviously have an ecological basis. When the best resources are clumped, pronghorn tend to be territorial, and bucks holding the best territories do most of the breeding. The breeding system shifts to harem formation as resources become more uniform, when population levels are low, or when sex ratios are unusually skewed (ratios of one male to 10 or more females). According to C. R. Maher, "Spacing systems are affected as much by the animals' internal state, i.e., physiology, as by external conditions, e.g., population density and food abundance. Physiological condition is reflected in metabolic processes and hormone levels, and hormones have important effects on reproduction and aggression."

Pronghorn sleep often, but without continuity or regularity, and usually for short periods. Activity is greatest just after sunrise and before sunset. Pronghorn spent the majority of their time feeding and the next greatest proportion of time reclining and ruminating. It has been observed that their time upright almost equals time reclining and greatly exceeds both feeding and reclining time for bucks during the rut. Time upright also exceeds that for reclining, and almost equals feeding for does during the rut.

Sizes of home ranges seemingly should reflect amounts of activity. However, large variations in sizes of pronghorn home

ranges result from differing habitat quality, population and group sizes, density, past history of land use, and season. Thus, sizes of pronghorn home and seasonal ranges and distances moved daily or seasonally vary so much with these factors and weather conditions that results from studies in one area seldom have application to another area, or even to another year.

As reported in 1927, pronghorn of Jackson Hole traveled 150 mi (240 km) southward to the Red Desert. Those on the prairies of Saskatchewan moved 100 mi (160 km) south and west. Those on the plains near the Rockies went toward the foothills, and those on open prairies around the Black Hills flocked there from all directions. Observations of these movements between Jackson Hole and the Red Desert were later confirmed by radio-tracking.

Because environmental factors can influence or trigger moves, the timing of movements is variable. Fences, interstate highways, reservoirs, and railways complicate movements and reduce, often drastically, the carrying capacity in areas where pronghorn must move long distances to procure the year-round necessities of life. Few truly migratory herds remained by the beginning of the twenty-first century.

Feeding ecology and diet

Pronghorn prefer succulent forbs to other foods. An adequate supply of forbs during spring and early summer generally assure a good fawn crop. During snowy winters and periods of droughts, the animals depend on shrubs, mostly sagebrush (Artemesia spp.)

Feeding pronghorn are described as walking slowly, occasionally pausing and holding their noses just above or brushing through vegetation. Food items appear to be found by scent as well as sight. Individuals commonly pass their noses along the length of a forb and then remove one or two leaves from the plant. At other times, individuals temporarily specialize in single items such as flowers, walking briskly from plant to plant, obviously guided by vision. Pronghorn use their mobile and dexterous upper lips to select plant parts, draw them into their mouths, and hold them in place until cut free by the incisiform teeth, which are jerked forward and upward by movements of the head.

Reproductive biology

Pronghorn does usually breed for the first time when 16–17 months of age, but fawns only occasionally conceive. The gestation period of approximately 250 days is unusually long for so small a ruminant. About 98% of does on good habitat produce twins. Southern pronghorn have longer rutting periods that begin earlier than those of animals on northern prairies. Northern pronghorn and those in Oregon generally breed during mid- to late September. All breeding seasons are timed so that fawns are born when nutritional content of forage is best for the does.

Pronghorn are polygynous, and a few robust bucks often do most of the breeding. Breeding and birthing generally take place in prime habitat, where the most succulent forbs are available. After giving birth, a doe licks the entire fawn, but grooming of the anogenital region soon takes precedence over

generalized licking and continues after each suckling bout until the fawn is two to three weeks old. During such grooming bouts, fawns assume a distinctive rump-up posture and eliminate, while the mother ingests the urine and feces. Fawns hide for the first three weeks of life and their dams feed them about three times a day. This is a predator-avoidance behavior involving both does and fawns.

Conservation status

Pronghorn are endemic to western North America and are common in suitable habitat. Numbers were reduced to less than 30,000 early in the twentieth century. The population rebuilt to more than a million animals by the early 1980s—perhaps the carrying capacity for the reduced habitat available.

The U. S. Fish and Wildlife Service has listed as Endangered two subspecies, the peninsular pronghorn in Baja California Sur, Mexico, and the Sonoran pronghorn in Arizona and Sonora, Mexico. The federal government of Mexico has listed all endemic subspecies in Mexico as Endangered. For the total pronghorn population, unrestricted hunting and competition with millions of domestic sheep (Ovis aries) reduced numbers. Managed hunting, protection, and reduction in sheep numbers helped restore the population.

Peninsular and Sonoran pronghorn persist in deserts where numbers were low even before the advent of European-Americans. Since then, desertification has put these animals in serious danger of extinction. The mexicana subspecies has

suffered from habitat loss, desertification, and poaching. During the last 20 years of the twentieth century, the total number of pronghorn has varied from more than a million to about 700,000. Droughty summers and severe winters reduced numbers during some years.

A recovery plan for pronghorn in Mexico was completed in 1999. It contained an estimate of 1,200 pronghorn in all of Mexico. A 1998 U.S. Fish and Wildlife Service recovery plan for Sonoran pronghorn estimated fewer than 300 of that subspecies in the United States.

Significance to humans

For many Native Americans within historic times, pronghorn mainly personified speed, grace, vigilance, good fortune, and peace. Indian myths were used to explain how pronghorn originated, why they lived on the plains, why they had a gallbladder while the deer did not, etc.

Apparently because of their beauty, paintings of pronghorn adorned prehistoric pottery and kiva walls in what is now New Mexico. Later, cowboy artist, Charlie Russell, and others frequently featured pronghorn in their western paintings.

The importance of pronghorn in late prehistoric Native American economies depended on their abundance and the availability of other food resources. Over wide areas, the abundant bison was overwhelmingly the major food source of Plains Indians. However, pronghorn contributed to dietary diversity as well as providing hides, horns, etc., for clothing and tools. They were worthy of large cooperative hunting efforts yielding up to hundreds of animals at a time.

After the near demise of bison in the late nineteenth century when rifles became readily available, pronghorn were common fare for both red and white peoples throughout the West. Great numbers of pronghorn were slaughtered for their skins, which were dried and sold by the pound.

As pronghorn increased in numbers during the 1930s, Canadian provinces and U. S. states opened hunting seasons. By the end of the hunting season in 2002, nearly five million pronghorn had been legally harvested. The harvest provided many tons of meat and days of outdoor recreation besides millions of dollars of profit for businesses throughout pronghorn country.

Pronghorn occasionally damage crops, especially alfalfa (Medicago sativa), extensively. Wildlife agencies sometimes pay for such damage or increase hunting pressure to reduce the number of animals.

Resources

Books

Byers, J. A. American Pronghorn: Social Adaptations and the Ghosts of Predators Past. Chicago: University of Chicago Press, 1997.

Geist, V., and M. H. Francis. Antelope Country, Pronghorns: The Last Americans. Iola, WI: Krause Publications, 2001.

Hill, R. B. Hanta Yo. Garden City: Doubleday and Co., 1979.

O'Gara, B. W., and J. D. Yoakum. Ecology and Management of Pronghorn. Washington, DC: Wildlife Management Institute, in prep.

Seton, E. T. Life-histories of Northern Animals. New York: Charles Scribner's Sons, 1909.

——. Lives of Game Animals. Garden City: Doubleday, Doran and Co., Inc., 1927.

Turbak, G. Portrait of the American Antelope. Hong Kong: Northland Publishing, 1995.

Periodicals

Heffelfinger, J. R., B. W. O'Gara, C. M. Janis, and R. Babb. "A Bestiary of Ancestral Antilocaprids." Proceedings of the Pronghorn Workshop 20 (2002).

Maher, C. R. "Quantitative Variation in Ecological and Hormonal Variables Correlates with Spatial Organization of Pronghorn (Antilocapra americana) Males." Behavioral Ecology and Sociobiology 47, no. 5 (2000): 327–338.

Organizations

The Arizona Antelope Foundation. P.O. Box 15501, Phoenix, AZ 85060 United States. Phone: (602) 433-9077. E-mail: [email protected] Web site: <http://www.azantelope.org>

North American Pronghorn Foundation. P.O. Box 1383, Rawlins, WY 82301 United States. Phone: (307) 324-5238. E-mail: [email protected] Web site: <http://www.antelope.org>

Bart O'Gara, PhD