Pronghorn

European discovery

Pronghorns were first seen and described by Spanish explorers in the 16th century, but the species was not formally recorded or scrutinized until the expedition in 1804–1806 by Captain Meriwether Lewis and Second Lieutenant William Clark. Following the discovery of a few subspecies of the sharp-tailed grouse, Lewis and Clark came across the pronghorn near the mouth of the Niobrara River, in present-day Nebraska. Clark was among the first Euro-Americans to publish the experience of killing a pronghorn, and described his experience as follows:

I walked on shore to find an old Vulcanoe [the Ionia Volcano?] ... in my walk I killed a Buck Goat of this Countrey, about the height of the Grown Deer, its body Shorter the horns which is not very hard and forks 2⁄3 up one prong Short the other round & Sharp arched, and is immediately above its Eyes the Color is a light gray with black behind its ears down the neck, and its face white round its neck, its Sides and its rump round its tail which is Short & white; Verry actively made, has only a pair of hoofs to each foot, his brains on the back of his head, his Nostrals large, his eyes like a Sheep he is more like the Antilope or Gazelle of Africa than any other Species of Goat.

The pronghorn was first officially described by American ornithologist George Ord in 1815.

Description

Pronghorns have distinct white fur on their rumps, sides, bellies, breasts, and across their throats. Adult males are 4.3–4.9 ft (1.3–1.5 m) long from nose to tail, stand 2.7–3.4 ft (82–104 cm) high at the shoulder, and weigh 88–143 lb (40–65 kg). The females are the same height as males, but weigh 75–106 lb (34–48 kg). The feet have two hooves, with no dewclaws. Their body temperature is 100 °F (38 °C).

Head

They have very large eyes with a 320° field of vision. Their orbits (eye sockets) are prominent and set high on the skull. Their teeth are hypsodont and their dental formula is 0.0.3.33.1.3.3. Unlike deer, pronghorns possess a gallbladder.

Each horn of the pronghorn is composed of a slender, laterally flattened blade of bone, which is thought to grow from the frontal bones of the skull, or from the subcutaneous tissues of the scalp, forming a permanent core. As in the Giraffidae, skin covers the bony cores, but in the pronghorn, it develops into a keratinous sheath, which is shed and regrown annually. Males have a horn sheath about 4.9–17 in (12+1⁄2–43+1⁄4 cm) (average 9.8 in or 25 cm) long with a prong. Females have smaller horns that range from 0.4–2.4 in (1–6 cm) (average 4.7 in or 12 cm) and sometimes barely visible; they are straight and very rarely pronged.

Males are further differentiated from females in having a small patch of black hair at the angle of the mandible. Pronghorns have a distinct, musky odor. Males mark territory with a preorbital scent gland on the sides of the head.

Scent glands

The preorbital gland's secretion contains the highly odoriferous compound, 2-ethyl-3-methylpyrazine, which is also the major volatile component found on the animal's back in the male's medial gland.

Male and female animals have glands that are exposed when the white hair on the rump stands up. 2-Pyrrolidinone, the major compound in the rump gland, has an odor reminiscent of buttered popcorn to humans. The flared rump hair and odor alert adjacent animals of a possible danger.

Pronghorns have well developed glands on each hoof. Like many ungulates, these interdigital (hoof) glands contain chemical compounds that are known to have antimicrobial activity against soil and mammalian pathogens.

Movement

The pronghorn is the fastest land mammal in the Western Hemisphere, being built for maximum predator evasion through running. Their top speed is dependent upon the length of time over which it is measured. It can run 35 mph (56 km/h) for 4 mi (6.5 km), 42 mph (68 km/h) for 1 mi (1.5 km), and 55 mph (88.5 km/h) for 800 metres (0.5 mi). Although it is slower than the African cheetah, it can sustain top speeds much longer than cheetahs. The pronghorn may have evolved its running ability to escape from now-extinct predators such as the American cheetah, since its speed greatly exceeds that of all extant North American predators. Carbon and nitrogen isotope comparisons between pronghorn, horses, bighorn sheep, bison, American cheetahs, American lions, and wolves of the Natural Trap Cave found that while American cheetahs seemed to subsist on pronghorns, they did not do so exclusively. In fact, pronghorns were also important prey of American lions and wolves. Compared to its body size, the pronghorn has a large windpipe, heart, blood volume, erythrocites, and lungs to allow it to take in large amounts of air when running. Additionally, pronghorn hooves have two long, cushioned, pointed toes, which help absorb shock when running at high speeds. They also have an extremely light bone structure and hollow hair.

Male pronghorns tend to have a higher level of physical activity than females and apparently also have a greater blood volume relative to body size.

Pronghorns are built for speed, not for jumping. Since their ranges are sometimes affected by sheep ranchers' fences, they can be seen going under fences, sometimes at high speed. For this reason, the Arizona Antelope Foundation and others are in the process of removing the bottom barbed wire from the fences, and/or installing a barbless bottom wire.

The pronghorn has been observed to have at least 13 distinct gaits, including one reaching nearly 24 ft (7.3 m) per stride.

When a pronghorn sees something that alarms it, the white hair on the rump flares open and exposes two highly odoriferous glands that releases a compound described as having an odour "reminiscent of buttered popcorn." This sends a message to other pronghorns by both sight and smell about a present danger. This scent has been smelled by humans 65 to 100 feet downwind from alarmed animals. The major odor compound identified from this gland is 2-pyrrolidinone.

Range and ecology

Prior to the arrival of Europeans, the pronghorn was particularly abundant in the regions west of the Mississippi River which is still its primary range today. The amount of wildlife was considered to be so vast at one time that the prehistoric American Prairie—and as recently as 200 to 300 years ago—has been dubbed the "American Serengeti", due to the once-millions-strong herds of bison, elk, and pronghorn, as well as many other now-extinct megafauna.

The present-day range of the pronghorn is west of the Mississippi, extending from southern Saskatchewan and Alberta, Canada, south into the western United States, primarily in the states of Arizona, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming, and into Mexico. In extreme Northern California, pronghorns can be found in inland counties, ranging from neighboring Nevada and Oregon, as well as the central coastal grasslands, further south. In Mexico, the Sonoran pronghorn (A. a. sonoriensis) subspecies may be found from the state of Baja California Sur east through Sonora to San Luis Potosí, in north-central regions of the country, albeit in gradually diminishing populations. They have been extirpated from Iowa and Minnesota in the United States, and from Manitoba in Canada.

Other regional subspecies include the Rocky Mountain pronghorn (A. a. americana), Mexican pronghorn (A. a. mexicana), the Oregon pronghorn (A. a. oregona), and the critically endangered Baja California pronghorn (A. a. peninsularis).

Pronghorns prefer open terrain at elevations between 3,000 and 5,900 ft (910 and 1,800 m), with the densest populations in areas receiving around 10–16 in (254–406+1⁄2 mm) of rainfall per year. They eat a wide variety of plant foods, often including plants unpalatable or toxic to domestic livestock, though they also compete with them for food. In one study, forbs comprised 62% of their diet, shrubs 23%, and grasses 15%. Another study similarly found forbs comprised 20%, cacti 40%, shrubs 18%, and grass 22%. Pronghorns chew cud (ruminate).

Healthy pronghorn populations tend to stay within 3–4 mi (5.0–6.5 km) of a water source. The majority are found within 5 mi (8 km) of a water source.

An ongoing study by the Lava Lake Institute for Science and Conservation and the Wildlife Conservation Society shows an overland migration route that covers more than 160 mi (260 km). The migrating pronghorn start travel from the foothills of the Pioneer Mountains through Craters of the Moon National Monument to the Continental Divide. Dr. Scott Bergen of the Wildlife Conservation Society says "This study shows that pronghorns are the true marathoners of the American West. With these new findings, we can confirm that Idaho supports a major overland mammal migration—an increasingly rare phenomenon in the U.S. and worldwide."

Cougars (Puma concolor), wolves (Canis lupus), coyotes (Canis latrans), grizzly bears (Ursus arctos horribilis), and bobcats (Lynx rufus) are major predators of pronghorns. Golden eagles (Aquila chrysaetos) have been reported to prey on fawns and adults. Jaguars (Panthera onca) also likely prey on pronghorns in their native range in the southwestern United States and northern Mexico. In the Pleistocene, jaguars likely were dangerous to pronghorns as a short-range ambush predator.

Social behavior and reproduction

Pronghorns form mixed-sex herds in the winter. In early spring, the herds break up, with young males forming bachelor groups, females forming harems, and adult males living solitarily. Some female bands share the same summer range, and bachelor male bands form between spring and fall. Females form dominance hierarchies with few circular relationships. Dominant females aggressively displace other females from feeding sites.

Adult males either defend a fixed territory that females may enter, or defend a harem of females. A pronghorn may change mating strategies depending on environmental or demographic conditions. Where precipitation is high, adult males tend to be territorial and maintain their territories with scent marking, vocalizing, and challenging intruders. In these systems, territorial males have access to better resources than bachelor males. Females also employ different mating strategies. "Sampling" females visit several males and remain with each for a short time before switching to the next male at an increasing rate as estrus approaches. "Inciting" females behave as samplers until estrus, and then incite conflicts between males, watching and then mating with the winners. Before fighting, males try to intimidate each other. If intimidation fails, they lock horns and try to injure each other. "Quiet" females remain with a single male in an isolated area throughout estrus. Females continue this mating behavior for two to three weeks.

When courting a female in estrus, a male pronghorn approaches her while softly vocalizing and waving his head side to side, displaying his cheek patches. The scent glands on the pronghorn are on either side of the jaw, between the hooves, and on the rump. A receptive female remains motionless, sniffs his scent gland, and then allows the male to mount her.

Pronghorns have a gestation period of 7–8 months, which is longer than is typical for North American ungulates. They breed in mid-September, and the doe carries her fawn until late May. The gestation period is around six weeks longer than that of the white-tailed deer. Females usually bear within a few days of each other. Twin fawns are common. Newborn pronghorns weigh 4–9 lb (2–4 kg), most commonly 7 lb (3 kg). In their first 21–26 days, fawns spend time hiding in vegetation. Fawns interact with their mothers for 20–25 minutes a day; this continues even when the fawn joins a nursery. The females nurse, groom, and lead their young to food and water, as well as keep predators away from them. Females usually nurse the young about three times a day. Males are weaned 2–3 weeks earlier than females. Sexual maturity is reached at 15 to 16 months, though males rarely breed until three years old. Their lifespan is typically up to 10 years, rarely 15 years.

Relationship with humans

In regions inhabited by the Plains Indians tribes, as well as in the Northwest Plateau, pronghorn were hunted as a principal food source by local people. The pronghorn has also featured prominently in Native American mythology and oral history.

Merriwether Lewis and William Clark made several other observations on the behavior of pronghorns and how the local tribes hunted them. They described the animal, which they referred to as the "Antelope" or the "Goat", as follows:

Of all the animals we have seen the Antelope seems to possess the most wonderful fleetness. Shy and timorous they generally repose only on the ridges, which command a view of all the approaches of an enemy ... When they first see the hunters they run with great velocity ... The Indians near the Rocky Mountains hunt these animals on horseback, and shoot them with arrows. The Mandans' mode of hunting them is to form a large, strong pen or fold, from which a fence made of bushes gradually widens on each side. The animals are surrounded by the hunters, and gently driven towards this pen, in which they imperceptibly find themselves enclosed, and are then at the mercy of the hunters.

Population and conservation

At the turn of the 20th century, members of the wildlife conservation group Boone and Crockett Club had determined that the extinction of the pronghorn was likely. In a letter from George Bird Grinnell, Boone and Crockett Club chairman of the game preservation committee, to Walter L. Fisher, Secretary of the Interior, Grinnell stated, "The Club is much concerned about the fate of the pronghorn which appears to be everywhere rapidly diminishing." By the 1920s, hunting pressure had reduced the pronghorn population to about 13,000. Boone and Crockett Club member Charles Alexander Sheldon, in a letter to fellow member Grinnell, wrote, "Personally, I think that the antelope are doomed, yet every attempt should be made to save them." Although the club had begun their efforts to save the pronghorn in 1910 by funding and restocking the Wichita Game Refuge in Oklahoma, the National Bison Range in Montana, and the Wind Cave National Park, in South Dakota, most of the efforts were doomed since experience demonstrated that after initial increases the pronghorns would die off because of the fenced enclosures.

In 1927, Grinnell spearheaded efforts along with the help of T. Gilbert Pearson of Grinnell's National Audubon Society to create the Charles Alexander Sheldon Antelope Refuge in northern Nevada. About 2900 acres of land were jointly purchased by the two organizations and subsequently turned over to the Biological Survey as a pronghorn refuge. This donation was contingent upon the government's adding 30,000 acres of surrounding public lands. On June 20, 1929, United States President Herbert Hoover included the required public lands upon request of the Department of Agriculture and the Department of the Interior after learning that the Boone and Crockett Club and the National Audubon Society were underwriting the private land buyout. On January 26, 1931, Hoover signed the executive order for the refuge. On December 31, 1936, President Franklin Roosevelt signed an executive order creating a 222,000-hectare (549,000-acre) tract; this was the true beginning for pronghorn recovery in North America.

The protection of habitat and hunting restrictions have allowed pronghorn numbers to recover to an estimated population between 500,000 and 1,000,000 since the 1930s. Some recent decline has occurred in a few localized populations, due to bluetongue disease which is spread from sheep, but the overall trend has been positive.

Pronghorn migration corridors are threatened by habitat fragmentation and the blocking of traditional routes. In a migration study conducted by Lava Lake Institute for Science and Conservation and the Wildlife Conservation Society, at one point, the migration corridor bottlenecks to an area only 200 yards wide.

Pronghorns are now quite numerous, and outnumbered people in Wyoming and parts of northern Colorado until just recently. They are legally hunted in western states for purposes of population control and food. No major range-wide threats exist, although localized declines are taking place, particularly to the Sonoran pronghorn, mainly as a result of livestock grazing, the construction of roads, fences, and other barriers that prevent access to historical habitat, illegal hunting, insufficient forage and water, and lack of recruitment.

Three subspecies are considered endangered in all (A. a. sonoriensis, A. a. peninsularis), or part of their ranges (A. a. mexicana). The Sonoran pronghorn has an estimated population of fewer than 300 in the United States and 200–500 in Mexico, while there are approximately 200 Peninsula pronghorn in Baja California. Populations of the Sonoran pronghorn in Arizona and Mexico are protected under the Endangered Species Act (since 1967), and a recovery plan for this subspecies has been prepared by U.S. Fish and Wildlife Service. Mexican animals are listed on CITES Appendix I. Pronghorns have game-animal status in all of the western states of the United States, and permits are required to trap or hunt pronghorns.