The pronghorn (Antilocapra americana) is a species of artiodactyl mammal endemic to interior western and central North America. Though not an antelope, it is often known colloquially in North America as the prong buck, pronghorn antelope, or simply antelope, as it closely resembles the true antelopes of the Old World and fills a similar ecological niche due to convergent evolution. It is the only surviving member of the family Antilocapridae. During the Pleistocene period, 12 antilocaprid species existed in North America. About five existed when humans entered North America and all but A. americana are now extinct.
Adult males are 1.3–1.5 m (4 ft 3 in–4 ft 10 in) long from nose to tail, stand 81–104 cm (32–41 in) high at the shoulder, and weigh 40–65 kg (88–140 lb). The females are the same height as males but weigh 34–48 kg (75–110 lb). The feet have just two hooves, with no dewclaws. The body temperature is 38 °C (100 °F).
Each "horn" of the pronghorn is composed of a slender, laterally flattened blade of bone that grows from the frontal bones of the skull, 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 on an annual basis. Unlike the horns of the family Bovidae, the horn sheaths of the pronghorn are branched, each sheath possessing a forward-pointing tine (hence the name pronghorn). The horns of males are well developed.
Males have a prominent pair of horns on the top of the head, which are made up of an outer sheath of hairlike substance that grows around a bony core; the outer sheath is shed annually. Males have a horn sheath about 12.5–43 cm (4.9–17 in) (average 25 cm (9.8 in)) long with a prong. Females have smaller horns, ranging from 2.5–15 cm (1–6 in) (average 12 centimetres (4.7 in)) and sometimes barely visible; they are straight and very rarely pronged. Males are further differentiated from females in that males will have a small patch of black hair at the angle of the mandible. Pronghorns have a distinct, musky odor. Males mark territory with a scent gland located on the sides of the head. They also have very large eyes, with a 320 degree field of vision. Unlike deer, pronghorns possess a gallbladder.
It can run exceptionally fast, being built for maximum predator evasion through running, and is generally accepted to be the fastest land mammal in the Western Hemisphere. The top speed is very hard to measure accurately and varies between individuals; it can run 35 mph for 4 mi (56 km/h for 6 km), 42 mph for 1 mi (67 km/h for 1.6 km); and 55 mph for .5 mi (88.5 km/h for .8 km). It is often cited as the second-fastest land animal, second only to the cheetah. It can, however, sustain high speeds longer than cheetahs. University of Idaho zoologist John Byers has suggested that the pronghorn evolved its running ability to escape from extinct predators such as the American cheetah, since its speed greatly exceeds that of extant North American predators. It has a very large heart and lungs, and hollow hair. Although built for speed, it is a very poor jumper. Their ranges are often affected by sheep ranchers' fences. However, 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 barb-less bottom wire.
The pronghorn has been observed to have at least 13 distinct gaits, including one reaching nearly 7.3 m (8 yards) per stride.
Range and ecology
Pronghorns were brought to scientific notice by the Lewis and Clark Expedition, which found them in what is now South Dakota. The range extends from southern Saskatchewan and Alberta in Canada south through the United States (southwestern Minnesota and central Texas west to coastal southern California and northern Baja California Sur, to Sonora and San Luis Potosí in northern Mexico.
The subspecies known as the Sonoran Pronghorn (Antilocapra americana sonoriensis) occurs in Arizona and Mexico. Other subspecies include the Mexican Pronghorn (A. a. mexicana), the Oregon Pronghorn (A. a. oregona), and the critically endangered Baja California Pronghorn (A. a. peninsularis).
Pronghorns live primarily in grasslands but also in brushland and deserts. They eat a wide variety of plant foods, often including plants that are unpalatable or toxic to domestic livestock (sheep and cattle) though they also compete with these for food. In one study forbs comprised 62% of the diet, shrubs 23%, and grasses 15%, while in another, cacti comprised 40%, grass 22%, forbs 20%, and shrubs 18%. 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 miles. 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 Wildlife Conservation Society says, "This study shows that pronghorn 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, wolves, coyotes and bobcats are the major predators. Golden Eagles have been reported to prey on fawns.
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 their groups and adult males living a solitary life. There are female bands which share the same summer range and bachelor male bands form between spring and fall. Females form dominance hierarchies with few circular relationships. Dominant females will aggressively displace other females from feeding sites.
Adult male pronghorns employ two different mating strategies during the breeding season. A pronghorn male will defend a fixed territory that females may enter or it might defend a harem of females. A pronghorn may change mating strategies depending on environmental or demographic conditions. In areas that have high precipitation, adult male pronghorn 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 will visit several males and remain with each for a short time before switching to the next male, at an increasing rate as oestrus approaches. "Inciting" females will behave as samplers until oestrus and then incite conflicts between males. The females watch and then mate with the winners. "Quiet" females will remain with a single male in an isolated area throughout oestrus.
When courting an estrous female, a male pronghorn will approach her while softly vocalizing and waving his head side to side, displaying his cheek patches. A receptive female will remain motionless and sniff his scent gland and then allow the male to mount her. When mating, the tip of a male pronghorn's penis is often the first part to touch the female pronghorn. The penis is about 5 inches long, and is shaped like an ice pick. The front of the glans penis is relatively flat, while the back is relatively thick. The male pronghorn usually ejaculates immediately after intromission.
Pronghorns have a gestation period of 235 days, longer than is typical for North American ungulates. They breed in mid-September, and the doe carries her fawn until late May. This is around six weeks longer than the white-tailed deer. Newborn pronghorns weigh 2–4 kg, most commonly 3 kg. In their first 21–26 days, a fawn spends time hiding in vegetation. Fawns interact with their mothers for 20–25 minutes a day and 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. Males are weaned 2–3 weeks earlier than females. Sexual maturity is reached at 15 to 16 months, though males rarely breed until 3 years old. The longevity is typically up to 10 years, rarely 15 years.
Population and conservation
By the 1920s, hunting pressure had reduced the pronghorn population to about 13,000. Protection of habitat and hunting restrictions have allowed their numbers to recover to an estimated population of between 500,000 and 1,000,000. There has been some recent decline in a few localized populations, due to blue tongue disease which is spread from sheep; however the overall trend has been positive since conservation measures were put in place.
Pronghorn migration corridors are threatened by habitat fragmentation and the blocking of traditional migration 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. There are no major range-wide threats, although localized declines are taking place, particularly to the Sonoran pronghorn, mainly as a result of, among others, 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). Populations of the Sonoran pronghorn in Arizona and Mexico are protected under the US Endangered Species Act (since 1967), and a recovery plan for this subspecies has been prepared by USFWS (U.S. Fish and Wildlife Service 1998). 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 shoot pronghorns.
A. americana, Pronghorn
A. a. americana, Common Pronghorn A. a. mexicana, Mexican Pronghorn A. a. oregona, Oregon Pronghorn A. a. peninsularis, Baja California Pronghorn A. a. sonoriensis, Sonoran Pronghorn