Antilocapra americana

Geographic Range

Antilocapra americana is endemic to North America and distributed throughout the treeless plains, basins, and deserts of western North America, from the southern prairie provinces of Canada, southward into the western United States and to northern Mexico. Distribution of populations within this range is discontinuous. In 1959, a population was introduced to Hawaii. However, by 1983 the population was roughly 12 individuals and headed for extinction.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Pronghorns are primarily found in grassland, sage scrub or chapparal, and desert. The southern portion of their range consists mainly of arid grasslands and open prairies. Throughout the rest of their range they are common in sage scrub and chaparral as well, areas of dense shrubs with tough leaves. Pronghorns are particularly dependent on sage brush for forage in these areas. Pronghorn feed primarily on sage, forbs, and grasses. They have also been known to consume cacti in some areas. There is an overlap in forage preferences with domestic sheep and cattle, so some competition for food occurs. Overgrazing by sheep has been implicated in pronghorn die offs, especially in winter. Pronghorn habitat ranges from sea-level to about 3500 m. Their need for free standing, fresh water varies with the moisture content of the vegetation they consume. They may have to travel a great distance to find a water source. In winter, northern populations depend heavily on sage brush. Pronghorn are commonly found along wind-blown ridges where vegetation has been cleared of snow, although they will dig through snow with their hooves to get to vegetation.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• savanna or grassland savanna or grassland
• chaparral chaparral

• Other Habitat Features
• suburban suburban
• agricultural agricultural

Physical Description

Pronghorns are small ungulates with barrel-shaped bodies. Females stand 860 mm at the shoulder and males 875 mm at the should. Females are approximately 1406 mm in body length and males are approximately 1415 mm. The tail is up to 105 mm long and ears are up to 143 mm long. Their body weight is from 35 to 70 kg, depending on sex and age. Their hair is dense and very coarse and is air-filled, providing excellent insulation. Guard hairs are hollow and underlain by finer, shorter underfur. Guard hairs are erectile for heat regulation. As more air becomes trapped in fur, the more they are insulated from external temperatures. Their dorsal fur is a rufous brown and they have creamy underbellies, rumps, and neck patches. Males have short black manes on the neck, from 70 to 100 mm in length, as well as a neck patch and a black stripe that runs across the forehead from horn to horn. Females lack these black facial patches, but have a small mass of black hair around their nose. Their ears are small and point slightly inward at the tip. Pronghorns have a patch of white, erectile fur on their rumps that is visible at great distances. The mucous membranes and eyelashes are coal black. Southern populations are paler in overall color than northern populations. The horns are erect, with a posterior hook and a short anterior prong. The prong gives rise to the common name “pronghorn”. This pronged pattern is unique to this species. The horn is a keratinized sheath, black in color, and is deciduous. Horn sheaths grow over a bony extension of the frontal bone, which is now called the cancellous bone in ungulates. A new sheath forms under the old, which splits and is dropped just after the rut each year. Both sexes have horns, although the horns of females are generally small or absent, and never exceed ear length. Female horns average about 120 mm and the prongs are not prominent. The horn begins to grow at the age of six months and will be shed by 18 months. The maximum horn height for males will occur within 2 to 3 years of age and will average 250 mm, exceeding the length of the ear.

Pronghorn limbs are specialized for cursoriality, giving them enhanced speed and endurance. They are the fastest known New World mammal, traveling at speeds of 98 km/h when sprinting, and can hold a sustained speed of 59 to 65 km/h. The advantages to having speed and endurance include the ability to forage over large areas, to seek new food sources when familiar sources fail, and the ability to escape predators. Pronghorns have unguligrade foot posture, which lengthens the legs by allowing them to stand on the tips of their digits. The length of the radius bone is as long, or longer, than the femur. The ulna is reduced and partially fused to the radius. The clavicle in ungulates has been lost and the scapula has been reoriented to lie flat against the side of their chest where it is free to rotate roughly 20° to 25° in the same plane in which the leg swings. The ulna and radius have been reduced to eliminate the twisting and rotating of the elbow. The reduction of bone and associated muscles in the distal limbs decreases limb weight, giving them more speed. Pronghorns have modified their joints to act as hinges allowing only motion in the line of travel. This has been done by introducing interlocking spines and grooves in their joints. All these adaptations have made pronghorns excel in cursorial locomotion, but they can no longer jump because they have lost the suspension mechanism that cervids have. This explains their apparent fear of fences.

The dental formula of Antilocapra americana is 0/3-0/1-3/3-3/3, where incisors and canines only occur on the lower jaw. Pronghorns have hypsodont crown height; discernable roots do not occur, allowing the cheek teeth to be ever growing. An approximate age when the molars erupt varies slightly; the first comes in at 2 months and the second and third come in around 15 months of age. Replacement of incisors varies as the first is replaced at 15 months, the second at 27, and the third at 39 months. Canines are replaced between 39 and 41 months. Premolars are all replaced at 27 months of age. The sequence of tooth eruption, replacement, and wear is used to estimate the age of pronghorns. Cementum annuli analysis of the first permanent incisor is used for older age classes.

Maximal rate of oxygen intake in pronghorns determines the peak at which the animal can synthesize ATP by aerobic catabolism. This then determines how intensely the animal can exercise. Pronghorns are an extreme example of evolutionary specialization for high oxygen consumption. When comparing body weight to weight-specific consumption of oxygen, pronghorns have values three times higher than the that expected for their body size. This high oxygen consumption makes pronghorns Earth’s fastest sustained runner. Unlike cheetahs , also one of the fastest animals on Earth, pronghorns produce ATP required to run fast aerobically. They have exceptionally large lungs for their body size and exceptional abilities to maintain high rates of blood circulation.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• male more colorful
• ornamentation ornamentation

Reproduction

Pronghorns are polygynous. Males defend territories from March through the end of the rut in early October. They defend a small harem of females in their territories during that time. Males with territories that contain a water source and have topographic features that help them corner does, do better than males without those features in their territories. Depending on a female's body condition, she will search among territorial males for potential mates. This behavior may last for two to three weeks. Pronghorns have scent glands that emit pheromones to attract or identify mates. These pheromones are important to interactions between sexes. Scent glands are located on either side of the jaw, between the hooves, on the rump, and above the tail. The glands on the neck are larger in males and are thought to be associated with sexual interaction as they are more active during the rutting season. Before mating, a male will approach a female from behind and shake his head to emit pheromones to attract the female. Males also use scent gland secretions to mark tall grasses on territorial boundaries. Males also mark territories with scrapes where they urinate and defecate, using a stereotypes "sniff, paw, urinate, defecate" sequence that may be repeated. Male interactions can include some or all of the following: 1) staring, 2) vocalization by the territory holder (a decrescendo snort-wheeze), 3) approaching an intruder, which can be accompanied by head thrashing, sneezes, and teeth grinding, 4) interacting with an intruder, and 5) chasing, which can be for only a few meters or up to 5 km. Male use of the snort-wheeze vocalization is often accompanied by erection of the mane, rump patches, and the cheek patches. If an intruder does not run away, then the two males walk in parallel to each other in a slow, deliberate manner with their heads held low. If a fight occurs, the males thrust their horns at each other in an attempt to do injury. Males end up in horn-horn or head-head pushing battles in which they try to knock the other off balance. Fights average only about 2 minutes long, but often result in serious injury.

• Mating System
• polygynous polygynous

Breeding occurs from mid-September to October in northern parts of the pronghorn range and from July to October in southern parts of their range. Females ovulate from 4 to 7 ova at the time of mating. These ova quickly travel to the uterus and form blastocysts, where they absorb nutrition for almost a month before implantation. Blastocysts develop long, thread-like walls that begin to twist together and form knots. One quarter to one third of blastocysts die of malnutrition when this knotting reduces the membrane surface area. As many as 7 embryos may still survive this knotted blastocyst stage. However, as the embryos develop, distal embryos are forced into the oviduct, where they perish from lack of nutrition and are reabsorbed. The gestation period is about 252 days and births are synchronous, with all females giving birth within a few days of each other. Females give birth to one or two fawns in the spring, typically they have a single young in their first year of breeding and twins in subsequent years. Females often labor on their sides, but stand as the front legs of the fawn begin to emerge from the vulva. Females and their young form bands in the summer that roam over the territories of one to several males. Pronghorns have 4 inguinal mammary glands. Young are partially weaned by 3 weeks old, at which point they begin to eat vegetation as well. Most female pronghorns breed in their second year, at about 16 months old, although some females can breed as early as 5 months old. Males can breed in their first year, but rarely do because older, dominant males monopolize breeding opportunities. Males typically begin to breed in their third year.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• viviparous viviparous

Female pronghorns care for their young from 1 to 1.5 years after birth, after which the young will become independent. At the time of birth, the mother will consume the afterbirth to prevent detection by predators. She also consumes any excrement of the young for the first few weeks of their life to prevent detection by predators. For several days after birth young are weak and unable to keep the pace with adults, so mothers and young rest near a source of water until they gain their strength. Females leave their young in a hidden location in vegetation while they forage, but remain within two miles of them. Within minutes after birth, young pronghorns can stand on their own and they nurse within 2 hours. Within days of birth, young pronghorns can outrun a human and begin to travel and forage with their mother and other females and young in summer bands. Siblings are generally on their own until they begin to travel with their mother. Fawns play extensively in the summer herds, developing strength and dexterity. Male pronghorns do not help in raising offspring.

• Parental Investment
• precocial precocial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • female
  • protecting
    • female
• pre-independence
  • provisioning
    • female

Lifespan/Longevity

Female pronghorn have been aged at 16 years in the wild, though they seldom live past 9 years. The most common causes of death are predators, hard winters with deep snow, lack of water, and hunting or car collisions. Pronghorns have been recorded living 11 years in captivity.

Behavior

The timing, length, and pattern of seasonal movements varies regionally. In general, pronghorns form large wintering herds and disperse in the spring. In the summer smaller herds are formed of up to 12 individuals. Young males form bachelor herds and older males claim territories that they defend against other males, typically from late March to October. Pronghorns may move up to 160 km from winter ranges to avoid very deep snow. Natural barriers, such as rivers and mountain ranges restrict movement and contribute to the discontinuous nature of their distribution. Manmade barriers (fences, highways, railroads) now have a significant impact on movements and reduce the carrying capacity of rangelands as pronghorn are forced to move longer distances to find everything they need.

• Key Behaviors
• cursorial
• diurnal diurnal
• nocturnal nocturnal
• motile motile
• migratory migratory
• territorial territorial
• social social
• dominance hierarchies dominance hierarchies

Home Range

Pronghorns in Wyoming were estimated to range throughout areas of 2.6 to 5.2 square kilometers in summer and early autumn. Daily ranges were from 0.2 to 0.4 square kilometers. Territories are widely spaced and do not overlap. Winter ranges are larger, 6.5 to 22.5 square kilometers. Home range size is strongly affected by local conditions and climate, which varies yearly.

Communication and Perception

Doe-fawn recognition seems to be through a combination of visual, vocal, and olfactory cues. Scent glands are widely used in male-male and male-female behavioral interactions. Scent glands are used to mark territories, attract potential mates, identify a mate, alert danger, or deter other males intruding in their territory. Both sexes have rump glands and interdigital glands; males also have a gland below each ear and on the back.

• Communication Channels
• visual visual
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• pheromones pheromones
• scent marks scent marks

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Pronghorns are herbivores, eating stems, leaves, grasses and shrubs. Pronghorns have been described as "dainty" feeders, feeding on small amounts of a wide variety of plants. Particularly important in their is browse, especially sagebrush in winter. Pronghorns on grasslands have been observed starving in winter, whereas nearby populations in sagebrush survive well. Forbs with high water content are preferred in the summer diet and grasses are generally eaten only when there is new growth. Cacti are also eaten to some extent, especially in southern populations. Pronghorns use foregut fermentation with rumination to break down cellulose. Their stomach is enlarged and compartmentalized into four chambers, as in other ruminants. Water consumption varies with the water content of the vegetation available locally. When tender leaves are available, with moisture content of 75% or more, pronghorns do not seem to need to drink free-standing water. In dry seasons or areas, pronghorns are typically found within 5 to 6 km of water and may drink up to 3 liters per day.

Pronghorns must compete with introduced cattle ( Bos taurus ) and sheep ( Ovis aries ) throughout most of their range. In some areas, pronghorn are excluded from areas used by sheep, because the sheep eliminate much of their preferred vegetation. In other areas pronghorn and sheep seem to be able to coexist well. However, pronghorns can do well on areas overgrazed by cattle because they prefer forbs and browse. It is estimated that 1 cow can eat as much as 38 pronghorns. Fences constructed to enclose cattle and sheep can prevent pronghorn movement across rangeland, resulting in starvation and dehydration. Pronghorns can be considered a valuable part of rangeland management because they eat noxious weeds.

• Primary Diet
• herbivore herbivore
  • folivore folivore

• Plant Foods
• leaves
• roots and tubers
• wood, bark, or stems

Predation

Fawns or weaker pronghorns are preyed on by coyotes, bobcats, wolves, mountain lions, golden eagles, and other predators within their range. Pronghorns can use their horns to help defend themselves, but they primarily use their speed to escape predators. They are capable of sprints up to 86 km per hour and sustained speeds of 59 to 65 km/hr, making them one of the fastest land mammals. Pronghorns also use their feet in fighting off predators. They have keen eyesight and can spot an object from approximately two miles away. Pronghorns are curious animals and will move towards an intruder until they can detect what it is. If they determine that it is a threat, they will flee. When disturbed, pronghorns erect the white fur on their rumps, which acts to warn others of a disturbance.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Throughout their range pronghorns co-occur with cattle , bison , sheep , and horses . Pronghorns can improve rangeland quality for these other speces by eating noxious weeds or invasive plants. Introduced livestock may overgraze areas they share with pronghorn, thus reducing cover and quantity of food. The reduction of cover may increase young mortality through predation.

Although there are few epizootic diseases that strongly affect pronghorn populations there are 33 species of roundworms, 21 genera of bacteria, 14 viral diseases, 8 species of protozoa, 5 species of tapeworms, 4 species of ticks, one fluke, and a louse fly that are known to infect them. "Bluetongue" disease has resulted in extensive mortality in some cases. It is an insect-borne viral disease (Bluetongue virus, BTV) that is transmitted by midges ( Culicoides imicola ). Worm infections have also resulted in extensive fawn mortality in some areas. Pronghorns that co-occur with sheep tend to have higher parasite loads than those in areas without sheep. Pronghorns are the definitive host of a nematode worm that also infects sheep and mule deer : Pseudostertagia bullosa (Nematoda: Trichostrongyloidea). They can also be parasitized by meningeal worms ( Parelaphostrongylus tenuis ) that are common parasites of white-tailed deer .

Economic Importance for Humans: Positive

Pronghorns are an important big games species in the western United States. Their use of open habitat means often hunters have success rates of up to 90 percent.

• Positive Impacts
• food food
• body parts are source of valuable material
• research and education

Economic Importance for Humans: Negative

Pronghorns are grazers that will take advantage of wheat or alfalfa fields during the winter if there is deep snow. This may negatively impact crop yield. However, most pronghorn populations occur in areas with little agricultural development.

• Negative Impacts
• crop pest

Conservation Status

It is estimated that up to 35 million pronghorns lived in North America before colonization by western Europeans. By 1924 this number had decreased to less than 20,000. Pronghorn populations have increased since that time and are now considered the second most numerous game species in North America.

The IUCN Red List lists Antilocapra americana as lower risk/least concern. Populations are stable, widespread, and relatively common throughout most of their range, with an estimated population size of 0.5 to 1 million. The U.S. Endangered Species Act recognizes two populations as endangered: Sonora pronghorns ( A. a. sonoriensis ) and peninsular pronghorns ( A. a. peninsularis ). Populations of Sonoran pronghorn in Mexico have been protected since 1967 and have undergone several recovery plans, the most recent in 1998. This population of pronghorn is listed under the Convention of International Trade of Endangered Flora and Fauna (CITES) Appendix I.

Other Comments

Common names for Antilocapra americana include pronghorn, pronghorn antelope, and berrendo (Spanish). Antilocapra americana has also been known by the synonyms Antilope americanus , Antilope (Dicranocerus) furcifer , and Antilocapra anteflexa . There are currently five subspecies recognized: American pronghorns ( A. a. americana Ord), Oregon pronghorns ( A. a. oregona Bailey), Mexican pronghorns( A. a. mexicana Merrian), peninsula pronghorns ( A. a. peninsularis Nelson), and Sonoran pronghorns ( A. a. sonoriensis Goldman).

The fossil record dates to the Miocene. Antilocapra americana is the only extant species of a group that was once much more diverse, with 13 extinct genera of antilocaprines known from the Pliocene throughout the current range of A. americana .

Additional Links

Encyclopedia of Life

Contributors

Kandace Krejci (author), University of Alaska Fairbanks, Tanya Dewey (author, editor), Animal Diversity Web, Link E. Olson (editor, instructor), University of Alaska Fairbanks.

References

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