Pronghorn, Antilocapra americana
TAXONOMY & NOMENCLATURE
(Byers, 1997)(Davis 2007)(Hernandez Fernandez et al 2005) (Murray 2006)
(Nowak 1994)(O’Gara & Yoakum 2004)(Janis 2000)
Ord (1815), as Antilocapra americana
Order: Artiodactyla* (Even-toed hoofed animals: includes pigs, sheep goats, cattle, deer)
Species: Antilocapra americana
Subspecies: A. a. americana, A. a. mexicana, A. a. sonoriensis, A. a. peninsularis
*New anatomical and DNA evidence on the relationship between Artiodactyla (even-toed ungulates) and Cetacea (whales and dolphins) recently led to a merging of the two orders into a new group, Cetartiodactyla (Montgelard, 1997; reviewed in Kulemzina, 2009). As of October 2012, experts had not agreed on whether to define Cetartiodactyla as an official taxonomic order that would replace Artiodactyla and Cetacea. Some continue to list camels in the order Artiodactyla (Franklin, 2011) or use the term Cetartiodactyla without defining it as an order (IUCN, 2008).
Taxonomic History and Nomenclature
Antilo = antelope; capro = male goat
- Pronghorn are the only surviving genus of the North American ungulate family Antilocapridae.
- The family to which the pronghorn belongs was originally divided into two subfamilies (one subfamily for extinct Miocene merycodonts and another group of all the other extinct forms plus the modern pronghorn). These distinctions are probably not valid. (Davis 2007)
- Debate remains over whether pronghorn belong in their own family or should be classified in Bovidae (O’Gara & Matson 1975), in Cervidae (Janis & Scott 1987). Recent genetic study says they are closer to Giraffidae forming a sister group to cervids and bovids. (Hernandez et al 2005; Murray 2006)
- Like bovids, pronghorn have horns consisting of a keratinous sheath over a bony core. But unlike bovids whose horn core is permanent, the horn sheath is shed annually and re-grown. Deer antlers don't have sheaths, but the antlers are lost and regrown yearly.
- Five subspecies were named because of minor differences in color, size, and location (O’Gara & Yoakum 2004):
- Antilocapra americana americana – type specimen
- A. a. oregona – DNA analysis determined this is not genetically distinct from americana, therefore populations in Oregon, California, Idaho, and Nevada should be considered americana (Lee, 1992)
- A. a. mexicana – more pale coloration, reduced mane, thinner muzzle than americana
- A. a. peninsularis – facial markings darker, shorter and rougher horns
- A. a. sonoriensis – smallest subspecies, smaller skull, but little difference from mexicana
- Cockrum (1981) felt that geographic variations between the populations do not warrant separate subspecific names.
- Mitochondrial DNA analyses since the early 1990s support the idea of clines within a wide-ranging species. (O’Gara and Yoakum, 2004).
- At present, subspecies are still used pending further study.
- Antilocaprid ancestors most likely evolved in the Old World and migrated to North American during the Late Oligocene. (Davis 2007)
- First antilocaprids seem highly specialized when they appeared in the fossil record, making it difficult to identify an Old World ancestor (O’Gara and Yoakum, 2004).
- Forked or branching horns are a distinguishing feature of the Antilocaprid family. (Davis 2007)
- Pronghorn evolved alongside predators such as dire wolves, the short-faced bear, the North American lion, jaguars, hyenas, and cheetahs; pronghorn were most likely prey for these species (Byers 1997).
- Existing pronghorn’s running speed and endurance are far greater than any living predator; likely that the species evolved these traits in response to evolutionary selection pressures from predators that became extinct at the end of the Pleistocene. (Byers 1997).
- Most anatomical characters used to describe the pronghorn and its fossil relatives seem
to place them close to the deer family (Cervidae). (Janis 2000)
- Recent phylogenetic analysis says pronghorn family is closest to Giraffidae, forming a sister group to cervids and bovids, or a sister group to all other horned ruminants.. (Hernandez Fernandez et al 2005; Murray 2006; Janis 2000)
- Variable number of chromosomes; the diploid number is 58 or 60 (Wurster and Benirschke, 1968; O’Gara & Yoakum 2004).
- Antilocapridae shows genetic similarities to both Bovidae and Cervidae.
- Studies showed the chromosome number and configurations in pronghorn similar to bovids (Todd 1975; Baccus et al 1983)
- Baccus et al. (1983) maintained that the pronghorn is genetically similar to cervids.
- DNA investigations indicate Antilocapridae is a legitimate family of its own (Cronin et. al, 1996).
- Occasional specimens have extra horns or teats, females may or may not have horns, and some females have a different type of uterus than others (O’Gara, 1968).
DISTRIBUTION & HABITAT
(Byers 1997) (O’Gara & Yoakum 2004)
- South-central Canada south to the high plains north of Mexico City
- Mississippi River and Gulf Coast of Texas west to the Pacific Ocean
- U.S.: southeastern Oregon, southern Idaho, Montana, and western North Dakota, south to Arizona and western Texas
- Canada: southern Alberta and Saskatchewan
- Mexico: northern Mexico
- Distribution has been reduced and fragmented by human development
- Seasonal migration and foraging prevented by fencing of rangelands and ranches
- Illegal hunting has reduced populations in Mexico
- Found in shrub lands, Mediterranean-type shrubby vegetation, grassland, temperate desert
- Deserts support less than 1 percent of the population
- Associated most frequently with treeless, flat terrain, most numerous in short-grass prairies
- Next most important habitat is steppe, preferably with vegetation ranging 5 to 30 in (13 to 76 cm) in height
- Herds range from near sea level to 11,000 ft (3,353 m) above sea level, but greatest concentrations are found 3,000 to 8,000 ft (914 to 2,438 m) above sea level
(Byers 1997) (O'Gara 1978) (O’Gara & Yoakum 2004)
Body Weight: male: 41 to 64 kg (90.4 to 141.1 lbs); female: 34 to 48 kg (74.9 to 105.8 lbs)
Body Length: 1.25 to 1.45 m (4.1 to 4.7 ft)
Tail Length: 7.5 to 10 cm (3 to 4 in.)
Coloration and Patterns
- Upper body and outside of legs light tan or reddish tan; chest, belly, inner legs, cheeks, lower jaw, sides, and rump patch are white
- Two broad white blazes across the throat. Male has broad black band from eyes down the snout and a black neck patch
- Rump patch is a large circular patch of white hairs 7 to 10 cm (2.8 to 4 inches) long, which can be erected as a signal
- Long black eyelashes help protect the eyes from the sun.
- Having forked or branching horns is a distinguishing feature of the Antilocaprid family. (Davis 2007)
- Both sexes carry horns. About 70% of females have horns, which may be stunted or undeveloped
- Females may have horns with no sheath
- Male horns 30 to 50 cm (11.8 to 19.7 inches); female horns, if present, seldom more than 7.5 to 10 cm (2.9 to 3.9 inches)
- Developed by three years of age
- Black and composed of a permanent, bony interior knob covered by a keratinous sheath, which is shed annually like antlers
- Lyre shaped, curving back and slightly inward near conical tips, each with one broad, short prong that juts forward and slightly upward, usually halfway from the base
- Used in male competitions during the rut, using thrusts and counter thrusts to try to gore opponents
- Prongs used to catch the thrust of opponent's horn and also to deliver serious piercing stabs.
- Short, erectile hairs, about 7 to 10 cm (2.7 to 3.9 inches) long
- Reduced or absent in mexicana subspecies
- Large, protruding eyes that appear to be located on the side of the head but are oriented forward enough to allow limited binocular vision
- Largest eyes of any North American ungulate in relation to its size; each eyeball is about 36 mm (1.4 inches) in diameter
- Nearly a 300-degree arc of vision without moving head or eyes; can detect movement up to 6.5 km (4 miles) away
- See movement more clearly than stationary objects
- Pointed double hooves, with cartilaginous padding to cushion shock in running over hard ground and rocks
- Front hooves are larger than back ones and carry most of the weight while animal is running
- Females strike with front hooves during agonistic encounters
- Both sexes have glands on the feet that secrete an oily conditioner for the hoofs.
Other Physical Characteristics
- Males average 10 percent larger than females
- Males have longer horns
- Black face and neck markings are not present in females
Physical adaptations for long-distance running and endurance:
- Long limbs with reduced number of toes
- Lightweight bones
- Small digestive tract to use less energy during locomotion
- Large tracheae, lungs, heart for rapid intake of oxygen and increased rate and power of circulation.
BEHAVIOR & ECOLOGY
(Byers 1997) (O'Gara 1990) (O’Gara & Yoakum 2004)
- Diurnal; activity is greatest at dawn and dusk (Einarsen, 1948, Kitchen, 1974)
- Sleep often, but for short periods without continuity or regularity
- Spend the majority of their time feeding, resting, and ruminating
- Activity schedule is highly variable, affected by season, weather, region, herd dynamics
- Daily area occupied by Wyoming Pronghorn was 2.6 to 5.2 sq km (1 to 2 sq mi) summer and autumn.
- Social organization is influenced by differences in quality of habitat and density and spatial dispersion of individuals
- Herd composition changes daily, even hourly; when food is plentiful, groups (bands) of 3 to 25 are spatially distant but still within visual contact of one another; in fall and winter, bands come together to form larger herd (as many as 1,000) as protection from predators
- Dominance hierarchies are maintained by female bands, bachelor bands, and territorial male bands that contain more than one male (see Breeding Systems)
Territorial Displays/Scent Marking
- Male pronghorn utilize different breeding systems within the same population, influenced by resources, population density, and sex ratio. (Maher, 2000)
- Territorial: When resources vary in quality or are clumped in defined locations, breeding males compete for the best territories (containing water and succulent vegetation) then defend a band of females within that territory
- Dominance: When resources are more uniform, bands with females and multiple males form; the males maintain a dominance hierarchy among them and the most dominant male conducts most of the breeding
- Harems: When resources are widely dispersed, population levels are low, or the sex ratio is skewed (1 male to 10 or more females), individual males defend a harem without regard to a defined territory
- Territorial males mark territory using a linked sniff-paw-urinate-defecate sequence
- Each element of the sequence is performed in exaggerated form with extreme postural changes
- Territorial males also mark tall grasses with secretions from cheek glands
- Territorial challenge usually composed of five phases:
- Stare at intruder
- Advertise presence with snort-wheeze and erect the mane and hair of rump patch
- Approach intruder, often trotting up to within 37 m (40 yards) then using stiff-legged gait to approach within 14 m (15 yards)
- Broadside displays, walking parallel with head low
- Chasing intruder out of territory, if he hasn’t already backed down
- Fights with horns are rare, and usually take place in the presence of a female ready to breed. Such fights consist of a series of thrusts and counter thrusts with the horns, locking horns and twisting necks, and lunging at the opponent. Serious injury and death can result from stab wounds.
- Alarm: erect hair on white rump patch, doubling the size of the patch and producing a “flash” visible up to 2 miles
- An aggressive male wishing to exaggerate the size of its horns will fold down the ears; the dark horns and dark facial markings will help emphasize the horns.
- Warning: snort-wheeze
- Male courtship: high-pitched whine
- Rutting male: roar
- Fawn: bleat
- Usual running speed of 40 mph (64 km/h); maximum speed of 57 mph (92 km/h) for up to 9 miles (15 km)'
- Leaping gait, with leaps covering 9 to 19 feet (3 to 6 m) at a time
- Good swimmer
- Instead of jumping over obstacles, prefers to go underneath them if possible
DIET & FEEDING
(Byers 1997) (Hansen & Clark 1977) (O'Gara 1978) (O’Gara & Yoakum 2004)
- Opportunistic herbivores; browse and graze; are "dainty" feeders choosing a wide variety of plants.
- Pronghorn are ruminants with
a four-chambered stomach; in ruminants, bacteria, protozoa, and fungi help digest tough plant cellulose
- Favor succulent, high-protein vegetation such as:
- shrubs (rabbitbrush, sagebrush, bitterbrush, Prunus sp.)
- grasses (bunchgrass, bluegrass, bottlebrush, ricegrass, squirreltail)
- cacti (Opuntia sp.)
- Grass makes up only about 12 percent of diet; Pronghorn during a Montana winter starved on grasslands but survived in sagebrush habitats. (
- During northern winters, Pronghorn are adept at pawing through
- Elongated head and pointed mouth parts aid in gathering of leaves and flowers, which are more nutritious that stems and twigs
- Peak foraging activity at dawn and dusk
- Spend 40 to 60 percent of time feeding
- Daily foraging movements of a herd vary from 0.1 to 0.8 km in the spring and summer to 3.2 to 9.7 km in the fall and winter
- Pronghorn practice forage switching from season to season to optimize dietary benefits from the most preferred, nutritious, and succulent species available
- 38 Pronghorn survive on the same amount of food required for one cow; 43.5 Pronghorn consumed the same amount of grass as one sheep (O'Gara 1978)
REPRODUCTION & DEVELOPMENT
(Byers 1997)(O'Gara 1978) (O’Gara & Yoakum 2004)
- Females direct mate choice, using three strategies:
- Sampling: females visit several harem-holding males, staying with each a few days before switching, usually returning to a male they visited within a week of estrus for mating
- Inciting: use sampling behavior until estrus, then run away from the male, inciting chasing and aggressive displays and fights among competing males. Female watches competition and mates with the winner
- Quiet: a month before estrus, female moves to an isolated, peripheral location occupied by a single male and stays with him throughout estrus and mating
- Breeding sequence:
- Male approaches female with a high-pitched whine
- Receptive female raises her tail and stands
- Male approaches waving his head from side to side in a pronounced gesture, making low sucking sounds, then mounts
- Copulation is brief, one forceful pelvic thrust
- Females may be harassed by bachelor males, with increasing aggression as estrus approaches; they often seek refuge with a territorial male, which chases off the bachelor
- Sexual maturity: females – 16 to 17 months; males – 1 year, although most will not breed until they are 3 to 4 years old and can compete to defend a territory or harem
- Breeding season:
- Northern populations: mid to late September, three-week rut is typical
- Southern populations: September to October, although breeding behavior is seen as early as July
- Synchronous birthing, most fawns born within a 10-day period
- 245 to 255 days
- After fertilization, ova can develop for nearly a month before implantation.
Infant (< 1 year old)
- Birthing season:
- northern populations: typically May to June
- southern populations: typically June to July
- birthing season is influenced by habitat and environmental conditions
- Litter size: Usually twins; subordinate females sometimes have one fawn
- Size at birth: 7 to 9 pounds, 20 to 29 inches
- Fawns are large; combined weight of twins at birth is about 18% of maternal weight
- Age at weaning: 4 to 5 months
- Reach full weight by 4.5 years, females reach this faster than males
- Sexual maturity for males by one year, but at this age they don't usually have an opportunity to breed.
- Females are sexually mature around 16 months, but have been known to conceive at 5 months.
DISEASES AND PATHOLOGY
(Thorne et al. 1988)
(Edwards et al. 2001) (Simmons et al. 2002)
In the wild
- Bluetongue virus, a.k.a. catarrhal fever
- Caused by Orbivirus serotype 17 and transmitted by insect vectors, particularly Culicoides sp.
- Occurrence in eastern Wyoming in 1976 killed at least 3,200 pronghorns; occurrence in northeastern Wyoming in 1984 resulted in 300 known deaths (Thorne et al. 1988)
- Virus transmission occurs during warm summer months and subsides with onset of cooler weather
- Parelaphostrongylus tenuis (Simmons et al. 2002)
- Parasitic nematode that infects the brain of many ungulates, resulting in severe neurological damage leading to death
- White-tailed deer are hosts, unaffected by the nematode but pass it on to other species through feces
- Gastropods act as intermediate host – crawl over the white-tailed deer feces and become infected with the larvae of the nematode; ungulate consumes vegetation and inadvertently ingests the gastropod
- Fusobacterium necrophorum has been contracted by wild-caught pronghorns brought into captivity and housed in close proximity with domestic bovid species
- Infection causes pododermatitis and/or stomatitis that can lead to septicemia and death
- Has occurred sporadically in the wild as well, in conditions where wild ruminants have been crowded within an area
- Wet pastures, warm temperatures, and presence of cattle feces increases chances of infection (Edwards et al. 2001)
POPULATION AND CONSERVATION STATUS
(O’Gara & Yoakum 2004)
- At one time there were thought to be 35 million or more pronghorns in their original range. During the latter half of the 19th century, numbers were reduced by 99% due to hunting and loss of habitat to agriculture, settlement, and fencing tracts of land, falling to 13,000 by 1915
- Conservation practices ended the decline in the early 20th century. From 1915 to 1925, the population increased to 30,500
- The population continued to increase into the 1980s, reaching more than 1 million in 1984, the highest number in the 20th century
- By 1997, the population had decreased again by about one-third
- In 2000, population estimates were 32,000 for Canada, 1,200 for Mexico, and 765,200 for the United States, a combined total of 799,200
- The Mexican subspecies (mexicana, peninsularis, and sonoriensis) experienced a continual decline of more than 80% throughout the 20th century
- Fewer than 300 Sonoran Pronghorn in the U.S. and 200-500 individuals in Sonora, Mexico. (IUCN 2008)
- Populations of Sonoran Pronghorn in Arizona and Mexico protected since 1967 under US Endangered Species Act.
- In 1998 US Fish and Wildlife Service developed a recovery plan for Sonoran Pronghorn.
Threats to survival
- With one exception, U.S. populations of Pronghorn (Antilocapra americana) are now excluded from CITES control.(http://www.cites.org/eng/resources/species.html) (2009)
- CITES I status for a small Arizona and Mexico population of Pronghorn (Antilocapra americana)
- This designation includes traditional subspecies A. a. sonoriensis.
- CITES I status for Mexican populations of Pronghorn (Antilocapra americana)
- This designation includes traditional subspecies A. a. peninsularis and A. a. mexicana
- IUCN least concern, Ver. 3.1
for Pronghorn (Antilocapra americana) (http://www.iucnredlist.org/details/1677)
- US Fish and Wildlife Service: Antilocapra americana peninsularis and A. a. sonoriensis: Endangered
- Habitat loss
from agriculture urban and mining expansion
- Removal of native vegetation by rangeland projects
- Fencing across seasonal movement routes
- Heavy livestock grazing
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