American Pronghorn

Antilocapra americana

Image courtesy Dave Menke: U.S. Fish and Wildlife Service www.fws.gov

Physical Characteristics

The pronghorn antelope is the smallest big game ruminant in North America, with females averaging 48 kilograms and adult males averaging 54 kilograms. They average 140 cm in total length and are about 90 cm tall. Their body color consists of white underside and rusty brown or tan back. They have the largest eyes in relation to body size of any wild ungulate in North America. They rely on these large eyes to scan the prairies for potential predators. Their most unique identifying characteristic is their horns. They have true horns that are covered with a keratinized sheath. These horn sheaths are shed every year, usually in November, and pronghorn are the only ungulate in the world that shed their horn sheaths. Male pronghorn, or bucks, have horns that average 25 to 38 cm in length that fork midway up, thus giving them their name. Females have horns that are much smaller, only 2.5 to 10 cm, and rarely have prongs (Yoakum and O'Gara, 2000).

Distribution and Habitat (see range map below)

Historically, pronghorn ranged from southern Canada through all western states to Mexico (Yoakum and O'Gara, 2000). They inhabit flat open areas of tall and short grass prairies and deserts. They rely on keen eyesight and speed to evade predators in these open areas. The pronghorn is the second fastest land animal in the world, reaching speeds of up to 72 km per hour (Wilson and Ruff, 1999).

Diet

The pronghorn diet consists of grasses, forbs, and shrubs. In grassland areas, forbs constitute 70% of their diet, with grasses and shrubs making up the remaining 30%. They are opportunistic foragers and shift their diet depending on situations. In the more arid areas of the shrub-steppe biome, pronghorn diet is comprised of mainly shrubs (64%), with forbs making up 29% of their intake and grasses the remaining 7% (Yoakum and O'Gara, 2000). Interestingly enough, they will eat many plants that are poisonous to livestock (Wilson and Ruff, 1999).

Reproduction

Pronghorns have long gestation periods, about 250 days. Females first breed at 16 months, but don’t usually produce fawns until 2 years old (Wilson and Ruff, 1999). Males spend the first three years of their lives in bachelor herds. During this time, they do not make any serious attempts to mate until the 3rd or 4th year (Byers, 1997). Females produce twins about 98% of the time. Newborn fawns usually weigh 3-4 kg, and spend the first couple of weeks hidden, rising only to nurse. Most populations have a short and fairly intensive breeding season, known as the rut (Wilson and Ruff, 1999).

Longevity

Pronghorn commonly live up to 9 years of age, and death is usually related to failing teeth, drought, or harsh winters. They often die as a result of malnutrition, disease, or predation (Yoakum and O'Gara, 2000).

Predation

Besides harsh winters, predation is the main cause of death in pronghorns. The primary list of predators include coyotes, bobcats, golden eagles, and dogs (Ockenfels, 1994). Hunting is also a cause for mortality, but is regulated and believed to have little impact in most situations (Maher and Mitchell, 2000). Since pronghorn are usually found in open prairies, mountain lions are not considered a major predator. However, some pronghorn in Arizona live in areas of rugged terrain with more vegetation than praries. This terrain and cover supplied by increased vegetation could provide mountain lions with the cover they need to hide and stalk pronghorn. In a study 11 out of 29 mortalities were attributed to mountain lion predation. Most of these incidents were within 100 m of rugged terrain, and no kills occured in short-grass prairie. The author obtained information from a number of accountable resources and found that mountain lion predation on adult pronghorn is very likely in areas of rugged, heavily vegetated terrain (Ockenfels, 1994).

Predators particularly target fawns. Berger (2005) provided two detailed accounts of a male pronghorn aiding females in chasing away coyotes in Grand Teton National Park. These observations are the first in suggesting that males defend fawns from predation by coyotes. They give several reasons why male pronghorn assist in fawn defense. 1) In areas where predation is the main source of mortality for fawns, the males can possibly increase their own fitness by defending the fawns they sired. This hypothesis requires that males recognize their offspring, and there is weak evidence to show that this is true. 2) Males may protect fawns to ensure future reproductive success, since more surviving females means more mating opportunities. 3) A selfish hypothesis entails that coyotes that are harassed will be less likely to attack pronghorn in the future. 4) Males may only look like they are harassing the coyotes, when they are actually only trying to keep the females in their territories (since females usually go out first to drive the coyote away). But according to one of the accounts presented in this article, this last hypothesis seems very unlikely.

Distribution:

Range Map: Based on Burt and Grossenheider, 1976

http://www.ultimateungulate.com/Artiodactyla/Antilocapra_americana.html

Image courtesy of John Byers

www.sci.uidaho.edu/ biosci/labs/byers/research/