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Diversity of Living Antilocapridae (Pronghorns)
Antilocapridae is one of a number of families in the mammal order Artiodactyla (even-toed ungulates). Antilocaprids are known only from North America and only one species, the Pronghorn (Antilocapra americana), survived the end-Pleistocene extinctions. Male Pronghorns have much larger horns than do females and only male Pronghorns have the forward-pointing tine on their horns that give the species its name. Unlike true antelopes (of the family Bovidae), male Pronghorns shed the keratinous sheath of the horn annually, after the rut. Females shed their horns more irregularly.
Ecology and Behavior
The extraordinary running ability of pronghorns is generally attributed to strong selection pressure from now-extinct North American cheetahs, lions, hyenas, and other predators. Adaptations for running include long, slender limb bones and a heart and lungs that are much larger than those in other mammals of similar size. Modern Pronghorns can sprint a close to 100 km/h and can maintain a speed of 70 km/h for ten or more kilometers. Pronghorns strongly prefer open country with unrestricted lines of sight. They are found mainly in high elevation grasslands and deserts of western North America, where conditions are too dry for agricultural crops. Pronghorns can survive even where there is no standing water.
Pronghorns are found in fluid groups, with the exception of solitary females around the time of giving birth and mature males in spring. Many Pronghorn populations are migratory, moving to high quality foraging areas in the summer and to warmer areas with less snow in the winter. Winter herds of up to 1000 individuals have been reported. Fights between rutting Pronghorn bucks are rare, but when they occur, a third of the time they result in serious injury or death. Males fight only in the presence of a sexually receptive female.
Pronghorns are highly selective browsers, consuming mainly forbs but also some grasses in early spring and browsing more shrubs in fall and winter. The resulting high quality diet may facilitate their unusually high reproductive investment. The Pronghorn's gestation period is around 248 days, a long time given their body mass, longer in relatively dry years than in wet ones producing more abundant food. Pronghorn females nearly always have twins. Predation by Golden Eagles (Aquila chrysaetos) and Coyotes (Canis latrans) is a major source of fawn mortality. Fawns are weaned at around 90 days.
Pronghorns and Humans
In nearly all states where Pronghorns occur, they are game species with a managed hunting season. Pronghorns are wary of humans and cannot be approached closer than around 500 m. When alarmed, herds flee in tight synchrony. In Yellowstone National Park and National Bison Range Wildlife Refuge, Pronghorns are not hunted and are habituated to humans and vehicles, allowing fairly close approach. Livestock fences seriously impair the movements of Pronghorns. Although Pronghorns are capable of substantial jumps, they avoid jumping when possible, so a 1 m fence will divert a migrating Pronghorn herd. Management guidelines call for fences to be constructed with a smooth bottom wire 40 cm above the ground, allowing Pronghorns to crawl under the fence.
Conservation Status of Pronghorns
Early in the 20th century, human hunting of both Pronghorns themselves and of Gray Wolves (Canis lupus) (which suppress populations of Coyotes, the primary predators of Pronghorns), likely came close to extirpating the Pronghorn. In the early 1800s, there were an estimated 35 million Pronghorns. By 1924, there were only around 13,000. Between 1924 and 1964 the population increased ten-fold. There are now around 1 million Pronghorns, distributed across 15 states and two Canadian provinces (and present but declining in Mexico). However, around half of these animals are in the state of Wyoming and populations in some of the other states are small and isolated. Byers (2011) suggests that Pronghorns (along with Bison [Bison bison] and other grassland species) would benefit would benefit from the establishment of a Great Plains National Park, 800 km on a side, in the northern Great Plains west of the 100th meridian, where crop agriculture persists only because of heavy government subsidies.
(Byers 2011 and references therein)