Pronghorns are common across the High Plains, from the western Dakotas to Mexico, and on sage grasslands of the Intermountain West. Natives of North America, they are the sole remaining species of a family that included up to 13 members during the Tertiary Period and Pleistocene Epoch. Most paleontologists believe that pronghorns are not directly related to Old World antelopes and some feel that they are more closely aligned with cattle, sheep and goats. Indeed, like bovids, they have a horn composed of a keratin sheath that overlies a bony core; however, unlike bovids, the pronghorn's sheath is branched (in males) and is shed each year.
Once numbering over 40 million individuals, the pronghorn population declined dramatically throughout the 1800s and early 1900s due to over hunting and the fencing of rangelands. Unable to leap over fences, many starved when they were prevented from migrating during periods of heavy snow. Pronghorns are otherwise well equipped to withstand the harsh winters that typify most of their range; though relatively slender animals, they have a dense undercoat and their outer hairs are hollow, providing superb insulation.
Male pronghorns are territorial from late spring through early fall and gather a harem of up to 20 females; younger males travel about in bachelor herds and occasionally challenge the breeding males, often to their own detriment. Breeding occurs in early autumn and females give birth (usually to twins) in early summer. The young are kept hidden for several weeks and, by then, are able to outrun coyotes, their primary predator; as adults, they will become one of the fastest mammals on Earth, able to reach speeds of 70 miles per hour. In winter, pronghorns congregate in large herds, feeding on the lee side of mesas and roaming about to find open forage.
