Wyoming is home to a substantial population of free-roaming horses, often referred to as Mustangs. These animals are not native wildlife but are considered “wild” under federal law because they are unbranded, unclaimed, and live without human supervision on public lands. They are the descendants of domesticated horses that either escaped or were released generations ago, establishing themselves as a fixture of the Western landscape. This population is subject to continuous public interest and complex governmental management.
The Scope of Wyoming’s Wild Horses
Wyoming hosts one of the largest populations of wild horses in the American West. Recent estimates place the total wild horse population in the state at approximately 6,000 to 8,800 animals, far exceeding the population goals set by land managers. These herds roam across nearly five million acres of federally protected land designated for their use.
The horses are descendants of various domestic breeds, including the original Spanish horses brought by explorers in the 16th century. Their bloodlines mixed with those of horses that escaped or were turned loose from ranches and military posts. They are often referred to as Mustangs, a term derived from the Spanish word mesteno, meaning stray or unowned.
Their presence is characterized by a high annual growth rate, which can reach 15 to 20 percent. This rapid reproduction creates a continuous management challenge for federal agencies responsible for the health of the rangelands.
Federal Authority and Oversight
The legal foundation for managing these herds rests with the Wild Free-Roaming Horses and Burros Act of 1971. This federal law recognizes the horses as “living symbols of the historic and pioneer spirit of the West” and grants them protection on public lands. The Bureau of Land Management (BLM) and the U.S. Forest Service are mandated to manage and protect these populations.
The BLM’s primary objective is to achieve and maintain the Appropriate Management Level (AML) for each designated area. AML is the maximum number of horses that can be sustained on the land in balance with other mandated uses, such as supporting native wildlife and livestock grazing. The AML for Wyoming is set significantly lower than the current population, typically between 2,566 and 3,725 horses statewide.
Maintaining the population at AML ensures a “thriving natural ecological balance” and prevents range deterioration. When horse numbers rise above the established AML, the BLM is legally required to remove the excess animals. The agency must balance horse protection with its responsibility to maintain healthy rangelands for all species, including sage grouse, elk, and pronghorn.
Key Herd Management Areas
Wild horses in Wyoming are concentrated in 14 federally designated Herd Management Areas (HMAs), primarily located in the southwestern portion of the state. These HMAs are the specific geographic zones set aside for the long-term management of the herds. The topography and character of the herds vary greatly.
The McCullough Peaks HMA, near Cody, is known for its distinct herd that displays a wide array of coat colors, including palomino and buckskin. These horses are believed to have bloodlines linked to stock from Buffalo Bill’s Wild West Show. The White Mountain HMA, near Rock Springs, features high-desert terrain where the horses are thought to possess genetics related to North American gaited breeds.
A significant management complexity arises in areas like the Adobe Town HMA, which is part of the Great Divide Basin. This region features a “checkerboard” land ownership pattern, where alternating sections of public and private land create boundary issues. The Salt Wells Creek HMA is also part of this complex, requiring constant coordination between federal managers and private landowners.
Controlling the Wild Horse Population
To bring population levels down to the AML, the BLM utilizes two main methods: physical removal and fertility control. The most visible method is the “gather,” commonly known as a roundup, which typically uses helicopters to herd horses into temporary corrals. Gathers are necessary because the high annual growth rate means herds can double in size in as little as four to five years.
Once gathered, excess horses are removed from the range and transported to off-range corrals or pastures. These animals are offered to the public through the BLM’s Adoption and Sale Program. The goal is to place horses into private homes, though many remain in long-term holding facilities due to limited adoption demand.
Fertility control is the other primary tool, often involving the Porcine Zona Pellucida (PZP) vaccine on mares. PZP is a non-hormonal, reversible contraceptive administered via dart gun or by hand after capture. This vaccine helps limit reproduction and slow the population growth rate, reducing the frequency of large-scale gathers.