Theodore Roosevelt free-ranging horses WE

This entry was contributed to Wild Equus by Dr. Jason Ransom of Colorado State University, member and specialist of the Wild Equus Network (WEN).


Species: Equus caballus         Subspecies/Breed/Type: American Mustang

Estimated Population size: Maintained at 120–160 horses

Country: United States

Region/Province/Range: Theodore Roosevelt National Park

Population type: Free-ranging-heavily managed

Management Authority: Government Agency – National Park Service

Images from Jason Ransom

Details of Home Range or Territory

Theodore Roosevelt National Park is located in south-western North Dakota, USA (45◦55’N/103◦31’W). The South Unit of the park, where horses range, is approximately 19,000 ha and consists of eroded badlands with gullies and ravines separated by large upland plateaus, and small buttes. The mixed-grass prairie vegetation is predominantly comprised of needle and thread (Hesperostipa comata), threadleaf sedge (Carex filifolia), western wheatgrass (Pascopyrum smithia),blue grama (Bouteloua gracilis) and little bluestem (Andropogen scoparius). Unlike most mustangs in the US, which are managed by the Bureau of Land Management, this herd is part of the National Park Service system. Horses in the park are preserved as part of the cultural landscape, but because they range with pronghorn (Antilocapra americana), bison (Bison bison), mule deer (Odocoileus hemionus) and elk, the result is a vignette of primitive North American landscape.

Details of Population

Details about the park and its resources can be found at www.nps.gov/thro. Research is on-going for this population and longer-term behavioral data are currently being analyzed. Additional details about this population, and specifically about behavior, can be found in: Ransom, J.I., Powers, J.G., Garbe, H.M., Oehler Sr., M.W., ,Nett, T.M., and D.L. Baker. 2014. Behavior of feral horses in response to culling and GnRH immunocontraception. Applied Animal Behaviour Science 157:81-92

Structure and demographics

This population is maintained at around 120–160 horses through periodic management removals and immunocontraception (GnRH). Predation by pumas is minimal, even though the large cats share habitat. The horses organize themselves into about 20 bands with an average band size of about 8.7. All individual horses in this population are identified and assigned individual identity numbers by managers and researchers, and age and reproductive data for each animal are maintained.

Issues worth noting and needed actions

Like most populations in the USA, available habitat for horses is finite and management is necessary to protect the full suite of natural resources in this National Park.  The on-going research on use of GnRH to reduce fertility is promising as it appears to have minimal effects on short-term behavior while slowing population growth. Like all management tools, the long-term effects of human actions need to be monitored in order to responsibly preserve the ecosystem.

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