America's Wild Horses and Burros—Research to Support Management

Science Center Objects

The wild horses that roam the west are feral descendents of domestic animals that either escaped from or were intentionally released by early European explorers and later settlers. As a result of both origin and contemporary management, the Spanish or Iberian influence remains strong in some wild horse populations (e.g., the Kiger, Pryor Mountain, and Sulfur Mountain herds). In other populations, escaped or released military, saddle, and draft horses dominated by the Thoroughbred, Morgan, Quarter Horse, and draft breeds have formed broad zones of introgression (intermixing), sometimes with Spanish bloodlines. These populations of mixed ancestry increased to inhabit large areas of the western U.S.

It vested the U.S. Bureau of Land Management (BLM) and the USDA Forest Service with responsibility for their management and directed these agencies to manage wild horses and burros for a “thriving natural ecological balance.” Through its National Wild Horse and Burro Program, the BLM manages about 38,000 wild horses and 5,500 wild burros roaming in 179 herd management areas, comprising almost 32 million acres in ten western states. Largely unchecked by natural predators, wild horse populations can grow at rates of 18–25 percent per year and strongly influence their habitat. In addition, wild horses share rangelands with wildlife and seasonal cattle.  

The primary means of managing herd size has been periodic “gathers.” Most herds were gathered (rounded up) every 3–5 years. Wild horses and burros removed from the range are offered for public adoption. Animals that are not adopted or sold are maintained in long-term holding facilities where they can continue to be available for adoption, or they simply live out the remainder of their natural lives. But decreased adoption demand coupled with holding facilities reaching capacity has forced BLM to evaluate management options and strive for longer lasting solutions to high population growth.

In the late 1990s, the BLM entered into a partnership with the U.S. Geological Survey, Fort Collins Science Center (FORT) to design and implement a research program that would investigate alternative approaches to address population growth as well as other management challenges faced by BLM. The U.S. Geological Survey (USGS) is the research arm of the Department of the Interior (DOI), and in 2000 the Fort Collins Science Center hosted a series of expert panels to discuss the subjects of fertility control, population estimation, herd genetics, habitat assessments, and health and handling issues. Based on reports produced by these expert panels and information from a variety of other sources, BLM, FORT, and U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) staff prepared a Strategic Research Plan for wild horse and burro management. The priority for management initially focused on fertility control and population estimation.

More recently, BLM needs have expanded to include broader ecological information to support wild horse and burro management. In 2013, the National Academies of Science (NAS) published a report, (National Research Council 2013) which recommended and prioritized research for the program. In response and support of the NAS report, USGS scientists proposed a number of new studies that are currently in various stages of proposal writing, peer-review, study approval, or project initiation. Proposed research projects include:

  1. Non-invasive genetic sampling of free-roaming horses to estimate population size, genetic diversity, and consumption of invasive species.
  2. Developing a suitable radio collar or radio tag for feral horses and burros.
  3. Development of a population model and cost analysis for managing wild horses (“WinEquus II”).
  4. Population demography and ecology of wild horses in two sentinel herds in the Western United States.
  5. Demography of two wild burro populations in the western USA.
  6. Developing and testing aerial survey techniques for wild burros.
  7. Evaluating the efficacy and safety of Silicone O-ring intrauterine devices as a horse contraceptive through a captive breeding trial.
  8. Effect of spaying females on the demography, behavior and ecology of a wild horse population.
  9. Evaluating behavior and ecology of geldings among a breeding population.
  10. Modeling carrying capacity of free-roaming horses.
  11. Assessing effects of wild horses, cattle, and wildlife on sagebrush habitat and ecosystem processes.
  12. Testing efficacy of contraceptives for female burros in a captive trial.

USGS seeks to continue the partnership and science support for BLM management of wild horses and burros. The resulting findings and products from these research studies will continue to provide BLM with the science necessary to guide decisions at the individual, population, and landscape level.

    Wild Horse Identification: Facial Markings

    Wild Horses Identification

    Horses have natural markings consisting of color and patterns. Patterns are formed by color patches and parts of the body that lack color. These natural markings, termed "signalment," are used to record a physical description of an individual horse. Although the range of horse color and markings is wide, use of color and markings to identify a particular horse is a fairly standard process:

    • To identify one horse from another on the range, wild horse managers rely on the color of the horse as well as body markings that are most distinguishable and easily observed.
    • Facial markings—such as star patterns on the head, stripes on the face, and snips on the nose—can be clearly seen and are commonly used to identify a horse.
    • Leg markings—such as the extent of white at the base of one or more legs—further distinguish individual horses.
    Wild Horse Body Color and Markings chart for identification

    Wild Horse Body Color and Markings chart for identification

    To build a record on an individual horse, managers collect and store pictures detailing the horse's particular markings in the Photo Catalog of WHIMS. Information on the horse and field observations are then added to the horse's record in the Wild Horse Database. Then, to ensure that information is added to the correct horse’s history, managers must review previous descriptions and photographs of the horse's markings. Managers can use the Application Database to retrieve information on individual horses as well as generate various reports. With a database of photos on the color and markings of individual horses, it is a simple matter to quickly identify horses for various purposes, including familiarizing new or volunteer staff with particular individuals and herds.

    Wild Horse Identification: Leg Markings

    Wild Horse Body Color and Markings chart for identification

















    • Sarah R.B. King, Ph.D., Research Scientist, Ecosystem Science and Sustainability, Colorado State University.
    • Randall Boone, Ph.D., Research Scientist, Ecosystem Science and Sustainability, Colorado State University.
    • Bruce Lubow, Ph.D., Natural Resource Ecology Laboratory, Warner College of Natural Resources, Colorado State University.
    • Dan L. Baker, Ph.D., Biomedical Sciences, Animal Reproduction and Biotechnology Laboratory, Colorado State University.
    • Jason E. Bruemmer, Ph.D., Department of Animal Sciences, Equine Sciences Program, Colorado State University.

    Web Resources

    1. BLM National Wild Horse and Burro Program
    2. Little Book Cliffs Wild Horse Area
    3. McCullough Peaks Herd Management Area
    4. Pryor Mountain Wild Horse Range
    5. Wild Free-Roaming Horses and Burros Act of 1971, Public Law 92-195

    Return to Herbivore-Ecosystem Interactions or Ecosystem Dynamics

    Selected Publications

    General Reading

    Berger, J. 1986. Wild horses of the Great Basin. The University of Chicago Press, Chicago, IL. 326 p.

    Dobie, J.F. (1952) 2005. The mustangs. Bison Books–University of Nebraska Press, Lincoln, NE. 376 p.

    Hansen, S. 1983. Roaming free: Wild horses of the American West. Northland Press, Flagstaff, AZ. 80 p.

    Ryden, H. 2005. America's last wild horses. 30th anniversary ed. The Lyons Press, New York. 352 p.

    Sponenberg, P.D., and B.V. Beaver. 1992. Horse color: A complete guide to horse coat colors. Breakthrough Publications, Emmaus, PA. 124 p.

    Alternative Population Estimation Techniques

    Borchers, D.L., W. Zucchini, R.M. Fewster. 1998. Mark-recapture models for line-transect surveys. Biometrics 54(4):1207-1220.

    Bowden, D.L., and R.C. Kufeld. 1995. Generalized mark-resight population size estimation applied to Colorado moose. Journal of Wildlife Management 59(4):840-851.

    Buckland, S.T., D.R. Anderson, K.P. Burnham, J.L. Laake, D.L. Borchers, and L. Thomas. 2001. Introduction to distance sampling: estimating abundance of biological populations. Oxford University Press, Oxford, UK. 432 p.

    Graham, A., and R. Bell. 1989. Investigating observer bias in aerial surveys by simultaneous double-counts. Journal of Wildlife Management 53(4):1009-1016.

    Guenzel, R.J. 1997. Estimating pronghorn abundance using aerial line transect surveys. Wyoming Game and Fish Department, Cheyenne, WY. 174 p.

    Johnson, B.K., F.G. Lindzey, R.J. Guenzel. 1991. Use of aerial line transect surveys to estimate pronghorn populations in Wyoming. Wildlife Society Bulletin 19(3):315-321.

    Little, D.K., K. Grissom, R. Oyler, C. Barnes, S. Elefritz, J. Christensen, G. Acheson, L. Thomas, J. Herrert, R. Lee, A. Fuller, M. Pierce, and S. Kohnke. 1999. Report of the Arizona Burro Census Team. Bureau of Land Management, Phoenix, AZ. 19 p.

    Quang, P.X., and E.F. Becker. 1997. Combining line transect and double-count sampling techniques for aerial surveys. Journal of Agricultural, Biological, and Environmental Statistics 2(2):230-242.

    Samuel, M.D., E.O. Garton, M.W. Schlegal, and R.G. Carson. 1987. Visibility bias during aerial surveys of elk in northcentral Idaho. Journal of Wildlife Management 51:622-630.

    Southwell, C., B. Mare, M. Underwood, F. Quartararo, and K. Cope. 2002. An automated system to log and process distance sight-resight aerial survey data. Wildlife Society Bulletin 30(2):394-404.

    Trenkel, V.M., S.T. Buckland, C. McLean, and D.A. Elston. 1997. Evaluation of aerial line transect methodology for estimating red deer (Cervus elaphus) abundance in Scotland. Journal of Environmental Management 50(1):39-50.

    Unsworth, J.W., F.A. Leban, D.J. Leptich, E.O. Garton, and P. Zager. 1994. Aerial survey: User’s manual (2nd ed.). Idaho Department of Fish and Game, Boise, ID. 84 p.

    Fertility Control in Mares

    Fraker, M.A., and R.G. Brown. 2011. Efficacy of SpayVac® is excellent: A comment on Gray et al. (2010). Wildlife Research 38:537-538.

    Gray, M.E., D.S. Thain, E.Z. Cameron, and L.A. Miller. 2010. Multi-year fertility reduction in free-roaming feral horses with single-injection immunocontraceptive formulations. Wildlife Research 37:475-481.

    Killian, G., D. Thain, N.K. Diehl, J. Rhyan, and L. Miller. 2008. Four-year contraception rates of mares treated with single-injection porcine zona pellucida and GnRH vaccines and intrauterine devices. Wildlife Research 35:531-539.

    Kirkpatrick, J. 1995. Management of wild horses by fertility control: The Assateague experience. NPS Scientific Monograph No. 26. National Park Service, Denver, CO. 60 p.

    Kirkpatrick, J.F., I.K.M. Liu, J.W. Turner, Jr., R. Naugle, and R. Keiper. 1992. Long-term effects of porcine zonae pellucidae immunocontraception on ovarian function in feral horses (Equus caballus). Journal of Reproduction and Fertility 94:437-444.

    Kirkpatrick, J.F., S.E. Shideler, and J.W. Turner, Jr. 1990. Pregnancy determinations in uncaptured feral horses based on free steroids in feces and steroidal metabolites in urine-soaked snow. Canadian Journal of Zoology 68:2576-2579.

    Kirkpatrick J.F., and A. Turner. 2008. Achieving population goals in a long-lived wildlife species (Equus caballus) with contraception. Wildlife Research 35:513-519.

    Kirkpatrick J.F., and A. Turner. 2002. Reversibility of action and safety during pregnancy of immunizing against porcine zona pellucida in wild mares (Equus caballus). Reproduction (Suppl. 60):197-202.

    Kirkpatrick, J.F., and J.W. Turner, Jr. 1991. Changes in herd stallions among feral horse bands and the absence of forced copulation and induced abortion. Behavioral Ecology and Sociobiology 29(3):217-219.

    Kirkpatrick, J.F., J.W. Turner, Jr., and A. Perkins. 1982. Reversible fertility control in feral horses. Journal of Equine Veterinary Science 2:114-118.

    Liu, I.K.M., M. Bernoco, and M. Feldman. 1989. Contraception in mares heteroimmunized with pig zonae pellucidae. Journal of Reproduction and Fertility 85:19-29.

    Powell, D.M. 1999. Preliminary evaluation of porcine zona pellucida (PZP), immunocontraception for behavioral effects in feral horses (Equus caballus). Journal of Animal Welfare Science 2:321-335.

    Turner, A., and J.F. Kirkpatrick. 2002. Effects of immunocontraception on population, longevity and body condition in wild mares (Equus caballus). Reproduction (Suppl. 60):187-195.

    Turner, J.W., Jr. 1999, 2000, 2001, 2002. Annual progress reports on USDI-BLM Assistance Agreement. Nevada Wild Horse Fertility Control Project, Department of Physiology and Cardiovascular Genomics, Medical College of Ohio, Toledo, OH.

    Turner, J.W., Jr., I.K.M. Liu, D.R. Flanagan, A.T. Rutberg, and J.F. Kirkpatrick. 2007. Immunocontraception in wild horses: One inoculation provides two years of infertility. Journal of Wildlife Management 71:662-667.

    1WHIMS metadata were created according to Biological Data Profile standards.