McCullough Peaks horses

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

Country: United States of America

Region/Province/Range: Park County – Wyoming

Population type: Semi feral-heavily managed

Estimated Population size: between 112-194 horses

Management Authority:  Bureau of Land Management -McCullough Peaks HMA

Images by Jason Ransom. Please respect © copyright!

Management Practices: 

The US Bureau of Land Management has managed this population with periodic round-ups, adopting removed horses to the public. Since 2004, management has more intensively been done using a time-released form of the immunocontraceptive PZP and periodic round-ups.

Details of Population

McCullough Peaks Herd Management Area is located Park County, Wyoming, USA (latitude 44°35‘N, longitude 108°40‘W), and consists of 44,400 ha of primarily open sagebrush steppe with badlands along the western edge. Vegetation consists of large expanses of small shrubs, grasses, and forbs. Pronghorn antelope and mule deer are sympatric with horses here and little natural depredation occurs. Elevations range from 1,200 m to 1,964 m. Mean annual temperature is 8.0°C (range -30.0– 37.8°C) and mean total annual precipitation is 271.2 mm (range=168.9–389.1 mm).

Structure and demographics

Population size reached a high of 495 horses before a large management removal in 2004, and now is maintained between 112 and 194 horses. Bands average 8 horses and many bands closely associate into herds; travelling, feeding, and resting together. At its largest population, bands with more than one stallion occurred, but are now infrequent. Bachelors form loosely associated ephemeral bands or range independently. Genetically, these horses are most related to draft breeds such as the Percheron, probably reflecting much of the early settlement activity around the old west town of Cody. Horses of all colors are in this herd, including Overo, Tobiano, and Sabino paint horses.

Issues worth noting and needed actions

Like most populations in the USA, available habitat for horses is finite and management is necessary to protect all natural resources while attempting to balance the multiple-use mandate for the federal lands where horses live.  The science needed for more-informed management is improving, but many obstacles persist. You can read much more in the 2013 National Research Council report “Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward

Bibliography and further reading

Additional details about this population, and specifically about behavior and fertility control, can be found in:

Ransom, J.I., Roelle, J.E., Cade, B.S., Coates-Markle, L., and A.J. Kane. 2011. Foaling rates in feral horses treated with the immunocontraceptive porcine zona pellucida. Wildlife Society Bulletin 35:343-352

Ransom, J.I., Cade, B.S., and N.T. Hobbs. 2010. Influences of immunocontraception on time budgets, social behavior, and body condition in feral horses. Applied Animal Behaviour Science 124:51-60

On-going behavior and ecology research from Dr. Ransom can be followed on Twitter @wildequids

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