Resource Brief - Horses

mare and foal grazing near Loblolly pine trees
Mare and Foal Grazing

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Evidence suggests that domestic horses (Equus caballus) were on Assateague as early as 1669 when settlers put their livestock, including horses, on the island. Descendents of these animals have been living in the wild on Assateague for well over 300 years.


Monitoring Trends

When most of the Maryland portion of Assateague Island became a national seashore in 1965, there were 10 horses living within the new park's bounda­ries. That number had increased to 28 when the sea­shore acquired ownership of the horses in 1968. It was not until 1975 that scientists began to study the seashore's horse population. Surveys continue to­day over a 35 km (21 mile) area between the Ocean City Inlet and the Virginia state line. During the sur­veys, which are completed six times a year by vehicle and foot, each horse in the population is accounted for.

A sighting record is completed as each horse is iden­tified by a combination of features such as color, sex, markings, and scars. This information is then added to a data base where scientists and seashore manag­ers track biological data such as age structure, mater­nal ancestry of the herd, the ratio of male to female horses, mortality and frequency of foaling.

Early surveys revealed that the original population of 10 horses present when the seashore was established was increasing at a rate of 10 to 15 percent annually. With a ballooning population, important concerns surfaced for managers: how to protect the long­term health and viability of the horses while at the same time minimizing their impact on the environ­ment.

Subsequently, a series of studies were launched to determine the impact of the horses on island vegeta­tion. The studies focused on the species of vegeta­tion that the horses prefer- saltmarsh cordgrass (Spartina alternafiora), American beachgrass (Ammophila breviligulata), and three-square sedge (Scirpus americana)- and revealed that these species were being over grazed. Additionally, scientists de­termined that the island could only support a limited number of horses if impacts to island vegetation were to be minimized. Later ecological studies revealed that heavy grazing contributed to a reduction in the abundance, density, size and diversity of plants in both wetland and dune habitats. Scientists also noted that fauna found within those habitats were impacted negatively by the reduction of habitat quality.

Graph showing horse population growth and foal births
Horse population growth and foal production on Assateague Island National Seashore, 1965-2015. Blue line - horse population, red line - foal births, blank space - unavailable data.

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Scientists and seashore managers then explored a variety of methods to reduce and stabilize the popu­lation, knowing that removing or selling animals for population control was not acceptable. Based on results of limited field trials with a wild mustang population in the west, managers agreed to allow scientists to test the feasibility of controlling the horse population with some type of contraception.

In 1986, the initial fertility control studies on As­sateague focused on using steroids to inhibit sperm production in stallions and ovulation in mares.

This approach proved ineffective with the As­sateague horse population. Researchers then fo­cused their efforts on the promising field of immunocontraception. Ideally, the contraceptive would be effective, reversible and safe. It would not affect social behavior and would be remotely deliv­erable so that no animal would have to be captured.

In 1988, scientists began field trials that focused on administering the contraceptive Porcine Zona Pellu­cida (PZP) to 26 mares. Six untreated mares were added to the trial for comparison. That fall, preg­nancy tests revealed that no treated mares were pregnant. However, three untreated mares were. Research continued on the effects of PZP, including reversibility, for the next 7 years.

With ecological studies and fertility control research in hand, it was time for seashore managers to set a horse population management goal. After carefully examining all habitat and biological studies available at the time, managers decided on an initial popula­tion goal of 150 animals.

Dart used to remotely administer PZP immunocontraceptive to female horses
Dart used to remotely administer 1 cc of PZP to Assateague mares. The dart is approximately 10.8 cm (4.25 in) long.

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PZP is an immunocontraceptive vaccine that works with a mare's immune system. One cc of the vaccine is loaded into a dart that is fired from a gun at a range of 25 to 50 meters from the receiving mare. The dart's impact with the mare's hip sets off a cap charge which injects the vaccine, and pops the dart off the animal.

The vaccine stimulates the mare's immune system to produce antibodies that work by blocking sperm re­ceptor sites on the zona pellucida. This thin protein­aceous membrane surrounds all mammalian eggs and contains sperm receptors that allow sperm to attach to the egg.

As these antibodies circulate through the mare's sys­tem, they attach themselves to the sperm receptors on the zona pellucida and distort the shape of the receptor. As a result, the receptor on the sperm no longer matches the receptor on the egg and the egg cannot be fertilized.

Initial protocol brought each mare into the contra­ceptive program at age two, with two treatments the first year and boosters for each of the next two years. The initial dose provided antigen recognition ( or "priming" of the immune system). Any subsequent booster doses provided full stimulation of the im­mune system to produce the antibodies that prevent­ed conception for a year.

illustration of unaffected horse ovum (egg) without distorted sperm receptor sites
Illustration of unaffected horse ovum (egg) without distorted sperm receptor sites.

After age four, each mare was left untreated until she delivered a live foal. At that time she was put back on PZP, for as long as the population remained above the goal range. An unexpected benefit of con­traception is that mares began living longer, healthi­er lives. Prior to the initiation of the contraception program, few mares lived beyond their late teens. Now it is not uncommon for them to live into their late twenties or early thirties.

Adaptive management of the seashore's horses is an evolving process. In 2006, based on current genetic and ecological studies, managers revised the initial population goal down to a range of 80 to 100 ani­mals. A population of this size will remain genetical­ly viable and still have minimal impacts on island vegetation and habitat. The population has since reached the new goal range, and the last mare to be contracepted was a single 2 year old, in 2016. As managers continue to monitor the horse population, they will be proactively engaged in protecting the island and its resources for future generations.



National Park Service. (2008). Environmental Assessment of Alternatives for Managing the Feral Horses of Assateague Island National Seashore. Retrieved from

National Park Service. (2006). Horses of Assateague Island Population and Habitat Viability Assessment Workshop. Retrieved from

Two of Assateague's wild horses on the beach
Wild Horses

Enjoy their beauty from a distance, and you can help make sure these extraordinary wild horses will continue to thrive on Assateague Island.

Graphic of horse showing outcomes (ouch, vacation over, nope, think again) of petting wild horses
Safety Tips for Viewing Wild Horses

Learn how to protect yourself while viewing the wild horses.

Last updated: January 16, 2021

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