Is the herd of wild horses at Theodore Roosevelt National Park big enough to survive?

MEDORA, ND The wild horses of Theodore Roosevelt National Park are enclosed within a perimeter fence which prevents the herd from breeding with outside horses to diversify their bloodlines.

For decades, official park policy has been to limit herd size to 35-60 horses, a size experts say is too small to guarantee a genetically viable herd without the risk of inbreeding.

Park officials, who are drawing up a new management plan for the horses, said they had no basis for keeping the horses, which they classified as livestock, and said their preferred alternative would be to phase out the herd. .

But other options in a range of alternatives might include making no changes to horse management divided between about 15 freely roaming bands in the South Unit and a small herd of longhorn steers grazing in the North Unit.

The herd is now estimated at 185 or more horses, a number that falls within the 150 to nearly 280 minimum that experts say is needed to ensure the horses’ genetic health.

Park Superintendent Angie Richman said if a decision were made to allow horses to stay, the park would have to reduce the herd from 35 to 60, a population target set by a 1978 environmental assessment.

Park officials declined to comment, however, on whether they would manage the horse herd to maintain numbers to ensure genetic variability and herd fitness.

Gus Cothran, a Texas A&M University equine geneticist who has studied the ancestry of the park herd, said that ideally the herd should have 150 to 200 horses to provide various bloodlines to maintain genetic diversity.

These are approximate numbers, he said. These numbers should be sufficient to maintain a loss of genetic diversity to 1% or less, a common goal of herd managers.

However, if the goal is to limit the loss of genetic diversity to 2 percent, the herd could be smaller, between 75 and 100 horses, Cothran said.

This is considered feasible, he said. It’s a little more risky.

Other experts have determined that a wild horse herd should be larger than a minimum of nearly 280 horses to ensure long-term genetic viability.

The horses stand on and at the foot of a hillock.

The horses of Theodore Roosevelt National Park had their origins in the “Indian ponies” and early ranches that roamed the Little Missouri Badlands. During the 1980s, the park targeted wild stallions for removal and introduced domesticated stallions, altering the bloodlines of the herd.

Patrick Springer / The Forum

The research found that 278 horses were needed to reach 100 breeding animals. A small herd results in reduced fortitude and genetic variability.

It’s not doing well because of inbreeding issues, said Erik Molvar, a wildlife biologist who directs the Idaho-based Western Watershed Project and has published research on the behavior, ecology and population dynamics of the Idaho elk. Alaska.

Over time, a small population may encounter a genetic bottleneck, with some traits becoming prevalent while others fade, especially when some herd members are removed or unable to reproduce.

An even higher number than 278 may be needed, Molvar said, adding that it may be the minimum number to prevent the loss of genetic variability over time. As a wildlife biologist, he recommended keeping that number.

Molvar noted that the question of whether the park is large enough to support a herd of that size is “a question not just for horses, but for all other species.”

A team of researchers writing on genetics in conservation management determined in 2014 that a population of 100 people is needed to limit total fitness loss to 10 percent or less, and even a population of 500 is too low to maintain evolutionary fitness perpetuates, which would require a population greater than 1,000.

Genetic diversity is important in maintaining the health of free-ranging horse populations, according to a report from the National Academies of Science for the Bureau of Land Management, which manages nearly 69,000 feral horses in 10 western states.

While there is no magic number above which a population can be considered viable forever, studies suggest that thousands of animals will be required for long-term survival and maintenance of genetic diversity, the report said.


The rugged badlands terrain of Theodore Roosevelt National Park has become a refuge that has isolated a distinctive group of wild horses descended from Plains Indian ponies, according to a Harvard researcher. The horses were a hybrid of Indian mustangs and early ranches.

Patrick Springer / The Forum

The threat of loss of genetic diversity is less for a managed population than for a truly feral population.

However, if new horses aren’t introduced to refresh bloodlines, genetic diversity will decrease over time, Cothran said.

Inbreeding among the horse herd at Theodore Roosevelt National Park has been a concern, he said. In fact, the herd has relatively low genetic variation, she added.

The park periodically harvests and removes the horses to prevent overpopulation, a practice that will reduce genetic variation over time, Cothran said.

Over the next few years, this will cause a further loss of variation, he said.

Since 2020, park policy has been to give all mares eight months of age and older a birth control drug, a practice that horse advocates have said further reduces genetic variation and threatens the genetic viability of the herd.

Any animal that doesn’t reproduce doesn’t really count in terms of maintaining genetic diversity, Cothran said. If they don’t reproduce, they don’t really contribute to the next generation.

On the other hand, he said, if mares are allowed to return to a reproductive state, they matter in terms of maintaining genetic diversity, but not if they are rendered sterile.

In 2019, Cothran warned of the low genetic variability of the park herds and the risk of inbreeding.

Five horses run in front of a rock face, their manes and tails flowing behind them.

Blaze and his gang are seen galloping into western North Dakota. The volunteers gave each of the horses a name and traced the 20 or so gangs.


They are at a level of genetic variability that should be of concern, he told the Forum four years ago. Their genetic variability is low enough to make the possibility of inbreeding real.

To address the vulnerability of inbreeding, Cothran and co-authors of a study published in 2018 recommended introducing outside horses, particularly mares, to increase the genetic diversity of herds. Blake McCann, a wildlife biologist and chief of science and resource management at Theodore Roosevelt National Park, co-authored the study.

The study, which explored the genetic ancestry of the park’s herd, attributed the low genetic diversity to a population bottleneck in the 1960s, when the park’s longstanding policy was to remove horses altogether.

A series of horse removals from 1964 to 1966 reduced the herd from about 25 to an all-time low of 16, according to a 1989 history of the park’s herd by Castle McLaughlin, an ethnologist who was a National Park Service employee at the time .

After the park was established in 1947, ranchers in the area still grazed horses in the park. By the mid-1950s, an estimated 200 to 300 horses were running free within the park, and 125 were captured in the park’s first roundup in 1954, the vast majority trespassing on ranch livestock, according to McLaughlin’s report.

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