Geneticist Weighs in on Significance of Pryor Mountain mustangs

Noted geneticist gives his two bits on significance of Pryor Mountain mustangs
by BRETT FRENCH | Billings Gazette Staff | September 9, 2009

In the tug of war between Pryor Mountain wild horse advocates and federal managers, Gus Cothran’s name gets tossed around frequently.

Cothran, a professor at Texas A&M University who specializes in horse genetics, said his views are sometimes distorted to suit the needs of special interests.

“I understand that people are looking for expertise,” he said. “But it would be nice to talk to people in person so things don’t get taken out of context.”

Here are Cothran’s thoughts on the Pryor Mountain wild horse roundup and the resulting reduction of the herd.


How many animals does it take to keep a wild horse herd genetically viable?

“One-hundred and twenty breeding animals was kind of the agreed upon number several years ago,” Cothran said.

In the short term, the Pryor mustang herd won’t be hurt by a reduction from 188 animals to around 130, he said. The Bureau of Land Management, the federal agency in charge of the herd, had originally proposed removing about 70 animals. But the roundup ended Wednesday before the agency reached its target. In its new management plan, BLM is proposing a maximum herd of 120 animals, excluding the current year’s foals.

“There’s always a greater risk when you reduce the herd, but you have to balance that with the environmental issues,” Cothran said.

BLM has justified its reduction of the herd based on a study of the Pryor Mountain Wild Horse Range that found the mountain and high desert area in ecological decline from overgrazing. The agency has proposed installation of water guzzlers to spread horses out on the mountain in hopes of easing the pressure on higher meadows, in addition to conducting prescribed burns to remove conifers.

The concern in keeping only 120 horses on the range, Cothran said, is the threat of a potential die-off that could reduce the herd even further, reducing the herd’s genetic diversity.

In the Pryor horses’ favor if such an event should occur, though, is that the horses that have been adopted out could be used to recharge the herd in such a dire situation.


The BLM also has experimented with using the contraceptive PZP to reduce the growth of the herd.

Cothran said the contraceptive has been used successfully. But he said there’s a danger in applying the drug to a small population of animals because it can control which genetics are passed on.

On the plus side, Cothran said, delaying reproduction – or what he called extending the genetic interval – is good for a herd. The genetic interval is the time between when an individual animal reproduces and when its offspring breeds. The longer the interval, to a point, the better for slowing the rate of genetic change.

“The roundups aren’t real popular things, either,” he said. “In the long term, PZP may be less disruptive for the horses.”

Wild family tree

The Pryor Mountain wild mustangs have been shown to be descendants of old Spanish breeds, Cothran said. These are the horses first brought to the Americas by the Spanish.

These genes aren’t pure, however. The Pryor horses probably bred with ranch stock and horses from other wild horse ranges were introduced out of concern over inbreeding in the Pryor horses, he said.

“There are a number of different strains of the colonial Spanish horse, each of them are different genetically now because they were isolated from each other,” Cothran said.

He did say, however, that the breed brought over by the Conquistadors to Central America no longer exists.

Despite any direct ties to a specific breed, Cothran said the Pryor mustangs are special horses.

“There are not many herds out there now that have the old colonial Spanish-type horse,” he said.

Because the Pryor herd contains this unusual genetic component, Cothran said they are unique.

“I do agree that we need to try and preserve this herd.”

Changing times

Cothran recognizes that management of the Pryor horses can be controversial, with people on all sides of the issue concerned about the horses but with different ideas on how to best take care of them. The situation is further complicated by variables such as changes in the environment and within BLM’s management.

There is one point maybe all can agree on.

“They are one of the most significant, if not the most significant, wild horse herds in the United States,” Cothran said.

Link: Billings Gazette: Noted geneticist gives his two bits on significance of Pryor Mountain mustangs

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  1. If I buy a Mustang from one of these Mustang Heritage sales, how do I know I am adopting a true mustang and not simply (genetically) a feral or wild horse?

    • Bess Carnahan says

      Pete: Excellent queston. I just found it today in working on info for my web site.

      There is a registry for the Pryor Mountain Mustangs and I am the registrar. Only a horse, proven to be pure Pryor Mountain Mustang can be registered. Requirements are: The horse was purchased during one of the few auctions at the Britton Springs (Pryor Mountain) auction and you have the papers, OR The horse can be shown to be the offspring of a pair of registered Pryor Mountain Mustangs OR you have the horse genetically tested by Texas A & M. There are at least two breeders who have bred and sell Pryor Mountain Mustangs. Red Tolman at Arrowhead Farms in Massachusetts and Bess Carnahan in Lyman NE. I do know that Red has some very pretty Pryor Mountain Mustangs for sale at the present time. I do not have any horses ready for sale. I should have a web site up and running in the next few months and may have more information then about horses for sale. In the meantime, you can contact me at:

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