It’s a long way—in miles and in habitat—from domesticated ponies in the verdant meadows of central Pennsylvania to wild mustangs in the parched high deserts of Arizona, California, Nevada, New Mexico, and Wyoming. However, research under way in our college is bringing them closer together.

Researchers hope that contraception methods that seem to work on a small herd of ponies kept in a lush, partially wooded pasture at Penn State’s horse farm near Beaver Stadium can be applied to hundreds of thousands of wild horses in the West, where overpopulation is straining habitats and wildlife management budgets.

Contraception research with these Penn State ponies may benefit the West's wild horses.

“A pony is considered to be any horse that is less than 14.2 hands, or 58 inches, high,” explains equine scientist Nancy Diehl. “Ponies are a lot closer in size to wild mustangs than Penn State’s 1,200-pound quarter horses, so it makes sense to do our research with ponies. We currently have a herd of 14 ponies–13 mares and a stallion.”

Most contraception work with wildlife—which is what the West’s wild horses are considered to be—has focused on vaccines. But Diehl, whose expertise is horse behavior, teamed up with animal scientist Gary Killian, who has done extensive contraception research with white-tailed deer, to test a new method: inserting intrauterine devices (IUDs).

“These devices are very similar to the ones that prove so effective in preventing human pregnancies,” says Diehl. “Vaccines only seem to prevent pregnancies for about a year, and the IUD appears to be a longer-lasting, more effective method of dealing with wild horse overpopulation in the West.”

Horses evolved naturally in North America, and no one is sure why they didn’t exist on this continent at the time of European exploration. Spanish explorers reintroduced horses to North America in the late 15th century, and American Indians helped to spread horses throughout the Great Plains and the West. Until as recently as the mid-twentieth century, horses continued to be released onto public lands by the U.S. Cavalry, farmers, ranchers, and miners.

Federal law prohibits the killing or slaughter of any wild horses or burros. Before federal protection was passed, wild horses were victims of ruthless and indiscriminate slaughter for meat and hides, and their numbers plummeted.

Today, however, wild horses are reproducing at an alarming rate—herds of hundreds of thousands have been increasing by 15 to 20 percent per year, doubling their population every four years. They can be collected for adoption by private citizens—and annually thousands are—but adoption is limited by the difficulty and unpredictability of the taming process.

“Not nearly enough wild horses are adopted to control the wild populations, so many western states collect thousands of the animals and keep them in holding facilities that are humane but crowded and demoralizing to the captive horses,” says Diehl, who journeyed to the Reno, Nevada, area last year to assess the situation.

“From a behavioral standpoint, the holding facilities are disasters for the horses. The states are doing the best they can, but the horses collected can never be released back into the wild, and most are there for as long as they live.”

Using IUDs eventually may alleviate the population problem, but inserting them in wild horses won’t be as easy as it is with domestic ponies. “The ponies are docile and just stand there,” says Diehl. “With the wild mustangs, IUDs would have to be inserted as part of a program that involves capturing them and administering anesthesia to take blood and other samples.”

Last year, Killian worked with the state wildlife agency in Nevada to insert IUDs in 15 captured mustangs. “They are presently being monitored,” Diehl says. “There are many questions to be answered. We want to know that the IUDs stay in and work under natural breeding conditions, and we need to know that they are safe for the horses.”

Diehl also wants to be certain the IUDs don’t damage the wild horses’ social structure. “Wild horses have a very important social structure, made up of small groups consisting of a stallion and a harem of mares,” she explains. “We want to see the effects of mares not getting pregnant. There no doubt will be more wear and tear on the stallions. We will be monitoring their behavior as well as their physiology. We must be sure not to alter the harem structure, because if we change the harem structure, we change wild horses as we know them.”

—Jeff Mulhollem