When Mustang Populations Run Wild

When there’s frequent “hanky-panky among the herd,” researchers harness wild horses’ own immune systems as a contraceptive.

The following is excerpted from Wild Horse Country: The History, Myth, and Future of the Mustang by David Philipps.

I was waiting outside Dr. Jay Kirkpatrick’s office in Billings when he pulled up, driving an ancient blue Subaru hatch-back with a faded Cornell sticker on the back. In his early 70s, he was wearing a khaki uniform like a man on safari. He opened the door and pushed back the front seat to let out an equally ancient sheepdog that followed him into the building.

A reigning expert on PZP, Kirkpatrick had short, gray hair parted neatly to the side, wire-framed glasses, and a number of one-liners well polished by having told the story of PZP to dozens of classes over the years. After shaking my hand, he put on his best fake sinister smile and told me, “I want you to become an emissary of PZP. Let the indoctrination begin.”

Related Segment

Big Trouble Managing Mustangs

Kirkpatrick is the accidental Johnny Appleseed of PZP. He is not a horse person, but he was motivated by his belief in wildness. In his decades of work with the substance, he has trained hundreds of people with a three-day course that explains the biology, politics, and practical application of PZP. It includes plenty of dart-gun practice.

On the morning I arrived, Kirkpatrick led me down a small hall in his lab to a room where his latest class was waiting. In a room filled with various models of dart guns sat three other students: a regional BLM wild horse manager Holmes had persuaded to take the class; a wildlife director at the Humane Society of the United States, which was using PZP on horses and urban deer; and a 20-year-old woman with a baseball cap pulled down low over her eyes and camouflage fleece zipped up to her chin. Her mother, like TJ Holmes, had started darting a local herd. This one was in the McCullough Peaks, in the Bighorn Basin, near Cody, Wyoming.

“I really like to hunt,” the woman said. “Since I like to shoot things, my mom said I should try doing it for good instead of just killing them.”

“What do you like to hunt?” Kirkpatrick asked. The girl shrugged as she weighed how to word what could be a very long answer, then she smiled and said, “Food.”

Wild Horse Country: The History, Myth, and Future of the Mustang


Next it was the BLM manager’s turn. He had just moved from the Midwest to take over range management in the region that includes Disappointment Valley. The first person to show him around the wild horse herd was TJ Holmes. “So of course she told me all about PZP,” he said, “and it makes sense. Right now, with these roundups we are just filling buckets and no one has turned off the spigot.”

The woman from the Humane Society of the United States said her group saw PZP as extremely promising and planned to expand its efforts with the vaccine.

When it was Kirkpatrick’s turn, he settled in for a long yarn that told the whole history of PZP. “I’ve been doing this for more than 40 years,” he said. “There is a lot to catch up on. It started in 1971, the same year the Wild Free-Roaming Horses and Burros Act was passed. I was a wet-behind-the-ears reproductive biologist, just a few years out of Cornell, and I had a job at Montana State University. One summer afternoon two BLM cowboys with sweaty hat bands and shit on their boots walked in.”

One of the men was the manager of the Pryor Mountain Wild Horse Range, which roamed a windswept ridge not too far from Billings. The Pryor Mountain Range was created by a separate decree in 1968, before the 1971 law. The herd was starting to grow, and managers doubted they could find adopters for all the horses they wanted to remove. The manager asked Kirkpatrick whether there was any way to stop horses from reproducing.

For the reproductive biologist, it was an odd question. People had studied animal reproduction for centuries, but nearly all of their focus had been on how to increase reproduction, not how to put on the brakes. Kirkpatrick thought for a minute. He had grown up during the introduction of the birth control pill, so obviously he knew there was no physiological reason something couldn’t be done. That no one had done it yet did not mean no one could. Not realizing that his answer would dominate his life for the rest of his career, he casually told the cowboys, “Yeah, I guess so.”

Not realizing that his answer would dominate his life for the rest of his career, he casually told the cowboys, “Yeah, I guess so.”

Kirkpatrick and his good friend from grad school, John Turner, started working on the problem. First, they realized they had to learn something about how wild horse herds reproduce. The closest wild horse area was in the Pryor Mountains, about 40 miles southwest of Billings, on an 8,800-foot windswept collision of mesas and ridges. Lewis and Clark had passed by the mountains in 1804, on their exploration of the West, and named the area after Nathaniel Pryor, a sergeant in the expedition who had gone south toward the mountains trying to recover horses stolen by the local Horse Nations. Wild horses have been living up in the Pryor Mountains—isolated from other herds—for as long as anyone could remember.

To Kirkpatrick and Turner, it seemed like the perfect place to understand the horse’s reproductive dynamics. With his wife and Turner, he began spending summers in a one-room cabin in the Pryors, studying behavior and reproduction rates. During the winters, they tried to come up with a practical strategy for birth control.

Their first attempt at controlling reproduction was to try vasectomies for dominant stallions—something Velma Johnston had suggested as an option in 1976, shortly before her death. Their theory was that the snipped stallions would retain their harems of females but not reproduce. While the procedure worked, doing the surgeries in the field was too costly and controversial to be practical. Vasectomies were also less than ideal because they were not reversible. If a natural disaster wiped out too many horses, there was no way to reverse the fertility control.

[What’s up with that lather that horses produce when they sweat?]

Next they tried to make stallions infertile with high levels of testosterone. Horses tranquilized by dart from a helicopter were then given a huge dose of slow-release testosterone in a hip. It also worked. But the steroid was expensive and the use of the helicopter pushed the price beyond what the BLM could afford. The team also soon realized that focusing on one dominant stallion had little effect, because there is a lot of hanky-panky among the herds, and mares became pregnant anyway. “It was a pharmacological success and a practical failure,” Kirkpatrick said.

After that, they turned to trying to control mares, testing different hormone treatments that worked much like human birth control pills. Still they ran into problems. Natural hormones, like the ones millions of women take daily, broke down quickly after being injected, providing only a few weeks’ worth of contraception. Synthetics lingered in the food chain too long, meaning the effects could get passed on to other animals. One hormone they tried actually made the birth rate go up. “We tried so many things, but none of them really were acceptable,” Kirkpatrick said.

In 1985, after years of work, Kirkpatrick and Turner were still trying to perfect hormone treatments when another researcher discovered a different approach—one that used a horse’s own immune system as a contraceptive. Irwin Liu, an immunologist at the University of California at Davis, was working with a slaughterhouse byproduct called porcine zona pellucida, or PZP. Zona pellucida is a naturally occurring sticky protein that coats all mammal eggs and allows sperm to bind to the egg. It has a unique molecular structure that acts like locks that only sperm can fit. Porcine zona pellucida comes specifically from pigs’ eggs. Dr. Liu found that the zona pellucida from pigs is different enough from a female horse’s zona pellucida that, when injected into the horse’s bloodstream, the horse immune system flags it as an outside pathogen. The immune system then begins producing antibodies to fight the foreign zona pellucida. The antibodies are designed to bind to the pathogen and neutralize it. But pig and horse zona pellucida are similar enough that the anti- bodies designed to bind to pig protein also bind to horse protein. The antibodies essentially jam the locks on the horse’s eggs so no sperm can enter. No sperm, no foals.

[How climate may have influenced horses’ snouts.]

To get a reaction, it takes only a small amount of PZP—a few drops that can easily be loaded into a small dart. The antibodies stay in the horse’s system for about a year, after which the effects wear off, so PZP is reversible in case there is a sudden drop in population, but it lasts long enough that it can be applied with an annual dart.

To determine whether PZP could really work in a wild herd, Kirkpatrick and Turner needed an isolated test population. They found it in an unlikely spot, far from the wild horses of the West. Along the coast of Maryland and Virginia is a long, narrow strip of sand called Assateague Island. For as long as anyone can remember, wild horses have roamed the island’s marshes. Locals say the horses—which records show have lived on the island since before the Revolutionary War—were marooned when a Spanish warship bound for Cuba was dashed on the beach by a storm. The National Park Service, which runs the thirty-seven-mile-long island as a national seashore, says the horses more likely are strays from early settlers. But, like most mustangs, they are both. Tests have found Spanish and domestic genes in the herd. Either way, they have lived wild for a very long time.

The Assateague horses became famous in 1947, when Marguerite Henry—the author who later wrote Mustang: Wild Spirit of the West, about Wild Horse Annie—published a book called Misty of Chincoteague, about a wild foal that grows up wild on the island and is captured by a pair of kids who befriend her, then eventually let her go because they see her yearning to be free. It’s a simple story that sold more than two million copies and was made into a movie in 1961. No doubt it influenced many young people who later pushed for the 1971 law.


The wild horses on Assateague were protected by the National Park Service in the 1970s, but with no plan in place to limit population. By the late 1980s, the narrow island had 175 horses—more than the Park Service thought the sandy spit could handle. Kirkpatrick got a call from a ranger on the island, asking whether he could do anything to control the horse population.

“I didn’t even know there were horses out there,” Kirkpatrick said, recalling the conversation, but he said he would give it a try. An isolated island was the perfect test for PZP. In February 1988, he and Turner arrived on the island with a gun and a box of darts. The plan was to bring the population of 175 horses gradually down over several years to 100 and hold it there through annual darting. Each horse would get shots for three seasons, then be allowed to have a foal. That would allow the genetics to be passed down, while limiting growth of the herd.

Kirkpatrick and other researchers began sloshing through the marshes and crashing through the brush. At first it was easy. Thehorses were so used to tourists that the researchers could walk within 10 yards, load a dart, and fire. But as the team started going back spring after spring, the horses got smarter. They started to recognize the men and remember the sting of his dart. They kept a wider and wider distance. The darters learned too, though. They began to feign a lack of interest. Kirkpatrick pretended to be a tourist clamming or bird-watching. He ambled toward each mare in a series of tangential sashays, until he was close enough to hit his mark. The team learned to recognize each horse, keeping a folder with the markings and age of each, and the date when they were darted. The herd stopped growing and gradually started to shrink. The data showed the drug was about 90 percent effective, and, just as important, a small group of people, or even one individual, could deliver it by dart. The cost was far lower than any other alternative, including roundups. The project continues to this day.

[Are we smart enough to understand how smart animals are?]

Excerpted from Wild Horse Country: The History, Myth, and Future of the Mustang by David Philipps. © 2017 by David Philipps. Used with permission of the publisher, W.W. Norton & Company, Inc. All rights reserved.

Meet the Writer

About David Philipps

David Philipps is a Pulitzer Prize–winning national reporter for the New York Times and the author of Wild Horse Country: The History, Myth, and Future of the Mustang.

Explore More

The High Cost Of Notifications

The more you're interrupted, the more likely you are to interrupt yourself. Can we win the war on our prefrontal cortex?

Read More

Recreate Darwin’s Experiments In Your Backyard

Biologist James T. Costa recreated some of Darwin's experiments that led to his theory of evolution. Now, you can try them in the comfort of your backyard.

Read More