MELISSA BLOCK, host:
From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.
MICHELE NORRIS, host:
And I'm Michele Norris.
Mustang horses often evoke nostalgia for the American West. But today, the government says there are just too many of them.
For decades, the U.S. Bureau of Land Management has tried to restrict the booming horse population through adoptions and roundups, but adoptions have dropped.
As NPR's Serri Graslie reports, the government is now trying a different tack - birth control.
SERRI GRASLIE: It used to be cowboys who rounded up wild horses. Now, it's helicopters. The Bureau of Land Management, or BLM, has used them for years.
This YouTube video shows a low-flying chopper as it chases a herd across rugged terrain in Nevada's Calico Mountains. The horses are funneled into a holding pen where they slow, then stop. Confused by the sudden confinement, they circle, looking for an exit, and kick up snow as they bang against the metal corral.
Roundups like these aren't perfect, but they're necessary according to Tom Gorey, spokesman for the BLM's Wild Horse and Burro Program. He says right now, there are thousands more horses than the range can support.
Mr. TOM GOREY (Spokesman, Wild Horse and Burro Program, Bureau of Land Management): When that happens, we are mandated under the law to remove the excess horses.
GRASLIE: Gorey says the population can double every four years. Now, there are nearly 40,000 wild horses and burros in 10 Western states. The BLM says there's not enough food or water to sustain them, and all those trampling hooves can destroy habitat for other animals.
But activists say roundups are dangerous and can be deadly. Panicked mustangs can trample other, exhausted ones, and worked-up stallions may fight.
Suzanne Roy is with the American Wild Horse Preservation Campaign. She says there's a better way to manage them.
Ms. SUZANNE ROY (Campaign Director, American Wild Horse Preservation Campaign): It would involve shifting resources away from roundup and removal strategies, to management strategies on the range.
GRASLIE: That means removing cattle on the same land, and introducing natural predators. But Tom Gorey, with the BLM, says allowing the horses to roam and reproduce freely isn't smart.
Mr. GOREY: Yes, Mother Nature could regulate the horses through mass starvation and dehydration, but we don't think that's a viable alternative.
GRASLIE: But there is one thing activists and the BLM both agree on, and that's the use of birth control. A vaccine has already been successfully tried by another government agency, the National Park Service.
For 20 years, Allison Turner has managed the wild horses at Assateague Island National Seashore in Maryland. During the winter, the horses here do their best to hide on the small island. We searched for them in sand dunes and salt marshes in her Park Service truck.
Ms. ALLISON TURNER (Biological Technician, Assateague Island National Seashore): This happens all the time.
(Soundbite of laughter)
Ms. TURNER: When you need to - they've been out every day for the last two weeks.
GRASLIE: She's hands-off with the island's 114 horses, and never provides food or vet care. The park wants them to remain as wild as possible.
Finally, Turner spots a pinto in the dunes.
Ms. TURNER: There's two bands of horses here, grazing just at the edge of the shoreline.
GRASLIE: Lately, she's been pregnancy testing the mares. And with horses you can't touch, you have to get creative.
Ms. TURNER: And so what I have to do is find the mare that I want tested, and just wait until she drops a pile. And then I go and collect it.
GRASLIE: Using a plastic bag, she collects the sample, freezes it, and ships it off for testing.
In the spring and summer, Turner uses a modified rifle to shoot the mares with birth-control darts. The vaccine she uses comes from a protein found in pig eggs. When it's injected in any female animal other than the pig, it temporarily blocks fertilization.
Jay Kirkpatrick is a scientist who pioneered use of the vaccine on Assateague Island more than 20 years ago.
Mr. JAY KIRKPATRICK (Director of Science and Conservation Biology, ZooMontana): We treated 26 mares, and we simply darted them. The dart would inject the vaccine, and then the dart pops out. And a year later, not a single foal was born. That's the first time that had ever been done.
GRASLIE: Each mare is allowed to have one offspring during her lifetime.
Mr. KIRKPATRICK: We glibly refer to it as the equal opportunity management plan because every mare has an equal opportunity to make a genetic contribution to the herd.
GRASLIE: Last year, only two foals were born on Assateague. The BLM hopes to see similar results in the west. They'll still round up and remove 10,000 horses this year, but they'll also vaccinate 900.
It will be a challenge, though. They'll have to track down the mares across millions of acres, and that's not the only difficulty. The vaccine is actually making those same mares live longer - up to 10 years longer.
Serri Graslie, NPR News.