What's happening at Assateague Island National Seashore on Maryland's Eastern Shore has been called "ponies on the pill."
In an effort to reduce the herd, the National Park Service, which runs the seashore, has been using a dart rifle to inject mares with a contraceptive drug.
Wild horses, a dying breed throughout the West, are proliferating on coastal islands from Maryland to Georgia. But scientists have determined this is not such a good thing.
The horses are being blamed for disrupting the islands' ecological balance.
The animals can't get enough of at two types of tall, broad-leaved grasses that perform important functions, say biologists who study the eating habits of wild horses. The horses feast on dune grass, which protects beaches and dunes from erosion by surging tides. They also are partial to cordgrass, a common East Coast marsh plant that serves as habitat for birds, crabs and small mammals, such as mice and foxes.
As more horses than ever devour these grasses, island beaches are eroding, and marshes are stubbier and contain less wildlife.
"They're really causing fundamental changes in the ecology here," said Carl Zimmerman, who manages the horse contraception program at Assateague.
The same can be said for coastal islands in North Carolina and Georgia, where university researchers and federal wildlife officials have been studying the behavior of wild horses.
The horses have not been studied at Chincoteague National Wildlife Refuge on Virginia's Eastern Shore. Refuge officials there have asked for money to do a study.
The National Park Service, meanwhile, is trying to come up with ways to manage the wild horse population on coastal islands throughout the Southeast. There has been no shortage of suggestions.
Drug them. Keep them inside barbed-wire fences. Round them up and sell all or some of them.
But some say the horses have gotten a bum rap and should be left alone.
"We're pretty fed up with all of this talk that the horses are causing problems," said Roe Terry, a Chincoteague firefighter who serves on the local Pony Committee. "It's just not so."
Mr. Terry blames the destruction of grasses at the Chincoteague refuge on snow geese, which are known to eat grasses -- roots and all.
Snow geese have presented a problem at the refuge, officials there concede, but not in recent years. For some reason, fewer have been coming to the refuge.
A recent study by two Pennsylvania State University researchers who studied horses' eating habits at Assateague found the park has fewer grass plants because of the horses, and the plants there are stressed and don't reproduce as well. The plants that horses don't like dominate the marshes, which never have been so short.
"When you walk into a marsh with cordgrass, it should be 18 to 24 inches high -- up to your knees," Mr. Zimmerman said. "When you walk into our marshes, the cordgrass is three to five inches high. You can see your ankles."
What's more, the marshes there contain fewer fiddler crabs and rails, birds that use tall grass as a cover while they lay and incubate their eggs.
"If the horses weren't causing these problems," Mr. Zimmerman said, "there would be no reason to blame them. After all, wild horses are an important heritage of Assateague."
Wild horses are not native to the coastal islands, and their origin is the subject of much debate.
But Mr. Zimmerman said wild horses are a vital part of Assateague Island National Seashore, which Congress established in 1965. "Congress wanted the horses to be there," he said. "Part of the appeal of this place is that there are wild and free-roaming horses here."
In 1968, 28 horses lived there. Today there are 180.
"We feel pretty strongly that we have already passed the point where we have too many horses," Mr. Zimmerman said, though no one has figured out an ideal number.
The horse contraception program involves shooting adult mares with a vaccine that creates antibodies, which bind to egg cells, making it difficult for sperm to attach to egg cells and fertilize them.
"We're hoping for zero population growth in 1995," Mr. Zimmerman said. "The management objective is to maintain the horses at Assateague so they'll always be there.
"The key will be finding a balance."