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Birth Control for Wild Horses

February 6, 2000

SHACKLEFORD BANKS, N.C. (AP) _ The first wild mare through the chute was Number 67, Darkface, swiftly pinned by handlers so biologists and veterinarians could descend on her with needles and syringes.

As Darkface and 30 other mares whinnied and snorted and the Atlantic surf thundered in the background, she was injected with an immunocontraceptive being used for the first time to manage the wild herd on this desolate, 9-mile-long barrier island, part of Cape Lookout National Seashore.

With the advent of the contraceptive PZP, adoption and natural attrition no longer are the only means of controlling the herd’s size.

In the spring, if biologists decide Darkface doesn’t need to get pregnant this year, she will get a PZP booster shot from a dart gun. So will Delta, Helena, Splish, Leila, Shag and the other mares. The procedure is 90 percent effective.

``This technique was designed to control wildlife populations where traditional lethal methods are no longer legal, wise, safe, or publicly acceptable,″ said Jay Kirkpatrick, director of science and conservation biology at Zoo Montana, a regional zoo in Billings, Mont.

Kirkpatrick’s group administers the contraceptive under an agreement with the Humane Society of the United States.

``We believe immunocontraception provides the best, most humane way to limit population growth,″ said John Grandy, senior vice president of the Humane Society. ``You can limit population growth without killing the animals.″

Grandy and Kirkpatrick said the contraceptive has been used to control deer populations in urban areas, where residents don’t want hunters shooting near their homes.

On the North Carolina island, if biologists decide a horse shouldn’t foal next year, they need only shoot her with a dart containing a booster good for one foaling season. That will help the Park Service keep the genetically unique herd at 100 to 110 as required by law.

Park Service biologist Sue Stuska said when the herd is at capacity, the contraceptive boosters will control its size and keep the horses healthy. But during a low birth year or if a hurricane kills some horses, ``we don’t give next year’s booster because we want to increase the foal count,″ she said.

The shots were administered during a roundup to thin the Shackleford herd from 133. Wild horses were selected by a private foundation for the first adoptions since the Park Service took over the island in 1966. Seashore Superintendent Karren Brown said before PZP and the adoptions, disease and the elements largely regulated the herd’s size.

PZP (the initials stand for Porcine Zona Pellucida) is made from pig eggs and costs about $20 a dose. In existence about 30 years, the drug’s use in wild animals is new. It produces antibodies that block fertilization by distorting sperm receptors on the animal’s eggs.

Adopted in 1995 for managing the wild horse herd at Assateague Island National Seashore in Maryland and Virginia after years of trials, it now is employed on herds owned by the U.S. Bureau of Land Management in Nevada, Oregon and Utah.

PZP also is used to control deer populations in Groton, Conn.; Fire Island, N.Y.; Newton, Mass.; at an arboretum in Morristown, N.J., and on the campus of the National Institute of Standards and Technology at Gaithersburg, Md.

African elephants in Kruger National Park in South Africa, water buffalo at the U.S. Navy base on Guam and elk at Point Reyes National Seashore in California also are dosed with PZP to prevent overpopulation.

``In the case of the elephants, they shot between 300 and 800 elephants a year to keep the population down,″ Kirkpatrick said.

The Humane Society soon will use PZP to control the 200,000 wild dogs in Bucharest, Romania.

``The people love them and reject killing them as a solution,″ Grandy said.

PZP sterilizes dogs, but the reasons why aren’t clearly understood, said Kim Frisbie, Kirkpatrick’s assistant who injected the Shackleford horses with PZP.

Kirkpatrick said the Shackleford horses, descended from Spanish horses brought to the island 400 years ago, are unique. They have adapted to drinking brackish water, living on a low island whipped by salty winds, and to surviving hurricanes, insects and parasites.

The horses also exhibit unusual territorial behavior, with stallions keeping herds of females and juveniles on certain parts of the island. Groups of young males, called ``bachelor bands,″ roam until they can overpower an older stallion and take over his harem.

``We’re on the growing edge of this,″ Kirkpatrick said. ``This is really a historic herd of animals. You have to keep the gene pool intact. Herds like this are a scientific resource.″

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